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Weather Station use in Vineyards

By
Stephen Vasquez and Gabriel Torres
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An anemometer, found at the top of the weather station, measures wind speed and direction and can be placed in or near the vineyard. (all photos courtesy S. Vasquez.)

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).

Biostimulant Regulations Evolving

By
Cecilia Parsons
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New guidance is being developed for regulating Biostimulants as their use in California crops increases.

Biostimulant products have become widely used in agricultural production, but there are still unresolved issues around definitions, labels and efficacy claims.

Nick Young, senior environmental scientist with CDFA’s Fertilizing Materials Inspection Program, provided an update on biostimulant labeling from a regulatory perspective as part of the UC Ag Experts Talk webinar

The pending USDA definition of a biostimulant is, “a substance, microorganism or mixtures thereof that, when applied to seeds, plants, the rhizosphere, soil or other growth media, act to support a plant’s natural nutrition processes independently of the biostimulant’s nutrient content. The plant biostimulant thereby improves nutrient availability, uptake or use efficiency, tolerance to abiotic stress and consequent growth, development, quality or yield.”

No state fertilizer program currently permits the term “biostimulant” on labeling, Young said, but there are established standards for many biostimulant ingredients and a path to distribution.

Per CDFA requirements, labels for biostimulant products do require that the specific ingredient be named. If the ingredient isn’t recognized, then efficacy data may be required. Recognized ingredients should appear in a guaranteed analysis.

The issue with some biostimulant products, Young said, is that they may be required to have registration both as a fertilizing material and a pesticide. Adding the pesticide label is more costly and time consuming, and manufacturers would rather avoid the pesticide label if possible.

Young noted that just because a product is registered with CDFA as a fertilizing material, that doesn’t preclude CDPR jurisdiction.

An update draft of the Guidance for Plant Regulators and Claims, including Plant Biostimulants, was released in November by EPA, and Young said it has clarified many issues with biostimulants for industry and regulators, but questions still remain.

“It was a step in the right direction and there are a couple of flaws, but it is going to help the industry and regulators know what to do,” Young said.

Claims are a key EPA consideration in determining if a biostimulant is a pesticide. Products that are not considered pesticides are plant nutrients and trace elements, plant inoculants, soil amendments and vitamin-hormone products.

The regulatory approach from CDFA for products that have multiple plant regulator and non-plant regulator modes of action is if it can be demonstrated that a particular product has the activity claims on the product label and does not make any plant regulator claims on the label, it can be excluded from FIFRA regulation.

Update on Sudden Vine Collapse

By
Cecilia Parsons
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Eskalen noted that sudden vine collapse vines he tested were planted on Freedom rootstock and had presence of vine mealybug as well as girdling at the graft union, lack of starch in the rootstock and lack of feeder roots (photo by A. Eskalen.)

Grapevines can die for a number of reasons, but increasing incidences of Sudden Vine Collapse across all California grape growing regions have sparked investigation into possible causes for this disease.

In the 2020 San Joaquin Valley Virtual Grape Symposium, UC Davis plant pathologist Akif Eskalen shared what he and other researchers know about Sudden Vine Collapse and some management options.

Eskalen said this condition causes sudden death of grapevines in the middle of summer. Sudden Vine Collapse disease has occurred in vineyards in the Lodi and Monterey grape growing regions as well as San Luis Obispo and Santa Barbara counties and San Joaquin Valley counties including Fresno, Tulare and Kern. Similarities in Sudden Vine Collapse include vines planted on Freedom rootstock and the presence of vine mealybug. In his investigation, Eskalen also found symptoms of girdling at the graft union, lack of starch in the rootstock and lack of feeder roots on infected grapevines.

Conclusions from a sampling study done by Eskalen found that in each of the vines with moderate to severe decline symptoms, GLRaV-3 and GVA, grapevine leafroll virus and grapevine virus A were present. Grapevine trunk disease pathogens and Fusarium solani were isolated from the symptomatic grapevine samples, but no consistent fungal pathogen was found among all vine samples.

He also noted that Freedom rootstock is most susceptible to co-infection by grapevine leafroll associated viruses and vitiviruses. Virus infections can also cause graft incompatibility in certain rootstock, Eskalen noted.

Results from the sampling study also showed that efforts of the rootstock to reject the scion following infection causes girdling at the graft union, preventing flow of starch throughout phloem of the vine. The inability of the plant to transport starch leads to starch depletion in the roots and subsequent lack of feeder roots, further preventing the vine to acquire nutrients from the soil. Finally, Eskalen said, interactions between the grapevine viruses and Fusarium solani may play a role in the vine death.

Management options for grape growers, Eskalen advised, include first testing to confirm presence of viruses. Lodi Grape Growers recommends testing both healthy and collapsed vines for common leafroll viruses in California.

Testing in the fall (before the leaves senesce) is the best time of year to sample for these viruses.

Mealybug control in the vineyard is also advised as is removal of infected grapevines.

When replanting, Eskalen recommended using less sensitive rootstocks and continued control of grapevine trunk diseases in the vineyard.

