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Soil Application of Fungicides in Strawberry

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Strawberry is a high-value specialty crop in California and is susceptible to multiple pathogens that infect roots, crowns, foliage, flowers and fruits. Verticillium wilt caused by Verticillium dahliae, Fusarium wilt caused by Fusarium oxysporum f. sp. fragariae and Macrophomina crown rot or charcoal rot caused by Macrophomina phaseolina are major soilborne diseases that cause significant losses if they were not controlled effectively. Chemical fumigation, crop rotation with broccoli, nutrient and irrigation management to minimize plant stress and non-chemical soil disinfestation are usual control strategies for these diseases. Botrytis fruit rot or gray mold caused by Botrytis cineaea is a common flower and fruit disease requiring frequent fungicidal applications. Propagules of gray mold fungus survive in the soil and infect flowers and fruits. A study was conducted to evaluate the impact of drip application of various fungicides on improving strawberry health and enhancing fruit yields.

Propagules of gray mold fungus survive in the soil and infect flowers and fruits.

 

Methodology

This study was conducted in an experimental strawberry field at the Shafter Research Station in fall-planted strawberry during 2019-2020. Cultivar San Andreas was planted on October 28, 2019. No pre-plant fertilizer application was made in this non-fumigated field which had Fusarium wilt, Macrophomina crown rot and Botrytis fruit rot in the previous year’s strawberry planting.

Both soilborne diseases were present throughout the field during late spring 2019 with symptoms of wilt or crown rot appearing in many plants. In the current study, each treatment was applied to a 300-foot-long bed with single drip tape in the center and two rows of strawberry plants. Sprinkler irrigation was provided immediately after planting along with drip irrigation, which was provided one or more times weekly as needed for the rest of the experimental period.

Each bed was divided into six 30-foot-long plots, representing replications, with an 18-foot buffer in between. Between November 6, 2019 and May 9, 2020, 1.88 qt of 20-10-0 (a combination of 32-0-0 urea ammonium nitrate and 10-34-0 ammonium phosphate) and 1.32 qt of potassium thiosulfate w ere applied 20 times at weekly intervals through fertigation. Treatments were applied either as a transplant dip or through the drip system using a Dosatron fertilizer injector (model number D14MZ2). The following treatments were evaluated in this study:

  1. Untreated control: Neither transplants nor the planted crop was treated with any fungicides.
  2. Abound transplant dip: Transplants were dipped in 7 fl oz of Abound (azoxystrobin) fungicide in 100 gal of water for four minutes immediately prior to planting. Transplant dip in a fungicide is practiced by several growers to protect the crop from fungal diseases.
  3. Rhyme: Applied Rhyme (flutriafol) at 7 fl oz/ac immediately after and 30, 60 and 90 days after planting through the drip system.
  4. Velum Prime with Switch: Applied Velum Prime (fluopyram) at 6.5 fl oz/ac 14 and 28 days after planting followed by Switch 62.5 WG (cyprodinil + fludioxinil) at 14 oz/ac 42 days after planting through the drip system.
  5. Rhyme with Switch: Four applications of Rhyme at 7 fl oz/ac were made 14, 28, 56, and 70 days after planting with a single application of Switch 62.5 WG 42 days after planting through the drip system.

Parameters observed during the study included leaf chlorophyll and leaf nitrogen (with chlorophyll meter) in February and May; fruit sugar (with refractometer) in May; fruit firmness (with penetrometer) in April and May; severity of gray mold twice in March and once in May; other fruit diseases (mucor fruit rot caused by Mucor spp. and Rhizopus fruit rot caused by Rhizopus spp.) once in May, three and five days after harvest (on a scale of 0 to 4 where 0=no infection; 1=1-25%, 2=26-50%, 3=51-75% and 4=76-100% fungal growth); and fruit yield per plant from 11 weekly harvests between March 11, 2020 and May 14, 2020. Leaf chlorophyll and nitrogen data for the Abound dip treatment were not collected in February. Data were analyzed using analysis of variance in Statistix software and significant means were separated using the Least Significant Difference test.

Fruit yield per plant from 11 weekly harvests between March 11, 2020 and May 14, 2020.

Results and Discussion

Leaf chlorophyll content was significantly higher in plants that received drip application of fungicides compared to untreated plants in February while leaf nitrogen content was significantly higher in the same treatments during the May observation. There were no differences in fruit sugar or average fruit firmness among the treatments.

The average gray mold severity from three harvest dates was low and did not statistically differ among the treatments. However, the severity of other diseases was significantly different among various treatments with the lowest rating in Abound transplant dip on both three and five days after harvest and only three days after harvest in plants that received four applications of Rhyme. Unlike the previous year, visible symptoms of the soilborne diseases were not seen during the study period to evaluate the impact of the treatments. However, there were significant differences among treatments for the marketable fruit yield.

Leaf chlorophyll and nitrogen content between February 4, 2020 and May, 15 2020.

The highest marketable yield was observed in the treatment that received Rhyme and Switch followed by Velum Prime and Switch and Rhyme alone. The lowest fruit yield was observed in Abound dip treatment. Unmarketable fruit (deformed or diseased) yield was similar among the treatments. Compared to the untreated control, Abound dip resulted in 16% less marketable yield and such a negative impact from transplant dip in fungicides has been seen in other studies (Dara and Peck, 2017 and 2018; Dara, 2020). Marketable fruit yield was 4% to 28% higher where fungicides were applied to the soil. Although visible symptoms of soilborne diseases were absent during the study, periodic drip application of the fungicides probably suppressed the fungal inocula and associated stress and might have contributed to increased yields. The direct impact of fungicide treatments on soilborne pathogens was, however, not clear in this study due to the lack of disease symptoms.

Considering the cost of chemical fumigation or soil disinfestation and the environmental impact of chemical fumigation, treating the soil with fungicides can be an economical option if they are effective. While this study presents some preliminary data, additional studies in non-fumigated fields in the presence of pathogens are necessary to consider soil fungicide treatment as a control option.

Marketable fruit yield on various harvest dates. Marketable yield was 4% to 28% higher where fungicides were applied to the soil with the exception of Abound dip, which resulted in 16% less marketable yield.

 

Acknowledgments

Thanks to FMC for funding this study and Marjan Heidarian Dehkordi and Tamas Zold for their technical assistance.

 

References

Dara, S. K. 2020. Improving strawberry yields with biostimulants and nutrient supplements: a 2019-2020 study. UCANR eJournal of Entomology and Biologicals. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=43631.

Dara, S. K. and D. Peck. 2017. Evaluating beneficial microbe-based products for their impact on strawberry plant growth, health, and fruit yield. UCANR eJournal of Entomology and Biologicals. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=25122.

Dara, S. K. and D. Peck. 2018. Evaluation of additive, soil amendment, and biostimulant products in Santa Maria strawberry. CAPCA Adviser, 21 (5): 44-50.

Alternative Nematicides for Root-Knot Nematodes in Melons

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California is one the largest producers of melons in the U.S., and melons most commonly grown in California are cantaloupes and honeydew types. Although some acreage is reported throughout the state, most are grown in the Southern desert valleys and San Joaquin Valley. Root-knot nematodes (RKN), Meloidogyne spp., are the most significant plant parasitic nematodes affecting melon production in California, especially in light-texture soils. The nematodes are widespread in Central and Southern California.

Damage results from feeding of second stage juveniles (J2) inside melon roots, and the roots respond to nematode invasion by formation of root galls. Nematode-infested plants become stunted and less vigorous with severe galling of roots. Deformed roots due to galls are unable to sustain the water and nutrient needs of the plant in hot weather, leading to wilting and poor growth of plants, reduced yield and poor fruit quality. Nematode-infested plants may also become more vulnerable to other soilborne pathogens.

Currently, there are no resistant cultivars in melons, and RKN management has mainly relied on the use of preplant soil fumigants and soil-applied nematicides. Management with these products is expensive and involves safety and environmental risks. New fumigant regulations by the Department of Pesticide Regulations (DPR) have been put in place to restrict the emissions of volatile organic compounds from the use of soil fumigants. These regulations include limits on the amount of soil fumigants a grower is allowed to use in a year, caps on the amounts allowed within a township and new expanded buffer zones, meaning large parts of a field may not be treated all. These new regulations by DPR may mean that there will be some fields not treated for nematodes because of caps placed on the amount a grower is allowed to use or caps on the amount of fumigants allowed in a township.

Alternative control options that have high efficacy, are economically viable and environmentally safe need to be evaluated under field situations. The goal of this project was to evaluate the efficacy of Salibro, an organic product and a developmental product (DP1) in comparison to Nimitz (fluensulfone) in melons applied through deep-buried drip tube. Nimitz is registered on melons in California.

Root galling at harvest. 1) Untreated control 2) Nimitz 3) Gropro 4) Salibro 5) DP1

 

2020 Field Trial

This study was conducted as a small plot field trial on our RKN-infested site at the Shafter research farm. A western shipper-type melon variety, ‘Durango’, was hand transplanted onto 60-inch beds on June 30, 2020. There were four replications and five treatments in this trial arranged in a randomized block design. Rates, timings and method of application for each treatment is listed in Table 1, on page 18. Each plot was 20 feet in length with a five-foot buffer between plots along the bed. Treatments were applied either as a pre-plant or post-plant application through buried drip. Before chemigation, water was run for ten minutes to ensure all treatment tubing was filled, and after, chemigation water was run for about 20 minutes to flush the lines. The plots were irrigated using a surface drip and maintained using standard agronomic practices.

Table 1: Treatments, rate/A, application schedule and timings.

Before applying the treatments, soil samples were collected from all plots using a sampling tube at a depth of eight to 10 inches and submitted for analysis to determine the RKN count. Soil samples were collected and analyzed for nematodes again at harvest. Melon roots were evaluated for galling at mid-season and at harvest. Data on nematode counts and root galling was analyzed using SAS (statistical analysis software).

 

Data and Results

Plant vigor for each plot was rated visually on August 11, 2020 on a scale of 1-5, with 1=worse and 5=best plants. Vigor included the size of the vines and general appearance or health of plants. On the same day, five plants from each plot were randomly selected and visually rated for severity of root galling on a scale of 0-10; (0=no galls, 10=completely galled roots). The average galling on these five plants was used to give a galling index for each plot. The fresh shoot weights of these plants (without fruits) were determined.

At harvest on September 22, 2020, soil samples were collected from each plot for RKN count. All plants in each plot were dug and the severity of root galls on these plants was visually rated on a scale of 0-10 (0=no galls, 10=completely galled roots). The average of the galling on these plants in each plot was used to give a galling index for each plot.

Plant Vigor
No obvious differences were observed in plant vigor among treatments (Table 2). Some plots were a little more vigorous than others, but these differences were not attributed to treatment effect.

Table 2: Mid-season average plant vigor of melon plots in five treatments during the 2020 growing season. Vigor on a scale of 1-5, with 1=worse and 5=best plants.

There were no significant differences observed in fresh shoot weight of melon plants during mid-season evaluation on August 11 (Table 3). However, Nimitz resulted in higher shoot weights than the other treatments.

Table 3: Average plant weight (g±SE) of melon vines in five treatments during the 2020 growing season.

Root Galling
The severity of root galling was assessed at mid-season and at harvest. At mid-season evaluations, root galling was moderate and ranged between 2.4 in the Nimitz treatment and 4.6 in the untreated control (Table 4). Root galling in Nimitz and Gropro treatments was significantly lower than the other treatments. At harvest, there was a little increment in root galling across all treatments, however there were no significant differences among treatments. Surprisingly, Salibro and the developmental product was not beneficial in the trial and had higher root gall ratings than the non-treated control plots at harvest.

Table 4: Average root galling of melon plots in five treatments during the 2020 growing season. Galling on a scale of 0-10 (0=no galls, 10=completely galled roots).

 

Conclusion

In our 2020 trial, there were some treatment effects on mid-season root galling with Nimitz and the organic product Gropro having statistically lower root galling index compared to other products. However, none of the treatments were significantly different at harvest, and the results indicate that none of the treatments had a long-lasting effect on RKN levels in the soil. Therefore, further evaluations are needed to better determine the efficacy of these products as sole treatments and in combination with other products and their potential and continued use by the melon industry.

This project was funded by the California Melon Research Board.

Weather Station use in Vineyards

Talk to a farmer; at some point, the discussion will focus on the weather. It is the number-one topic discussed amongst farmers because weather can impact farming in significant ways. A rain event in June may benefit one crop but negatively impact another. Weather forecasts are important for scheduling farm activities, and for many years, farmers relied on the Farmer’s Almanac for long-range forecast.

Fortunately, today’s California farmers have multiple options to obtain weather forecasts and real-time data to make farming decisions. One of California’s oldest publicly accessible weather networks is the California Irrigation Management Information System (CIMIS). Built to improve irrigation efficacy, UC Davis and the California Department of Water Resources (DWR) established CIMIS in 1982. CIMIS has provided California growers with weather data accessed via the internet at cimis.water.ca.gov/. Growers can select one of the 145 stations near their property, download the data to a computer and manipulate it for their use (e.g. calculating ET). However, todays farmers are busier than ever, and having to rely on downloading climate data throughout the season can be challenging, especially when data from multiple locations are needed or when there are not any CIMIS stations nearby.

Growers can log onto CIMIS once an account has been setup

The CIMIS weather station network has been, and continues to be, a valuable tool for agriculture. But growers and researchers need weather data that better represents their farm or research location. Some of California’s first “local weather stations” used in vineyards were hygrothermographs that recorded temperature and humidity. Several UC grapevine pest and disease models were developed and tested in the late 70s and early 80s using hygrothermographs placed throughout California’s vineyards. However, in the late 80s, hygrothermographs were replaced with weather stations equipped with radio telemetry and controlled by a data-collecting base station. Since then, as technology improved, weather stations have become more sophisticated and provide real-time, on-demand data, 24 hours/day.

Some of the first weather stations recorded temperature, humidity, precipitation and wind speed and direction, displaying the data as graphs that needed some interpretation. Today’s grape growers have access to the same data, but data presentation is clearer with user-friendly web portals and phone and tablet apps. Additionally, climate, environmental and irrigation sensors have been improved or newly developed to generate data that can be used to make farming decisions (e.g. solar radiation, atmospheric pressure, leaf wetness, plant and fruit growth, plant health, soil moisture, electrical conductivity, pH, nitrogen, phosphorus, potassium, microbial activity, water pressure and usage, and more). Having access to numerous types of data allows growers to manage their vineyards in a much different manner than just a decade ago. However, access to that much data can be overwhelming if it is not understood or managed properly. Growers should be trained or have dedicated personnel to help interpret the information and share it with colleagues that will be making farm management decisions.

 

Some Practical Uses of Vineyard Weather Station Data

Advanced sensor technology has made it easier for growers to install, build, maintain and expand a reliable in-house network of stations collecting different information. Access to local data helps decision-making that impacts crop yield and quality. For example, in addition to knowing how much precipitation a vineyard experienced during a rain event, information such as leaf wetness, relative humidity, soil moisture and wetting depth can help forecast diseases (i.e. bunch rot), future fertilizer and irrigation applications and general vineyard activity that involves tractor work. Combine that information with vineyard characteristics (i.e. variety, soil type, etc.) and suddenly growers can evaluate more acres with fewer vineyard visits that save them time and money. The improved collection and transmission of data from base (aka weather) stations equipped with unique sensors have become valuable tools for managing vineyards. Base stations now incorporate weather, phenological, pest and disease models developed by university researchers to enhance their offerings via portals and apps. The following are some additional applications that can benefit grape growers interested in designing their own network of sensors.

 

Pesticide Applications

Pesticide applications must follow California’s laws and regulations. Prior to any application, climatic conditions must be checked so pesticide applications are optimized. Having a base station within a vineyard can improve pesticide application efficiency and efficacy. An anemometer, which measures wind speed and direction, can help pesticide applicators decide if wind conditions will permit a pesticide application. Wind speeds need to be between 2 to 10 mph to make a legal application. Knowing the wind direction can help decide the potential movement of a pesticide to an undesirable target (i.e. organic field). Tracking vineyard temperature can help determine if temperatures are hot enough to cause spray mist evaporation or phytotoxicity. In that situation, waiting for daytime temperatures to cool or spraying at night could help solve the issue. Being able to check a vineyard’s temperature prior to sending a crew to apply pesticides will save time, money and improve pesticide planning. Temperature sensors can also be used to detect inversion layers that can contribute to pesticide drift. Placing temperature sensors at multiple heights (e.g. 5 feet and 30 feet) will determine if the lower layer is cooler than the upper layer. When this happens, pesticides can move horizontally from thousands of feet to miles from the original point of application. When a vineyard experiences inversion layers, temperature sensors and a base station can detect the scenario and send an alarm to the person planning pesticide applications.

 

Degree Days and Temperature Modeling

Temperature, measured in degree-days (DD), influences grapevine growth throughout the season. From grapevine bud dormancy to fruit maturity, temperature regulates vine and fruit development. DD model predictions can help growers prepare for seasonal cultural practices (i.e. bloom sprays). For grapes, temperatures greater than 50 degrees F have been determined as the developmental DD threshold. In California, Thompson Seedless development as a function of DD has been determined. Approximately 50% bud break for this cultivar is observed when 155 DD are obtained with a February 20th start date. To reach 50% bloom, an additional 741 DD are needed, with maturity reached between 2880-3240 DD. A grower can use this information to track and identify DD specific to the varieties that they grow that have similar growth characteristics to Thompson Seedless. UC has also developed DD models for western grape leafhopper, omnivorous leafroller, powdery mildew and other pests and diseases to help grape growers make management decisions.

 

Chilling Hours

Grapes require a specific number of chill hours to complete dormancy, break bud and begin a new season. The minimum number of chill hours required for grapes to produce a commercially viable crop averages approximately 150 hours, which is low compared to stone fruit (i.e. cherries, peaches, etc.) that need ≥800 hours. Without adequate chill hour accumulation, bud break and yield become erratic, increasing farming costs significantly. Knowing when chill hour requirements are not being met can help a grower decide when to apply chemicals that improve bud break. A local weather station can better define chill hour accumulation than regional weather data (i.e. CIMIS). Most weather stations can automatically calculate the chill hours and portions. The chill portions algorithm accounts for warmer times of the day and presents a clearer forecast for predicting cold temperature needed for uniform bud break. More information about chill hour accumulation can be found here: fruitsandnuts.ucdavis.edu/Weather_Services/chilling_accumulation_models/about_chilling_units/.

 

Worker Protection

Worker protection is undoubtedly one of the most important responsibilities a grower has when people are working in the vineyard. Excessive UV or heat exposure can result in chronic and acute health issues. Weather stations not only track and monitor adverse weather conditions, but can also be programmed to send warning emails prior to critical UV or heat events. Since growers must record these events, weather station data is an easy way obtain documentation.

Weather stations have many uses in the vineyard beyond temperature and precipitation. The complexity of the station will depend on a grower’s need. They can generate an enormous amount of data that will need to be interpreted correctly so good decisions can be made. Table 1 shows what types of information can be determined from the different kinds of sensors.

Types of sensors needed to make management decisions.

 

Selecting a Weather Station that Makes Sense

Purchasing the right weather station will depend on the type of data needed to meet your needs. With multiple weather station options, growers can design a simple or complex weather station network that best suits their farm operation. Working with a weather tech company will help a grower determine their specific needs. After an initial needs assessment, a few follow-up meetings with a vendor will help finetune a weather network design. A 40-acre vineyard might easily be covered by a single station, especially when site characteristics are similar (i.e. climate, soil). However, using a single station to cover a 400-acre vineyard could result in misinterpreted climate data that may vary over a large property. Advancements in communication and sensor technologies now make it possible to have multiple sensors communicate remotely with a base station and have the information organized into an easy-to-read format.

 

Hardwired vs Solar Panel

The decision to use hardwired or solar power will depend on where the station will be located. Easy access to electricity and/or communication lines is usually the determining factor. Open space near buildings or rooftops are sometimes preferred because electricity is in close proximity. If in-field hardwired installation is preferred, wires placed in conduit to the correct depth and properly wired will be needed to avoid damage or interference with data collection. Solar-powered stations have become more affordable and are also easy to install. Solar panels and batteries will need to be checked and maintained biannually to avoid data collection interruptions. Whichever type you choose, it’s important to avoid installing stations near tall objects (e.g. trees, utility poles, buildings), paved roads or bodies of water because they will interfere with data collection. Poor site selection can result in poor data collection.

 

Number of Soil Moisture Sensors Per Acre

Temperature, humidity and wind speed and direction are less variable than soil moisture and can be located in one or two areas that represent the property (e.g. pump station). However, the number of soil moisture sensors required per block will depend on how variable the soil texture is. If soil and topography are homogeneous (i.e. uniform), and several blocks have similar characteristics (e.g. cultivar, rootstock, age, irrigation and management), two or three sampling points should be sufficient. Soil moisture measurements should be set at a minimum of one, three and five-foot depths to determine water movement. However, soil moisture sensors can take measurements every foot, which may be needed for certain situations. Additionally, if a vineyard has different types of soils that represent different blocks, multiple soils moisture sensors may be needed to collect accurate data for irrigation scheduling.