Conditions and Cures for Puff and Crease in Citrus

By
Cecilia Parsons
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An uneven appearance and cracking on the outer surface of citrus fruit called puff and crease, also called crease and split. This condition impacts fruit quality and can be caused by environmental conditions and nutrient imbalance (photo by Robert G. Platt, UC Statewide IPM Program.)

Nutritional status and environmental conditions contribute to puff and crease in citrus.

This disorder, seen as an uneven surface on the fruit, affects fruit appearance and quality.

Puff and crease indicates a separation of the albedo (the white material under the outer peel) from the outer surface of the fruit. This internal damage is visible externally as an uneven appearance of the rind surface, where some of the fruit surface appears puffy while other areas are indented. Satsuma mandarins, early navel varieties, and Valencia’s are susceptible to this rind defect.

Dustin Stewart, a crop advisor with Ultra Gro in Fresno, Tulare and Kings counties, said due to recent warm fall days, puff and crease is more prevalent on some early maturing Satsumas.

Puff and crease can be caused by late nitrogen applications in an orchard. Evidence linking nitrogen to puff, crease, smaller fruit size and staining exist. These negative effects are most significant at nitrogen levels greater than 2.6% N, Stewart said, but it is more likely that nutritional imbalance combined with warm daytime temperatures in the fall drives this condition in citrus. Once puff and crease has begun, it can’t be reversed, Stewart said, but this condition can be mitigated by making sure calcium, molybdenum, zinc and sulfur levels in the plant are adequate to assuage puff and crease.

These nutrients play a role in retarding senescence, which in the early stages is an organized phase of metabolism and not just a breakdown of tissue.

Application of plant growth regulators cytokinin and auxin or gibberellic acid can also delay development of puff and crease in citrus fruit. Cytokinin is a plant hormone that influences growth and stimulation of cell division. Cytokinin moves in the xylem and pass into leaves and fruit. Cytokinin also acts in conjunction with auxin, another plant hormone, to retard senescence.

Naturally occurring plant growth regulators must be absorbed by plant tissue to be effective. UC IPM guidelines advise spray applications to be done when climatic conditions are favorable. Good spray coverage is also important. Uptake of naturally occurring plant growth regulators (PGRs) by the trees is improved in warm and humid conditions. Use of a naturally occurring plant growth regulator to delay maturity and prevent puff and crease will be more effective in healthy, well-watered orchards with adequate nutrition. Tree size, canopy density, location of fruit and suitability of spray equipment for those conditions are other considerations for an effective PGR application.

Gibberellic acid treatments, applied at the correct time in fruit maturation, delays rind aging and softening. Timing of treatment in early mandarin varieties is two weeks prior to color break in orchards where harvest will be delayed.

Choosing a Site for a New Pistachio Orchard

By
Cecilia Parsons
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Evaluation of a potential pistachio orchard site calls for understanding pistachio life cycle, tree water use, rooting characteristics, tree spacing and canopy structure as well as harvest requirements and field traffic (photo by C. Parsons.)

Production potential of a pistachio orchard is maximized with deep, well-drained soils, but this hardy tree nut species can also produce on marginal ground.

During the 2020 UCCE Pistachio Short Course, UCCE Nut Crops Farm Advisor Mae Culumber in Fresno County outlined considerations in site selection for pistachio production. Pistachio trees’ tolerance for alkaline and saline soils has led many orchards to be planted in less-than-ideal soils or locations.

In her presentation, Culumber noted that site selection for an orchard should begin with understanding pistachio life cycle, water use, rooting characteristics, tree spacing and canopy structure as well as harvest requirements and field traffic.

Evaluation of the potential orchard site should include land cost, soil texture, drainage, chemistry and amendments. Development considerations include the cost of land leveling as well as irrigation system installation, energy requirements, pressure, filtration and maintenance. Distribution patterns and the necessary irrigation set frequency should also be evaluated. A UC publication that includes sample costs to establish a pistachio orchard can be found at coststudies.ucdavis.edu/en/current/.

When considering a site, Culumber said that cost of a soil evaluation represents only a small portion of total orchard development costs, but the information will be valuable in terms of management decisions. Soils should be evaluated for structure, permeability, stratification, drainage and salinity/fertility. The UC Soil Resource lab and the SoilWeb Earth in Google Earth provide a range of soil information including historical images of past crops at the site and their effect on the site and the soil. Assessment of historical images of the site can reveal if the ‘problem’ is soil or management related. Culumber noted that online survey data might not match ground observations due to the scale of the surveys and the influence that agriculture land management and irrigation can have on salinity levels.

On-site observations are the best means of determining conditions that will reduce tree performance.

Characterization of the soil profile can be accomplished by digging backhoe pits or auguring. Samples should be taken at several depths from at least one backhoe pit to six feet for each 40 acres in one soil type identified with web soil survey tools. Depths of layers, texture, lime, hardpan, rooting and drainage should be recorded, which will help in choosing equipment for land modification and amendments for reclamation if needed.

Permeability of soils can be improved by deep ripping prior to planting, using calcium supplying amendments, adding organic matter and planting cover crops. Choosing an irrigation system that matches the water infiltration rate is also important to establish and maintain tree health.