 

Basic vs High-End Stations

All weather stations will provide some type of climate data, but vineyard size (acres), one’s knowledge and confidence in interpreting the data and data access frequency will help you identify they type of weather station that’s right for your operation. A basic station will offer traditional climate data like temperature, humidity and, in some cases, rainfall amounts. In addition to traditional climate information, more sophisticated weather stations will include wind speed, soil moisture, water pressure and amounts, etc. that will result in a lot of data that someone will need to track if weekly decisions are going to be made. It’s important to evaluate the presentation of the data that weather station vendors offer. Portals and apps have simplified the way that growers see and use large amounts of data, which makes it easier for vineyard decisions to be made. If you have a larger operation or want to have a more automated system, a station that is connected to the internet can be a better option. With this station, you will be able to collect the data in real time in the field or remotely. General and more detailed data can be accessed from your computer or phone app. Some manufacturers offer services to set alarms for pest and diseases based on models, degree days and chilling days. The alarms can be sent to your phone via text, email or automated call. Services varies from providers.

General and detailed weather station information from an app.

No matter what type of base station and sensor configuration you choose to purchase, it is important that you first identify what information will help you make better management decisions. Additionally, you should identify someone that will be tasked with monitoring the system and sharing information with farm personnel. This person should be involved in the discussions with the vendor(s) since they will also be in constant contact with the vendor’s customer service representative.

Disease model (left) and degree days threshold option (right).

Making Nitrogen & Potassium Fertilizer Decisions in Vineyards

The goal of fertilization for any crop is to ensure the optimum levels of nutrients are available to the plant at key stages in the growth cycle. Balancing these factors is an art as well as a science. The first step is identifying what nutrients to apply. The second step is deciding how much fertilizer to apply. The third step is choosing the best time to make the application.

 

Tissue Analysis

In an ideal world, all of the plant’s needs would be met by nutrients available from the soil. In wine grapes, however, soil tests are not a useful predictor of fertilizer needs as the vine’s uptake of nutrients is affected by soil chemistry such as pH and the dynamics of different soil types. Therefore, tissue analysis in the forms of leaf blade and petiole analysis are required. Tissue should be sampled twice per year. Bloom time petiole analysis describes the vine’s nutritional needs during the growing season. Tissue sampling at early veraison is useful for making decisions about macronutrient adjustments postharvest as nitrogen, phosphorus and potassium are all mobile in the vine between harvest and leaf fall.

Bloom time tissue analysis provides a good picture of shortages in micronutrients such as zinc, magnesium and boron. Leaf blades and petioles are separated and analyzed separately. For bloom time sampling, leaves should be selected opposite the first cluster, ideally at 50% bloom. When sampling at veraison, select the most recently matured leaf — usually the fifth or so leaf from the tip. The most important things to consider are sampling at the same time each year and accurately reflecting variation within the block.

Tissue analysis results need to be interpreted in the context of other information. Are vines exhibiting symptoms of a nutrient deficiency or excess? What is the overall vegetative growth of the vine? Is fruit set less than desired or uneven? Nutrient deficiencies are relatively easy to spot, but not always easy to diagnose. Tissue analysis will identify or confirm what those deficiencies are. Assessing vegetative growth is relatively easy. These data are only meaningful when compared year to year, so several years of data collection may need to be conducted before the relationship between fertilizers applied and their effects can be identified.

Therefore, record keeping is an important part of making fertilizer decisions. Information including tissue analysis and fertilizer applications from previous years provide insight into how in-season and post-season fertilizer applications affected nutrient status over time. Data on pruning weights and estimates of canopy growth at bloom can also show trends suggesting whether too much or too little nitrogen is being supplied.

 

Fertilizer Rates

As every vineyard and every block within a vineyard is different, the only way to accurately (i.e. efficiently) determine amounts of nutrients to apply is to adjust published ranges for the deficiency or excess of nutrients based on personal experience. Translating tissue analysis results into specific amounts of fertilizer to apply is not an exact science. Differences in soil can have a sizable impact on nutrient uptake and, therefore, fertilizer requirements between areas. Consider flagging rows or using GPS coordinates to sample the same areas. Year to year comparisons will tell you if your fertilizer decisions are accurate and effective.

Nitrogen and potassium are the primary nutrients that need to be supplied with fertilizers, although phosphorus and calcium are also important. Both have downsides if oversupplied, however. Excess N results in excessive growth and overcropping while excess K yields an unacceptably high pH in the juice at harvest. Replacing minerals is very important as they are transported off-site in the crop and not recycled back into the soil like leaves or canes. Every situation is different, but believable ranges are 3 to 5 pounds N, 5 to 8 pounds K, and 1 to 2 pounds Ca are removed from the vineyard each year. Most of these nutrients are taken up by the vine during the postharvest period if they are available.

Grapevines are composed of 1 to 2% N. 30% of N that the vine uses is taken up during the period between harvest and leaf fall. The same is true for K, although the rate of uptake drops off dramatically about a month after harvest. N is an important requirement for the production and function of proteins and is a major component of chlorophyll. Wine grapes will consume 40 to 50 pounds N per acre during the growing season. Much of this will be returned to the soil in the form of prunings and leaves, but the breakdown of pruning wood into bioavailable forms of N takes years, and much may be lost as nitrous oxide during the process. N is released from soil organic matter at the rate of approximately 20 lbs/acre/percent organic matter. Therefore, on soils with less than 2% organic matter, the rate of N provided through fertilization should be increase by 10 to 20%. On soils with greater than 3% organic matter, consider reducing the amount of N delivered. Nitrate levels in irrigation water can be meaningful contributors to total N supplied to the vine. In some areas of California, nitrate levels are high enough that they could be considered fertilizers.

Rates of N application in wine grapes vary from 0 to as much as 60 pounds per acre per year. Making decisions about the rate of nitrogen fertilizer to apply is complicated as excessive applications can result in nitrates leaching into groundwater and/or the generation of the greenhouse gas nitrous oxide. In leaf blades at bloom time, less than ~2% total N is considered deficient. N deficiency is expected with levels less than ~1.5% in leaf blades at veraison.

K uptake is negatively affected by high magnesium in the soil. Therefore, even if K is plentiful in the soil, additional K may need to be provided to avoid deficiency. K levels are deficient below 1% in both petiole and leaf blades during the spring and ~0.7% in the fall. A reasonable target is 2 to 3%.

 

Timing Application

N may be applied in a split application, with half being applied just after berry set and the other half being applied postharvest. In-season applications of N may not be necessary at all depending on the results of tissue analysis. This is an especially important consideration given that too much N in the vine during the growing season can result in excessive growth, shading inside the canopy and higher disease incidence. To avoid this, spoon feeding vines during the growing season can afford more control.

The goal of postharvest fertilization is to deliver N, K and Ca to the root zone during a time when the vines will take up the nutrient and store them in the trunk so that they are available when the vine breaks dormancy and the demand for these nutrients is the highest. For example, one study found that 50% of N in the canopy comes from N which had been stored in the trunk and roots of the vine. Timing is critical. Too soon and N may kick off a new flush of growth and delay dormancy. Too late and the vine is no longer moving water into the trunk. The canopy needs to be healthy and functional. Hitting this window with late ripening varieties may be difficult, especially if a frost knocks the leaves off. Also, care must be exercised in cold areas because excessive N postharvest can cause hardiness issues. Fertigation is the preferred method of delivering N and K fertilizers as foliar applications can result in an overly rapid uptake. Also, there needs to be sufficient soil moisture for vine roots to take up the elements.

The amount of time required for vines to restore their carbohydrate and mineral reserves varies by crop load. Vineyards with a crop load of 2 to 4 tons per acre need very little time to recover and restore after being harvested. Vineyards in the 4 to 8 tons per acre range require about a month for restoration. Vineyards cropped above eight tons per acre need between 4 and 8 weeks to build their reserves back up.

 

A Note on Compost

A grower may want to apply compost to their vines for multiple reasons, including delivering nutrients, inoculating the soil with microbes, increasing soil organic matter and improving soil structure. When using compost as a fertilizer, one must know the chemical analysis of the material and take into account the range of nutrients compost will contribute to the vine, including but not limited to P and K. Good quality, finished compost will have a C:N ratio of less than 20:1. Of the total N per ton in the compost, about 30% is available to the vine. Of that, approximately half is available the first year after application, with the remaining N becoming available over the next three to four years.

Although grapevines can survive in depleted soils, maintaining adequate crop loads and vine health requires replacing the elemental nutrients removed in fruit and to account for the long time required to recycle N and K from prunings and leaves back into the soil. A successful fertilization program provides enough of the required elements without producing excessive growth, high juice pH or generating pollution. Scientific data and historical records combined with experience can achieve the goals of the fertilizer program.

Thanks to Dan Rodrigues of Vina Quest LLC for his contribution to this article.

Pierce’s Disease in Grapevine

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Pierce’s disease is caused by the bacterium Xylella fastidiosa. These bacteria live within xylem, the vascular tissue through which water travels in a plant. As the bacteria population grows, it stimulates the plant to produce tyloses. The combination of bacteria and tyloses cause vessel plugging, which restricts water movement in the plant, thus causing many of the disease symptoms. These blockages will eventually lead to the vine’s death. It is estimated that Pierce’s disease costs the California grape industry $56.1 million a year in lost productivity (Tumber et al., 2014). To minimize losses, it is important to understand the biology of the disease, including the bacteria’s host range, how the bacteria moves from plant to plant, and how to identify infected plants will help growers prevent losses and control the disease.

 

About the Bacterium

The bacterium X. fastidiosa has a large known host range. The European Food Safety Authority maintains a database of known hosts for X. fastidiosa (their updated list approved in April 2020 can be found at doi.org/10.2903/j.efsa.2020.6114.) Research done throughout California has identified many hosts, including weeds such as shepherd’s purse (Capsella bursa-pastoris), filaree (Erodium spp.), cheeseweed (Malva parvifolia), burclover (Medicago polymorpha) and annual bluegrass (Poa annua) among many others (Shepland et al., 2006 and Costa et al., 2004). Overall, at least 350 host plants have been identified from over 75 plant families as hosts for X. fastidiosa. From a control standpoint, once X. fastidiosa has been introduced to a geographic area, it will be virtually impossible to eliminate it from that location with such a wide variety of possible hosts.

X. fastidiosa does have another level of complexity. To date, four distinct subspecies of X. fastidiosa have been identified. X. fastidiosa ssp. fastidiosa is the subspecies that causes Pierce’s disease in grapevine, while X. fastidiosa ssp. multiplex is the subspecies that causes almond leaf scorch (Rapicavoli et al., 2018). This does mean that almond trees with almond leaf scorch would be unable to be the source of Pierce’s disease in a vineyard. To narrow down the definitive host range, grape-specific PD (X. fastidiosa ssp. fastidiosa) was inoculated into a range of possible host plants using glassy-winged sharpshooters as a vector. After an incubation period, multiple positive ELISA results were obtained for several plants including black mustard (Brassica nigra), black sage (Salvia mellifera), mirror plant (Coprosma repens), Spanish broom (Spartium junceum), Mexican elderberry (Sambucus mexicana), almond (Butte) (Prunus dulcis), white sage (Salvia apiana), sycamore (Platunus racemose) and coast live oak (Quercus agrifolia), confirming that they could host the bacteria (Costa et al., 2004). This does ultimately lower the number of possible plant hosts for X. fastidiosa ssp. fastidiosa; however, it still includes a long list of common plants found in and around vineyards, enough that even including this reduced host list, managing only these species, and other hosts yet to be identified, is still outside the ability in most growing regions.

 

Insect Vectors

Bacteria within the xylem tissue of one plant may be spread to another plant through the feeding activities of certain xylem-feeding insects. In vineyards, two groups of insects have been identified as possible vectors: sharpshooters and spittlebugs. Spittlebugs have been shown to vector X. fastidiosa in controlled settings, but their importance as a Pierce’s disease vector in vineyards is unclear. Sharpshooters, on the other hand, are known to be effective vectors of Pierce’s disease in vineyards.

There are several different sharpshooters in California that vector X. fastidiosa. The most important of these in the coastal portions of California is the blue-green sharpshooter. This sharpshooter is not adapted to the hotter climate of the San Joaquin Valley (SJV). In the SJV, there are three other sharpshooters: the green sharpshooter (Draeculacephala minerva), the red-headed sharpshooter (Xyphon fulgida) and the glassy-winged sharpshooter (Homalodisca vitripennis).

From left to right: green, red-headed and glassy-winged sharpshooters. Size comparison measurements are in the bottom right of each individual image (photos by Jack K. Clark, Regents of the University of California.)

Green and Red-Headed Sharpshooter
The green sharpshooter and the red-headed sharpshooter are both small and prefer to feed on grasses. The red-headed sharpshooter is specifically drawn to and reproduces on Bermudagrass. Both the green and red-headed sharpshooter can be found in irrigated pastures and along waterways such as streams, creeks, canals and ditches. Neither of these sharpshooters prefers to feed on grapevines, however they may do so under certain conditions and thus transmit Pierce’s disease. Since they don’t prefer grapevines, they tend not to spread deeply into vineyards; thus, when these vectors transmit Xylella, it is usually only to grapevines along the edges of a vineyard, whereas vines in the middle or the sides away from the green or red-headed sharpshooters’ preferred habitat are not affected.

Glassy-Winged Sharpshooter
The glassy-winged sharpshooter is twice the size of either of the other two sharpshooters. Their large size makes them more effective as a vector for Pierce’s disease because they can travel further than smaller sharpshooters and feed more effectively on a wider variety of plants, including woody plants such as grapes. To date, over 350 plants have been identified as hosts of glassy-winged sharp shooter (cdfa.ca.gov/pdcp/Documents/HostListCommon.pdf). Many of the hosts for glassy-winged sharpshooters are also hosts for X. fastidiosa. One of the key hosts for both glassy-winged sharpshooters and X. fastidiosa in the SJV, and for local control of Pierce’s disease, is citrus. The large feeding range of the glassy-winged sharpshooter also means that it can spread the disease throughout the vineyard instead of just to the edges.

The glassy-winged sharpshooter is not a native California insect, only arriving in California in the late 1980s (first recorded in 1989). As this non-native pest is such a dangerous vector, the CDFA tracks their distribution. Most of Kern county, parts of Tulare and Fresno counties, and a very small sliver of Madera county just over the San Joaquin River are all hosts to naturalized populations of glassy-winged sharpshooters within the SJV. In SoCal, Ventura, Los Angeles, Orange, San Bernardino, Riverside and San Diego counties as well as portions of Santa Barbara county and a small section of Imperial county all play host to endemic populations.

CDFA map for April 2020 of glassy-winged sharpshooter distribution in California (photo courtesy CDFA.)

Identification of glassy-winged sharpshooters within and near these areas is important for controlling their spread as well as the spread of Pierce’s disease. At the top, the insect has a deep brown color with creamy white dots on the head and thorax. These colors and dots continue onto the abdomen; however, here they are covered with transparent wings (the source of their glassy name). Highlighting the glassy wings are red lines and patches which can be seen from both the top and side.

The other main identifying mark is the flat white marking along the side of the of the abdomen. When sitting on a stem, this white mark stands out under and through the wings of this sharpshooter. Younger nymph glassy-winger sharpshooters have yet to develop their namesake wings. Their bodies are a lighter grayish-brown with very small white dots. In this stage, the standout feature is their red eyes. The red is the same color that will soon highlight the parent’s wings.

Later-stage nymphs have started to transition to the adult body color, and the red color in the eye is mostly lost. However, the red color has transitioned onto the wing pads in a pattern that has started to develop the adult wing’s patterning.

Images of glassy-winged sharpshooter. A. Top view of adult glassy-winged sharpshooter. B. Side view of adult glassy-winged sharpshooter. C. Top view of young nymph glassy-winged sharpshooter. D. Side view of late-stage glassy-winged sharpshooter (photos by K.T. Lund, UCCE.)

 

Vector Monitoring and Treatment

Monitoring for glassy-winged sharpshooters can be done using yellow sticky cards. It is recommended to use cards that are at least 5.5” x 9” in size. One card should be placed for every 10 acres and checked weekly for recent activity. Monitoring should be done from budbreak through November. If a glassy-winged sharpshooter is found, and you are outside of a known population center, please contact your local agriculture commissioner’s office or cooperative extension office. Green and red-headed sharpshooters are not attracted to yellow sticky cards, so to monitor their populations you will need to use a sweep net. Sweep lush green grasses near and within your vineyard in April and May to assess population size.

For both green and red-headed sharpshooters, finding two adults in 50 sweeps warrants a response. Unfortunately, as both sharpshooters are only incidentally on grapevines, treating the grapevines will not help the situation. The preferred habitat (lush grassy areas) will need to be addressed. Due to the overlapping generations seen in these sharpshooters, insecticide treatments are often ineffective. Removal of preferred habitat is a more effective treatment option for these sharpshooters.

For glassy-winged sharpshooters, a single find warrants a response. A list of treatment options for glassy-winged sharpshooters can be found on the UC IPM webpage at ipm.ucanr.edu/PMG/r302301711.html. The most common insecticides used for glassy-winged sharpshooter control contain the active ingredient imidacloprid. As with all chemical control, it is important to rotate active ingredients regularly.

Research conducted in 2017 showed that several other insecticides had long-term control of glassy-wing sharpshooters. These included Sivanto (a.i. Flupyradifurone) and Assail (a.i. Acetamiprid), which still showed greater than 90% mortality 7 weeks after application; Actara (a.i. Thiamethoxam), which still showed greater than 90% mortality 5 weeks after application; Harvanta (a.i. Cyclaniliprole), which still showed greater than 90% mortality 4 weeks after application; and Sequoia (a.i. Sulfoxaflor), which still showed greater than 90% mortality 3 weeks after application (Haviland and Rill 2019). This research was conducted in citrus because relying on vineyard-only management is not enough for glassy-winged sharpshooters. With the larger range of the glassy-winged sharpshooter, it is important to focus on an area-wide approach. A pilot program with cooperation between grape and citrus growers has shown great promise in Kern county. Citrus groves are a primary overwintering spot for glassy-winged sharpshooters. When treatments can be applied to these locations, it can lower the number of glassy-winged sharpshooters and, thus, the presence of PD in the area.

Pierce’s disease symptom on shoots. Patchy bark maturation on current year’s shoots leaving green islands (photo by K.T. Lund, UCCE.)

 

Vine Infections

Early identification of infected vines is the final step in preventing a larger problem from Pierce’s disease. Infected vines can be a source of the disease for vectors to spread to neighboring vines. They are also a strong indicator that the bacteria and a vector are present in your location.

The leaves of infected vines will turn yellow (for green varieties) or red (for red varieties) along the margins. This discoloration will then work inwards from the margin, with the discoloration quickly turning to brown/dried dead tissue. This often happens unevenly or in sections. Shoot tissue also shows an uneven maturation process, leaving green islands within lignified brown tissue.

Affected leaves eventually fall off, but will sometimes leave the petiole still attached to the shoot. Not all these symptoms will be found on every infected vine. If you suspect a vine is infected with Pierce’s disease, you can contact your county’s viticulture advisor for corroboration. Ultimately, a diagnostic analysis is required to confirm the presence of X. fastidiosa in the suspected vine. Table 1 lists laboratories within California that offer Pierce’s disease testing.

Table 1: California Laboratories that offer Pierce’s disease testing.

 

References

Costa, H. S., Raetz, E., Pinckard, T. R., Gispert, C., Hernandez-Martinez, R., Dumenyo, C. K., and Cooksey, D. A. 2004. Plant hosts of Xylella fastidiosa in and near southern California vineyards. Plant Dis. 88:1255-1261.

Haviland, D. and Rill, S. 2019 Evaluation of glassy-winged sharpshooter mortality following exposure to aged insecticide residues, 2017. Arthropod Management Tests, 44(1), 1–1. doi: 10.1093/amt/tsz075

Rapicavoli, J., Ingel, B., Blanco-Ulate, B. Cantu, D. and Roper, C. 2018. Xylella fastidiosa: an examination of a re-emerging plant pathogen. Mol. Plant Pathol., 19(4), 786–800

Shapland, E. B., Daane, K. M., Yokota, G. Y., Wistrom, C., Connell, J. H., Duncan, R. A., and Viveros, M. A. 2006. Ground vegetation survey for Xylella fastidiosa in California almond or orchards. Plant Dis. 90:905-909.

Tumber, K. P, Alston, J. M, & Fuller, K. 2014. Pierce’s disease costs California $104 million per year. California Agriculture, 68(1-2)

Dormant Season Yield & Disease Management

Pruning and disease management are important vineyard practices that need attention in the dormant season. Pruning directly affects the upcoming season’s potential yield and quality and can directly and indirectly affect the vineyard’s long-term productivity. Pruning practices, and the care of pruning wounds, can also help manage trunk diseases. Pruning practices that optimize productivity and quality, and protect against trunk disease, will help extend the productive lifespan of vineyards.

 

Dormant Pruning and Yield Management

Grapevines are pruned for three main reasons:

  1. To keep the vine in a shape that conforms to the trellis system and facilitates vineyard operations.
  2. To remove old wood and retain fruiting canes or spurs for the current season crop, plus spurs for future wood placement.
  3. To select a quantity and quality of fruiting wood that is in balance with vine growth and capacity.

The choice of pruning method is largely influenced by fruitfulness characteristic of vine variety. For instance, most raisin varieties are cane pruned because their basal buds produce shoots with fewer and smaller clusters than apical buds (Fig. 1). In contrast, most wine varieties are spur pruned, since most wine varieties have adequate basal bud fruitfulness and spur pruning is less laborious and costly than cane pruning. However, cane pruning is sometimes preferred for certain wine varieties, like Carmenere, which have low basal bud fertility, or in cool climate regions where cool spring weather might reduce basal bud fruitfulness in other varieties. Understanding the factors determining the bud fruitfulness provides insight as to the best pruning practice for a given vineyard.

Figure 1: Bud fruitfulness from different nod positions on 15-node and 20-node canes of common raisin varieties in California. Figure is elaborated from Cathline et al. 2020. Catalyst: Discovery into Practice. 4: 53-62.

Grapevine yield is formed over a two-year cycle that begins with the initiation of cluster primordia within compound buds. Cluster primordia are initiated in basal buds first, around bloom time, with more apical buds forming cluster primordia in succession, and most buds having formed whatever cluster primordia they will have by veraison (Fig. 2). Sunlight promotes cluster initiation, so sunny, warm weather between bloom and veraison helps maximize cluster primordia formation, whereas cool and cloudy weather can lead to less fruitful buds (Fig. 3). Because basal buds tend to form cluster primordia earlier than the apical buds, spring weather can have a greater impact on the fruitfulness of some varieties than others. For example, cane-pruned varieties initiate cluster primordia over a longer time period than spur-pruned varieties.

Figure 2: Timing of cluster differentiation in the primary bud along one 15-node cane of Thompson Seedless. Figure is elaborated from Williams 2000, Raisin Production Manual (UC ANR Publication 3393).
Figure 3: Bud fruitfulness generally correlates with the light exposure. Note different varieties respond to light exposure differently. Figure is elaborated from Sanchez and Dokoozlian 2005, American Journal of Enology and Viticulture. 56: 319-329.

Raisin growers have long been advised to retain “sun” canes, which generally have more fruitful buds than “shade” canes. Cane and spur morphology can also indicate potential fruitfulness. Mature, round canes and spurs having moderate thickness and internode length are often the most fruitful. Narrow canes and spurs often indicate weak growth with inadequate starch content to support cluster primordia formation. In contrast, exceptionally thick canes with long internodes and a flattened shape, commonly referred to as “bull” canes, may also be expected to have poor fruitfulness. Stressed vines may have insufficient carbohydrate content to support maximum bud fruitfulness. Insufficient water, inadequate nutrition and poorly managed pest or disease issues (e.g. nematodes and powdery mildew) can all stress the vines and reduce bud fruitfulness. As previously mentioned, node position also affects fruitfulness, cluster size and fruit quality of cane-pruned varieties. Leaving longer canes could increase yields if the vines do not become overcropped.

Cluster initiation is generally completed by veraison, so, by late summer, the number of clusters a designated bud may have in the following season has already been determined. Therefore, before pruning, growers can collect and dissect representative buds from a vineyard and observe and count the cluster primordia with the aid of a dissecting microscope. This information may be used to help predict yield potential and adjust their pruning severity to help achieve a desired number of clusters per vine. As growers gain experience with this method, it might also help them adjust their canopy or irrigation management practices to help improve fruitfulness, since shoot exposure to light improves bud fruitfulness.

After a pruning strategy has been decided on, and the vines were pruned, the maximum potential number of clusters per vine has been fixed. One of the main goals of pruning is to retain the optimal number of buds per vine to regulate the crop size. If too many buds are retained after pruning, the vines may become overcropped, leading to poor canopy growth, unripe fruit and possible carry-over effects on the following year, resulting in erratic and delayed budbreak, slow canopy growth and poor yield and fruit quality. In contrast, if too few buds are retained, the vines may be undercropped, resulting in suboptimal yield and excessive canopy growth, which can cause self-shading, reducing fruit quality. Therefore, understanding bud fertility and potential crop load can help inform pruning decisions and thereby optimize yield and quality.

 

Disease Management

Pruning practices also have implications for grapevine trunk diseases, which can seriously reduce vineyard productivity. Trunk diseases are caused by different fungi including Esca or black measles, Botryosphaeria (Botryosphaeria canker), Eutypa lata (Eutypa dieback) and Phomopsis viticola (Phomopsis cane and leaf spot). All of these fungi can enter the vines through pruning wounds, especially after precipitation. After a fungal infection has been initiated, it grows toward the roots, slowly killing the vascular tissue, decaying the wood and eventually killing the vines. The typical symptoms from trunk-diseased vines are cankers, dead arms and cordons, and trunks, with vines collapsing in a few years. The economic loss can be dramatic, and trunk diseases may significantly reduce the productive life of vineyards.

Top left: Dead arms/cordons on vines with trunk disease. Top right: Botryosphaeria canker on trunk cross section. Bottom left: Esca symptoms on grape leaf. Bottom right: Eutypa dieback symptoms on grape (photos courtesy G. Zhuang.)

Pruning methods can affect the potential disease risk. Cane pruning typically has less trunk disease risk than spur pruning systems since cane pruning leaves fewer pruning wounds than spur pruning. The best mitigation strategy for trunk disease is prevention. Selective pruning, sometime referred to as “vine surgery”, can remove infected wood, sometimes resolving an established infection. However, vine surgery is laborious and will not be effective unless all the diseased wood is removed. Retraining may be needed to replace arms or cordons removed in surgery, and the surgery will result in large, open pruning wounds that could easily become infected if not protected with pruning protectants. The labor cost to renew the cordon or trunk is typically economically prohibitive in the San Joaquin Valley, and it also does not offer the long-term solution, since those fungi can slowly reinfect the vine if complete elimination of diseased wood was not achieved by the vine surgery. The current preventative measures include double pruning or delay pruning, pruning wound protection and vineyard sanitation.

Double pruning or delayed pruning helps prevent the exposure of final pruning wounds until February or March when most rain events finish and weather is warming. Less rain with warm weather helps the vines seal the pruning wounds and prevent the fungi entering through pruning cuts. However, double pruning or delayed pruning does have some barriers for some growers to adopt (e.g. pre-pruner and labor availability). For growers who can adopt it, double pruning or delayed pruning offers an effective way to minimize trunk disease.

Pruning wound protectants (mostly fungicides) are another option when double pruning or delayed pruning is impractical. Dr. Akif Eskalen, UC Davis, has been evaluating different pruning wound protectants in California since 2019, and the results from those trials can be found here: ucanr.edu/sites/eskalenlab/Fruit_Crop_Fungicide_Trials/. Fungicide efficacy is variable, but the application of pruning wound protectants before the rain event can help prevent the fungal infections. However, pruning wound protectants cannot provide complete protection, and we still do not know how long the protection lasts after the spray. More than one spray might be needed if rain events occur more frequently after pruning.

Vineyard sanitation should be also integrated into the trunk disease management plan. Because numerous fruiting bodies can be found on pruning debris left in the vineyard, complete destruction is desirable to reduce the source of inoculum and avoid new infections. An extensive sanitation of the vineyard should be practiced, keeping the inoculum level as low as possible. This can be accomplished by pruning out all diseased wood, removing it from the vineyard and destroying it by burning or burying.

Recently, mechanical pruning has become more popular due to the increased cost and declining availability of farm labor. However, mechanical pruning may leave more than double the number of spurs per vine compared to traditional hand pruning. Delayed pruning or pruning wound protectants should be applied after pruning to reduce the risk of trunk disease. In all, trunk disease does not only affect this year or next year’s yield and general vine health, but also reduces the longevity of vineyard production life.

Nutrient Management and Crop Disease Prevention

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Farmers and agronomists have long observed that well-fed crops tend to suffer fewer pest and disease symptoms compared with nutrient-deficient plants. However, few of us turn to nutrient management to actively prevent disease. Just as a healthy diet rich in vitamins and minerals is the first measure in disease prevention in human health, proper fertilization should be your crop’s baseline of defense against pathogens. Balanced nutrient applications and good-quality soil produce vigorous crops capable of warding off disease.

Plants require a balanced supply of 18 essential elements throughout their lifecycle to sustain growth, repair tissue, prevent and resist disease. Deficiencies in macro and micronutrients often lead to impaired cell wall structure and accumulation of metabolic products that attract pathogens. Weakened and leaky cells walls give pathogenic fungi and bacteria easy entry points into the plant and plenty of food to fuel their proliferation.

Disease prevention requires understanding the environmental conditions that favor the pathogen as well as the nutritive additions that can upregulate plant defenses. Macro and micronutrient applications can support disease-free growth by building strong structural support and developing a biochemical environment unfavorable to pathogenic growth.

 

Nitrogen

In most cropping systems, nitrogen (N) is the most limiting nutrient to plant growth. Adequate N at key growth stages must be supplied throughout the season to help the crop meet its growth and yield potential. N management can also impact disease susceptibility. The type of nitrogen applied, the amount and the timing of the application all affect the soil chemistry and plant physiology in ways that can either favor or suppress infection. N management decisions should be based not only on the crop’s N requirement, but also on the field’s history of disease. The type of pathogens the plants will likely face may determine how to fertilize the crop.

Obligate and facultative parasites are affected by the crop’s N status differently. Obligate parasites, which require a living host for survival, benefit from a high concentration of N in the plant tissue. Stem rust in wheat, club root in cruciferous vegetables and powdery mildew on many crops feed on succulent, living tissue. High N applications increase vegetative growth, increasing the proportion of young susceptible tissue. High N also increases the concentration of amino acids in cells and on leaf surfaces, attracting and feeding pathogenic organisms. Nitrogen-rich plants decrease their production of phenolics and lignin, weakening both structural and chemical defenses against infection. Overfed plants weaken their defenses.

While obligate parasites such as downy and powdery mildew proliferate at high N levels, facultative parasites are sometimes suppressed by higher N content in the crop. Facultative parasites such as Fusarium, Alternaria and Xanthomonas feed on dead or damaged plant tissue. Management practices that can keep plants vigorous and prevent senescence, or cell death, will reduce damage from facultative pathogens. If Fusarium is a known problem in the field, preventing N deficiency should be the first preventative action against infection.

Starving the crop of N will surely weaken the plant, making it more vulnerable to attack. However, more N is not always protective, even against facultative pathogens. Some facultative bacterial and fungal foliar diseases proliferate under high N applications because they preferentially feed on young, succulent tissue. N fertilization increases vegetative growth, attracting pathogens with tender new leaves and stems.

Evaluating the crop’s nitrogen status by taking frequent leaf samples and conducting the soil nitrate quick test can help farmers and advisers determine how much N to apply to stay in the proverbial goldilocks zone.

 

Potassium, Calcium and Magnesium

Potassium (K) fertilizer is another critical defense against crop disease. K applications reduce both facultative and obligate parasite infections through two main mechanisms. First, K can thicken and strengthen cell walls, helping to keep pathogens out. Second, K deficiency impairs protein, starch and cellulose synthesis, leading cells to accumulate amino acids and sugars. Weak cell walls leak these metabolites, attracting and feeding parasitic organisms.

K applications can decrease both disease incidence and severity. Researchers have shown that K fertilization reduces damage from bacterial leaf blight in wheat, rice and cotton. Others have demonstrated reduction in leaf spot, sheath blight and stem rot in a variety of crops (Dordas, 2008).

Calcium (Ca) deficiency promotes pathogen growth in similar ways as K deficiency. Ca is a critical component of cell wall structure and membrane integrity. When Ca levels are low, the plant is vulnerable to fungal infection via the xylem. Once inside the plant, pathogenic fungi damage vascular tissue and the crop wilts.

Ca deficiency can cause severe physiological disorders such as bitter pit in apples and blossom end rot in tomatoes and peppers. Former UC plant pathologist and current director of TriCal Diagnostics Dr. Steven Koike warns against black heart in celery. Black heart, a Ca deficiency disorder, causes young growth in the heart of the plant to brown and blacken. Koike explains that as the celery grows, the necrotic tissue is pushed up and exposed to air where it can easily come into contact with pathogenic fungal spores. Sclerotinia spores aggressively attack damaged tissue leading to severe infection. Ca deficiency in other crops creates similar damaged entry points for pathogens.

Ca fertilizer applications can prevent infection in the field and can continue supporting the yield postharvest. Amending the soil with Ca and increasing pH can suppress clubroot disease in cruciferous vegetables such as broccoli and cabbage. Ca applications can also suppress Pythium, Rhizoctonia and Botrytis. Foliar Ca sprays shortly before or after harvest increase shelf life and help fruit resist rotting (Gupta et al., 2017).

Magnesium (Mg) deficiency is less common and perhaps less obvious than Ca or K deficiency, but Mg is no less important. Magnesium is at the center of chlorophyll and facilitates photosynthesis. When Mg is low, photosynthesis is reduced, causing a cascade of negative consequences. Among other issues, Mg deficiency causes a buildup of sugars and amino acids in leaves, attracting and feeding pathogens. Mg, K and Ca are all positively charged cations and compete for uptake via the roots. Mg deficiency often occurs in high-potassium soils and can be worsened by K fertilization. Pre-plant soil testing can help determine whether Ca, K and Mg fertilization are advisable.

 

Phosphorous

After N, phosphorus (P) is the second-most widely used mineral nutrient. The effects of P applications on disease suppression are less clear than the other nutrients. P fertilization is most helpful in fighting root rot when applied to seedlings. Plenty of P helps young plants quickly develop strong root systems and escape infection during their most vulnerable early stages of development (Dordas, 2008). P foliar applications can also provide local protection and systemic resistance against powdery mildew in some crops. However, other studies have shown increased disease after P fertilization (Dordas, 2008). Providing adequate P for growth is critical, but applying additional P does not necessarily confer any protection against disease.

 

Micronutrients

Micronutrients including boron, copper, manganese and zinc affect disease incidence in many ways. Micronutrients are critical to plant metabolism, cellular structure and stress responses. Manganese fertilization can suppress disease by supporting production of lignin and suberin, which provide chemical defense against fungal infection. Low zinc can cause accumulation and leakage of amino acids and sugars as seen with macronutrient deficiencies. Boron is critical to cell structure and lignin formation. Micronutrient applications can also prevent disease by activating the plant’s defense mechanisms against a broad range of pathogens. Deficiencies in any of the micronutrients increases the crop’s vulnerability to infection.

 

Optimizing Nutrient Management

Complex interactions between the pathogen, the host and the surrounding environment determine disease incidence and severity. We can use nutrient management to strengthen the host and manipulate the soil in ways that suppress pathogens. Whenever possible, determine which diseases your crop will likely face. Review field history and send soil and infected plant tissue to a pathologist like Dr. Steven Koike at TriCal Diagnostics. Conduct preplant soil tests to determine pH and baseline nutrient levels. For example, if you anticipate Fusariam pressure, adjust pH to neutral or slightly alkaline using lime. Plan foliar micronutrient applications to compensate for decreased manganese and iron availability at elevated pH. Other diseases and soil conditions will require different techniques.

Increasing beneficial microbial activity and increasing soil organic matter can build a suppressive soil environment and simultaneously increase nutrient availability. In addition to nutrient fertilizer applications, consider amending the soil with composts and biostimulant products such as seaweed extracts, humic acids and beneficial microbial inoculants.

Many strategies can increase nutrient availability and help crops defend themselves against disease. The CCA and PCA should develop a plan that will meet the unique conditions on each ranch.

 

References

Dordas, C. (2008). Role of nutrients in controlling plant diseases in sustainable agriculture. A review. Agronomy for Sustainable Development, Springer Verlag/EDP Sciences/INRA, 2008, 28 (1), pp.33-46. ffhal-00886444f

Gupta, N., Debnath, S., Sharma, S., Sharma, P., Purohit, J. (2017). Role of Nutrients in Controlling the Plant Diseases in Sustainable Agriculture. In: Meena V., Mishra P., Bisht J., Pattanayak A. (eds) Agriculturally Important Microbes for Sustainable Agriculture. Springer, Singapore. https://doi.org/10.1007/978-981-10-5343-6_8.

Huber, D.M. & Haneklaus, S. (2007). Managing nutrition to control plant disease. Haneklaus/Landbauforschung Völkenrode. 4.

Update on Potential Insect Vectors of Grapevine Red Blotch Virus in California Vineyards

Over the past 10 years, wine grape growers, researchers and UCCE have been working together to control the spread of grapevine red blotch virus (GRBV), which is the causal agent of grapevine red blotch disease (GRBD), a red leaf disease that negatively impacts wine grape yield and quality.

 

Symptoms, Impacts and Spread

Symptoms of GRBD typically first appear around mid-season, although timing can vary across cultivars and between years. In red varieties, red blotches form outward from the leaf margin or within the leaf blade, and primary/secondary leaf veins will often turn red (Figure 1a). In white varieties, the blotches manifest as pale green or yellow patches with no reddening of the veins (Figure 1b). Symptoms typically first originate on basal leaves then over time progress to additional leaves further up the shoot. In the early season, these foliar symptoms are distinct from grapevine leafroll disease (GLD), but by late fall, leaf blade coloration of vines with GRBD may be similar to vines with GLD.

The unique symptoms of GRBD were first noticed in Napa County in 2008, and in 2011 testing confirmed the existence of GRBV, a previously uncharacterized Geminiviridae. Subsequent vineyard surveys across the U.S. revealed that GRBV is widespread nationally, and the virus has also been reported from vineyards in Canada, Mexico, Argentina, India and South Korea. The wide geographic range suggests that GRBV was likely initially spread through the propagation of infected plant material. That said, secondary spread has been recorded in some regions, indicating there may be unique insect vectors and/or non-crop reservoirs of GRBV.

Plant Hosts and Insect Vectors of GRBV

Data so far indicate that GRBV appears limited to the genus Vitis, which can include non-cultivated wild-type grape vines that typically grow in riparian habitats. Multiple insect surveys have shown that certain species or genera tend to frequently test positive, although none of which are considered significant pests of grape vines. To clarify, insects that test positive for GRBV do not necessarily have the ability to transmit the virus. Rather, these surveys help researchers identify candidate insect species for further testing. At present, transmission experiments with common vineyard pests such as leafhoppers, mealybugs and sharpshooters have not shown any transmission.

The only insect to date that has been reported by two different labs to successfully transmit GRBV is the three-cornered alfalfa hopper (Membracidae: Spissistilus festinus) (TCAH) (Figure 2a), which was able to move the virus between potted vines in a greenhouse setting. As its name implies, TCAH is primarily a pest of leguminous crops such as alfalfa, peanuts and soybean. While adults can oviposit into grape vines, the immature TCAH cannot completely develop on this host. In contrast, TCAH thrive on many types of legumes, which are critical for their development. Although TCAH can be found in California vineyards, its pest status is negligible and densities in vineyards typically tend to be very low. TCAH are pierce-suck feeders that, when present on vines, can girdle lateral shoots and leaf petioles, which leaves a distinct dark ring and leaves distal to the girdle turn red (Figure 3).

TCAH activity in vineyards (and in the vine canopy specifically) appears to be closely tied to the presence and quality of legumes in vineyard ground covers. TCAH overwinter as adults in protected areas in and around vineyards, and in the early spring begin to deposit eggs onto ground covers (likely legumes.) Nymphs (Figure 2b) pass through five juvenile stages as they develop. Ideal temperature range for nymph development is 65 to 95 degrees F; however, they can develop at temperatures as low as 55 degrees F. The appearance of first-generation adults roughly coincides with the seasonal dry-down and/or mowing/cultivation of vineyard ground covers. As ground covers are diminished in the late spring and early summer, TCAH adults may opportunistically feed in the vine canopy, which could potentially lead to transmission of GRBV between vines. That said, a recent study found that TCAH appear to have a very strong preference for ground covers, and significant activity in the vine canopy was only observed once >90% of ground covers were eliminated or dead. In the North Coast, TCAH likely complete 1 to 2 generations per year depending on climate and availability of food resources, whereas in warmer areas such as SoCal TCAH can complete 3 to 4 generations per year. Adults have been observed in vineyards until as late as leaf fall, after which they seek shelter in protected areas.

Given TCAH generally have a low affinity for grape vines, that other insects have tested positive in surveys, and the occasional rapid rates and extensive spread of disease, it is worth exploring other candidate vectors. Furthermore, field transmission by TCAH remains unclear, as does transmission efficiency (that is, how quickly TCAH can acquire and transmit the virus,) both of which are critical pieces of information to better understand disease ecology. As such, there are no existing recommendations to use insecticides to control TCAH in vineyards. Rather, a management strategy that concentrates on the removal of vines and reduction of persistent leguminous ground covers (such as burclover and Spanish clover) may be more effective. While the specific economics of individual vineyards will vary, the general suggestion is that individual vines be replaced when overall infection is <20% of the block, whereas with >20% of vines infected it may make more sense to replace a larger section or the entire block.

 

Testing Additional Vectors

One insect that is currently being investigated as a potential vector is the leafhopper Scaphytopius graneticus (Cicadellidae), which has frequently tested positive for GRBV – but again, this does not mean it can necessarily transmit the virus. Aside from frequently testing positive, what makes S. graneticus unique from other candidate vectors is its strong affinity for grape vines. Very little is known about this insect, and it is not considered a significant vineyard pest, but the sparse records that exist all report it from grape vines in the western U.S. Furthermore, recent vineyard surveys have documented relatively high populations of S. graneticus that are almost exclusively found in the vine canopy. As such, studies are currently underway to evaluate S. graneticus ability to transmit GRBV in both a greenhouse and field setting as well as characterize its host plants and reproductive biology.

Previous greenhouse research also indicated that the Virginia creeper leafhopper (Cicadellidae: Erythroneura ziczac) might also be a vector of GRBV. However, this insect is not always found in vineyards where GRBD spreads and other researchers have been unable to verify GRBV transmission by this leafhopper species or the related western grape leafhopper (Erythroneura elegantula) or variegated leafhopper (Erythroneura variabilis). There are other leafhopper species of interest such as the mountain leafhopper (Colladonus montanus) that have tested positive for carrying GRBV in the field but have not yet been proven to be a vector.

 

Conclusion

There are currently no thresholds or management recommendations for TCAH in vineyards. The existing information does not support the use of insecticides for population control. Strategies that remove infected vines and reduce persistent leguminous ground covers may be more effective. This insect is generally in low abundance and prefers to reside on leguminous ground covers. While some opportunistic feeding in the vine canopy can occur, it remains unclear whether this is sufficient to spread GRBV under field conditions. Additional studies are currently underway to verify TCAH field transmission of GRBV. In the meantime, monitoring TCAH populations on ground covers (using a sweep net) and in the vine canopy (using yellow sticky cards) can be combined with disease mapping to build a picture of annual changes in pest densities and disease incidence within the vineyard and drive management decisions.

Bacterial Leaf Spot in Lettuce

Bacterial Leaf Spot (BLS) of lettuce was first reported in the U.S. in 1918 in South Carolina. The disease then expanded to other production areas in California, Arizona and ultimately in Florida. The disease causes losses of entire production when outbreaks are significant. This disease is particularly devastating to the leafy vegetable industry because it is favored by warm and humid conditions. Besides the U.S., the disease has been reported in lettuce production in Europe, Asia and South America as well.

BLS is caused by the bacterium Xanthomonas hortorum patovar. vitians, formerly described as X. campestris pv. vitians (the pathovar (pv) vitians is unique to lettuce.) The bacterium can attack any type of cultivated lettuce; no relationship between lettuce type and immunity to the pathogen is known. The bacterium has three races identified thus far: races 1, 2 and 3, but race 1 has been reported in western and eastern lettuce production areas in the U.S. BLS is sporadic in lettuce with losses up to 100% in subtropical productions areas. Xanthomonas hortorum pv. vitians reproduces quickly when high humidity, leaf wetness and high temperatures are conducive for disease development. The infection starts as small brown spots that in later stages of disease development coalesce to form bigger spots.

Here in Florida at least one small outbreak is reported by growers during the growing season from October to May. Lettuce growers in Florida are the most affected by this disease because the warmer and more humid conditions in the state are more conducive for disease development; after a severe infection occurs, lettuce cannot be commercialized.

In the last five years, growers have been able to contain the disease from spreading to other lettuce farms nearby by destroying infested crop areas with lettuce BLS; therefore, small losses were manageable, and growers did not lose entire crops.

 

Control Methods

The disease is not prophylactically controlled as other lettuce diseases such as downy mildew, sclerotinia drop and powdery mildew because it is uncertain when the pathogen population will increase and develop to cause diseases. There are no bactericides that can eradicate BLS from lettuce production. Copper-based compounds can be effective in reducing the severity and incidence of outbreaks of BLS when the disease first appears. However, there is a potential for development of resistance to copper in the pathogen population following repeated applications.

The high variability of disease outbreaks each year makes it impossible to predict when preventative applications of copper should be used. A combination of a copper fungicide with mancozeb may be effective to reduce BLS in lettuce; both compounds have some efficacy against bacteria but are protectants and not curative. Systemic Acquired Resistance based fungicides probably have limited activity but can be used as preventative as well.

Several practices may help reduce outbreaks of lettuce BLS. These practices, however, should be part of an integrated disease management program instead of recommended alone.

Crop destruction at the point of infection and surrounding areas have proved to be effective to avoid other neighboring lettuce fields from becoming infected. This strategy has been successful in containing disease spread to other fields during recent small outbreaks in Florida. However, when the disease is highly spread, this strategy may not be economically feasible. This recommendation may only help in early detection of the disease.

The bacterium is believed to be transmitted in infested seed, which is the most common avenue of disease introduction. The use of disease-free seed is highly recommended, but to date there is no effective method to detect the pathogen on seeds and assure cleanliness from BLS. Seed production should be conducted in dry, cool environments with less likelihood of bacterium development.

The BLS pathogen spreads by rain and overheard irrigation. Drip irrigation can be used to mitigate spread of the disease by keeping foliage as dry as possible. In California and Arizona, most lettuce fields are drip irrigated. However, drip irrigation is not economically feasible in Florida’s commercial field production currently.

An adequate weed control in nearby areas of lettuce fields is highly recommended because the pathogen may be epiphytic on weeds. Many weed species such as those in the families Asteraceae, Amaranthaceae, Aizoaceae, Chenopodiaceae, Portulacaceae, Solanaceae and Malvaceae may host the pathogen, X. hortorum pv. vitians.

Ultimately, host resistance is the most efficient control method against the BLS disease. BLS resistance can be found in certain heirloom lettuce cultivars that are not acceptable for commercial production. Disease resistance towards race 1 strains of the pathogen can be easily transferred to romaine, iceberg and leaf lettuce cultivars using traditional breeding methods. There have been releases from the USDA Agricultural Research Service of lettuce lines with resistance to race 1 for the California/Arizona lettuce production system. The University of Florida is developing such resistances for the Florida production system. Resistance to races 2 and 3 against X. campestris pv, vitians remains to be reported.

 

Further Research

A partnership was formed between plant breeders, geneticists, plant pathologists, weed management scientists and extension agents from the University of Florida Institute for Food and Agricultural Sciences, Pennsylvania State University and USDA-ARS. This partnership will investigate the lettuce BLS interactions using several approaches that include breeding, lettuce genetics, pathogen genetics and detection. Researchers will improve lettuce cultivars against several races of X. hortorum pv. vitians and provide information on pathogen and lettuce genetics that will help the industry to efficiently manage this detrimental disease; this information will be released to growers, producers, the seed industry and other stakeholders in English and Spanish.

Baterial leaf spot symptoms in romaine lettuce(top) and iceberg lettuce(bottom).

Ultraviolet-C Light and Robotics for Autonomous Control of Strawberry Diseases, Insects and Mites

This article summarizes the recent work of a multi-disciplinary research team on strawberry disease and arthropod pest management using a robotic Ultraviolet-C (UV-C) irradiation machine. The team’s desire to develop novel approaches for more sustainable strawberry culture (e.g. non-chemical and safer use of ultraviolet light) was based on the following factors: Reliance on fungicides for management of diseases (e.g. development of resistance to more synthetic pesticides, phytotoxicity, environmental issues, application residues, etc.,) make chemical management of Botrytis gray mold of strawberry often challenging and costly, especially when the harvesting period exceeds six months; Pestiferous insects and mites are developing resistance to synthetic pesticides, increasing demands by consumers for pesticide-free berries; Need for automation to reduce labor input in the field; Need for systematic approaches for disease and pest management that are less intrusive to workers in the field, harvesting and myriads of seasonal cultural operations.

 

Sustainable Approach Through UV-C

The idea of using UV light to control fungal diseases of strawberry fruit came from a discussion between Drs. Janisiewicz and Takeda in 2010 about problems with use of fungicides and needs for alternative control methods. UV light includes electromagnetic radiation with wavelengths between 100 and 400 nanometers (nm), which is shorter than that of visible light but longer than X-rays. UV radiation is present in sunlight, but almost all the UV light produced by the sun is filtered out by the ozone in the upper atmosphere. UV light can also be produced by specialized lights such as mercury-vapor lamps and tanning lamps, LEDs and specific wavelength excimer lamps. UV radiation is subdivided into three general classes based on their wavelengths (i.e. UV-A ranges from 315 to 400 nm, UV-B ranges from 280 to 315 nm, and UV-C ranges from 100 to 280 nm.)

Research with UV-A and UV-B has shown that insect behavior could be altered by modulating UV irradiation levels. Research on UV-B showed that a nightly ~3-hour exposure time was necessary to achieve adequate control of diseases such as powdery mildew on grapes and strawberry. UV-C irradiation is used to kill microorganisms in different situations including sterilization of air in hospitals, water treatments, sterilizing laboratory benches and treatment of meat and poultry products. The FDA has determined that UV light can be used to surface sterilize food products, and considers UV light to be safe when precautions are taken by the user.

The use of UV light in crop production, however, has been limited because the doses required to kill plant pathogens usually can cause damage such as leaf burn and defoliation. The goal of the team’s research program has been to develop UV-C treatment methods that have high efficacy for disease and pest control without damaging plants and are cost effective.

What is known is that UV-C irradiation kills microorganisms and arthropod pests by damaging their DNA. What other agricultural researchers had not considered in their treatment protocol was the fact that microorganisms and mites have a special mechanism for repairing the damaged DNA. This occurs when light is present, e.g. the repair process is “light activated”. Thus, there is a simple question: What happens if the microbes are irradiated with UV-C at night and they are kept in the dark for certain periods of time? The reasoning for this approach was a) That the dark period following UV-C irradiation will prevent the activation of this DNA repair mechanism that needs sunlight to become active and b) Without the ability to repair the damage to DNA, the organism would not be able to replicate and further infect plant, eventually causing the microbes to die. The night application of UV-C light for control of pathogens and pests allowed the team to use much lower doses of UV-C light that was still effective and, more importantly, was below the threshold that causes damaging effects on the strawberry plant.

 

Robotic Application Methods for Pest and Disease

In the May/June 2018 issue of Progressive Crop Consultant, the team reported on the use of germicidal UV-C irradiation for the control of major fungal diseases of strawberries (e.g. gray mold, anthracnose and powdery mildew.) Currently, the team has prototype autonomous platforms applying UV-C light to in-soil strawberry plants at night. As the research on diseases progressed, the project expanded to include management of insects and mites and development of an autonomous vehicle to apply UV-C treatments. The team is aware that research on using UV-B and UV-C irradiation to control diseases and arthropod pests on various crops had been conducted for more than two decades in various laboratories around the world, including several in the U.S., the Netherlands, Belgium, Brazil, Canada, China, Japan, Norway and the United Kingdom. Their research consistently reported plant damage described as burns, leaf curls and defoliation after irradiating the plants with doses required to control diseases and arthropod pests. Recently, some of these laboratories have transitioned to using UV-C lamps in their research.

The effect of treating strawberry plants with UV-C light on subsequent infection by anthracnose fungus (Colletotrichum gloeosporioides 162). Two agar plugs with fungal mycelia were placed on each leaflet. The top row has three detached leaflets from UV-treated plants. Note that very narrow band around each plug with fungal inoculum has turned yellow after nine days. The bottom row shows leaflets taken from control plants (not exposed to UV-C irradiation.)

Our research approach has focused on new ways of using UV-C light to kill strawberry pathogens and arthropod pests (insects and mites) without damaging the strawberry plant. Initial studies clearly demonstrated that a dark period of 2 to 4 hours (depending on which pathogen was being controlled) immediately after a UV-C irradiation of one minute or less prevented microbes from repairing DNA damage (e.g. repair mechanism requires daylight) caused by UV-C irradiation and prevented the disease to develop on fruit and leaves. This method of UV-C irradiation at night resulted in lowering the effective doses needed to kill the microbes that cause strawberry gray mold, powdery mildew and anthracnose. The reduced doses required to kill plant pathogens caused no apparent damage to strawberry plants. Further analyses revealed there was no damage to chlorophyll, no reduction in photosynthetic activity, no loss in pollen viability (ability of pollen tube to grow through the style) and development of fruit, and no reduction in fruit phenolic content as determined by metabolomic analysis.

Another aspect of the team’s research program is to study other direct effects of UV-C light on plants. Since research was being conducted on diseases, the team asked, “Can UV-C light make the strawberry plant more resistant to fungal infection?”

It is known that when a plant is perturbed by abnormal environmental or biotic factors such as insect feeding, it responds by producing more phenolic and other chemical compounds such as jasmonic acid to make itself resist infection by a pathogen or an attack from insects and mites. For this test, the team harvested leaves from strawberry plants that had been treated with brief (15 to 60 second) nightly applications of UV-C irradiation for about a month and then inoculated each leaflet with two agar plugs with actively growing fungus-causing anthracnose (Colletotrichum). Over a period of 7 to 10 days, the team observed the infection (black and yellow lesion) develop on the leaves.

12A close-up of an autonomous vehicle which has a UV-C light array mounted on its frame operating in a commercial strawberry field. For the purpose of photography, the robot was operated shortly after sunset.

On the leaves taken from UV-C irradiated plants, the dark area was confined to the agar plug, while on the leaflets taken from untreated plants, the lesion radiated out from the plugs and caused large areas of the leaflet to turn black and yellow. The team’s research has shown that multiple levels of biological effects occur on disease-causing pathogens as well as make plants exposed to night-time UV-C light treatments more resistant to fungal infection.

The team also showed that night-time UV-C treatments were effective against insects and mites. A detailed study with plants artificially infested with ~100 two-spotted spider mites (TSSM) revealed that nightly 60-second UV-C irradiation of plants for six weeks had reduced mite populations to below a commercial treatment threshold of five mites per mid-canopy leaflet, while mite population on untreated strawberry plants exploded to over 300 mites per leaf. Tomato plants artificially infested with greenhouse whitefly (GWF) were irradiated for just 16 seconds nightly. The results showed a decline in adult, nymph and egg populations on UV-C treated plants within two weeks.

The team also conducted studies on spotted wing drosophila (SWD). In that study, store-bought strawberries were artificially infested with adult female SWD and then were subjected to a pulsed UV-C light treatment. Over time, 12 SWD on average emerged from each untreated control fruits compared to only 1 SWD among 40 UV-C treated fruits. A better way to use UV-C irradiation is to prevent fungal pathogens from infecting susceptible plants, pestiferous insects and mites from infesting the crops, and prevent egg laying in the fruit. The team has plans to refine the UV-C light regimen, quality and application technology to make the UV-C technology more practical for commercial berry producers. Currently, the team is exploring ways to use UV-C irradiation more effectively and to further reduce the time required to treat each plant so that the robot can travel over a larger area each day.

 

Things to Consider

Substantial costs can be incurred through application of synthetic pesticides, scouting or monitoring, especially if the annual crop is being harvested over six months. UV-C irradiation provides the opportunity to reduce chemical and labor inputs for management of T. urticae, gray mold, anthracnose, powdery mildew and pestiferous insects in the production of specialty crops. In the team’s research, the engineering aspects of the robot technology and optimizing the UV-C system to make the technology economically competitive with conventional chemical control approaches are being refined.

Further field-based validation is needed to confirm the efficacy of the system on a broader scale to compare with conventional management practices (e.g. pesticide application). Prototype autonomous mobile UV-C irradiation units that could traverse multiple strawberry rows have been developed and evaluated under field conditions. To date, the team has used strawberry plants and their fruit as a model system, but this UV-C technology may be widely applicable to other specialty crops.

 

Future Directions

Currently, the team is evaluating ways for reducing the duration and frequency of exposure, comparison of daytime and night-time applications and addressing the need for deeper penetration of UV-C light into the plant canopy. Mites prefer to colonize and
lay eggs on the undersides of leaves and in the lower portions of canopies. Therefore, UV-C penetration is critical. The team’s study has shown that UV-C ir radiation reaching the lowest portions of the strawberry plants was reduced by ∼45% compared to that received by the upper surface of leaves at the upper canopy. This reduction in intensity may have played a role in supporting the survivorship of mites recorded in the lower canopy. Japanese researchers have reported that reflective mulches enhance UV light penetration to the undersides of the leaves. Also, arthropod management with UV-C light should be examined in conjunction with our technology to increase management efficacy and bring pest populations below the economic threshold levels.

The positive results obtained from night-time application and relatively low doses of UV-C irradiation for controlling fungal pathogens and arthropod pests to date indicate the potential of UV-C treatments to reduce disease incidence and alter insect behavior impacting their herbivory and egg laying. The team will continue to explore the efficacy of the UV-C irradiation system for management of pestiferous mites and insects on other specialty crop plants. To make UV-C technology commercially useful for open-field strawberry growing systems and nurseries, a prototype robot for applying UV-C was developed and field-tested in 2019 and 2020. USDA is now collaborating with TRIC Robotics (tricrobotics.com), which has highly skilled robot engineers, manufacturing capacity, and provides on-the-go solutions for autonomous UV-C application for specialty crops. The robotic platform is agricultural equipment built to control pests and diseases. As such, UV-C application equipment will be certified by EPA as a pesticide device. The videos of their robot applying UV-C treatments in the field during night time and day time can be accessed on YouTube by searching the key words “TRIC Autonomous UV-C Treatment.”

Now, it is not so much a question of whether UV-C light treatments can be applied effectively to field-grown specialty crops, but how soon application equipment and UV-C technology will be available to large and small growers. The team is currently conducting R&D work to develop a commercially viable UV-C delivery system that is affordable, user friendly, generally compatible with current production practices, as effective in controlling diseases and pests as currently used pesticides and reduces the overall costs of disease and pest control practices.

It is important to develop a platform that is useful and affordable to small-acre strawberry growers. The Northeast has over 200 certified organic farms with five acres or less of strawberries. To fund this aspect of the UV-C research project, the team received a grant in 2020 from the USDA Northeast Sustainable Agriculture Research and Education (SARE) program to improve control of diseases and arthropod pests of strawberry. Also, the robotic UV-C application system is being developed for large-scale strawberry growers and nurseries in California and elsewhere. Cooperating growers in California have been identified to conduct large pilot studies that will begin in early 2021.

 

Summary

The management of disease-causing fungal pathogens and arthropod pests is a complex task for specialty crop producers, particularly given the occurrence of latent infection by Botrytis cinerea and the highly perishable nature of fruits and berries, and the need for a complete control of SWD in berry fields. This requires comprehensive knowledge of disease etiology, host-plant biology and the physical environment determining the severity of infection and pest infestation.

Failure to manage Botrytis cinerea at bloom will have an impact on subsequent stages of fruit development, especially when the fruit reaches harvest maturity. Future research will certainly focus on multi-pronged strategies instead of searching for a single “silver bullet” answer to produce effective, long-term solutions for specialty crop growers. These solutions will surely include genetic improvements, the integration of night-time UV-C applications for a range of cropping systems along with use of “clean” nursery plants, good sanitation practices, new chemicals, and knowing how environmental conditions affect the biology of pathogens and pests to reduce disease pressures and pest infestation.

Additional information on UV-C light technology can be requested from F. Takeda (fumi.takeda@usda.gov). Further discussion on the UV-C technology can be found in the additional resources section.

 

Additional Resources

Janisiewicz, W. J., Takeda, F., Glenn, D. M., Camp, M. J., and Jurick, W. M. II. 2016. Dark period following UV-C treatment enhances killing of Botrytis cinerea conidia and controls gray mold of strawberries. Phytopathology 106: 386-394. https://doi.org/10.1094/PHYTO-09-15-0240-R

Janisiewicz, W. J., Takeda, F., Jurick II, W. M., Nichols, B. and Glenn, S.M. 2016. Use of low-dose UV-C irradiation to control powdery mildew caused by Podosphaera aphanis on strawberry plants. Can. J. Plant Pathol. 38:430-439. https://www.tandfonline.com/doi/full/10.1080/07060661.2016.1263807

Takeda, F., Janisiewicz, W.J., Smith, B.J., Nichols, B. 2019. A new approach for strawberry disease control. Eur. J. Horticult. Sci. 84:3-13. https://www.pubhort.org/ejhs/84/1/1/index.htm

Short, B., W. Janisiewicz, F. Takeda, and L. Leskey. 20/18. UV-C irradiation as a management tool for Tetranychus urticae on strawberries. Pest Management Science 74(11):2419-2423. https://doi.org/10.1002/ps.5045

Sun, J., W. J. Janisiewicz, F. Takeda, B. Evans, W.M. Jurick, M. Zhang, L. Yue, and P. Chen. 2020. Effect of nighttime UV-C irradiation of strawberry plants on phenolic content of fruit: Targeted and non-targeted metabolomic analysis. J. Berry Res. 10(3):365-380. https://www.doi.org/10.3233/JBR-190482

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