Soil and water samples should be submitted to a certified ag lab to determine salinity levels. Long-term productivity may be impacted if the water source is 4.5 to 6 dS/m EC. Salt increases osmotic potential, costing plants energy and interfering with water uptake.

Bacterial Leaf Spot in Lettuce

By
Germán Sandoya-Miranda
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Bacterial leaf spot, shown here in iceberg lettuce, is found in most lettuce growing regions of the country, including California, Arizona and Florida (photos courtesy Richard N. Raid UF/IFAS.)

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).

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

By
Houston Wilson, Monica L. Cooper, Rhonda Smith, Alexis Billings, Kelsey McCalla and Kent M. Daane
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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.

Nutrient Management and Crop Disease Prevention

By
Eryn Wingate
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Celery black heart, a calcium deficiency disorder, causes young growth in the heart of the plant to brown and blacken (photo by S. Koike.)

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.

Foliar Feeding of Plant Nutrition

By
JW Lemons
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Foliar nutrient applications offer a number of benefits over, or in addition to, soil applied applications under the right conditions (photo by Cathy Merlo.)

According to studies from the United Nations, the world population will increase by 1 billion people over the next decade, reaching an estimated 8.6 billion people by 2030. By 2050, Earth will have 9.8 billion inhabitants. This enormous population growth will be coupled with a corresponding increase in demand for food. Projections from the FAO have indicated that agricultural production worldwide will have to increase by an estimated 50% by 2050 to meet the needs of a growing population.

Proportionately, agricultural land is becoming scarce. Therefore, one of the biggest challenges is to improve the yield, quality and shelf life of crops by using the best fertilizers, applied in very precise doses and with proven methods. One of the best procedures to reach those goals is foliar application of nutrients in customized doses based on critical timing for each crop.

Foliar feeding is the entry of small amounts of liquid fertilizer through the surface of plant tissue. This can allow for rapid nutrient utilization by the plant. Foliar feeding provides the consultant and applicator the ability to blend the fertilizer with other products such as pesticides and micronutrients.

Current formulations of liquid fertilizers are believed to penetrate mostly the transcuticular pores on foliage, which are open virtually all the time compared to stomata. Nutrients also enter stomata, but these often are closed due to environmental stresses and darkness. Most stomata are located underneath leaves away from fertilizer spray patterns. Drawbacks to foliar feeding include the inability to apply large amounts of N, phosphorous (P) and potassium (K) without potentially burning the foliage and possibly scarring the fruit. Therefore, frequent applications of the right fertilizer and fertilizer blends at a low volume are required to maintain optimum tissue levels of key nutrients, resulting in consistent plant growth and functions like the production and transference of assimilates.

 

Foliar Nutrient Applications

Selecting the right fertilizers will continue to be an important part of the equation to improving yield, quality and shelf life of crops. How we apply the nutrient is also vital. Let us examine foliar spraying more closely. Crops treated with foliar spray tend to have a higher nutritional value. Improved Total Soluble Solids (TSS) have contributed to increased sugar, vitamins, minerals and proteins in the harvested crops. It is not enough just to grow more crops, but those crops must have better nutritional value for us as well.

We can see very positive effects by examining just one mineral nutrient such as K. K regulates plant functions and increases nitrogen efficiency. By increasing the saline concentration of the cells, the plants are better able to resist frosts. Applied at the right time and in the correct amount, K promotes the development of a stronger cell structure, which allows the crops to better withstand drought, disease and pests. A better internal structure promotes a prolonged shelf life. In many of our soils, we lack adequate available K to meet the crop demands. One way to meet those phenological growth stages is to supplement K sprays. This could affect the production levels and quality of numerous crops such as tuber fill and skin condition in potato, nut meat density in almond and even oil production in mint.

Originally, it was thought that using foliar nutrient applications was only appropriate when a nutritional deficiency was present. This is definitely not the case. Ongoing scientific research has consistently shown that periodic applications of certain nutrients in various crops can have a positive effect on both quantity and quality of fruits, nuts, vegetables and grain crops. Although most supplemental nutrients are absorbed through the root system, it is also important to note that leaves (and to a lesser degree stems,) flowering plant tissues and even fruit surfaces can absorb limited amounts of nutrients. It is crucial to understand which nutrients can be supplied effectively by foliar applications if the soil-supplied nutrients are inadequate or impaired in some way.

In agriculture, our scientists have identified many cases where nutrient supplementation using a foliar application may be considered more effective than an in-crop topdressing application. One such situation that has had success is during late stages of fruit development where Ca sprays applied through the season to apples has shown ability to combat Ca deficiency. This deficiency is directly connected with the physiological disorder called bitter pit. The application of Ca making direct contact with the fruit has shown good results in controlling this disorder. Also, Ca being sprayed on cherries preharvest amounted to less cracking in fruit during late maturation stages.

Research trials done in almonds have also demonstrated where a foliar-applied nutrient can have a huge impact on a crop. Knowing that fruit set is crucial in almond trees, it has been proven that micronutrients, zinc (Zn) and especially boron (B), have a significant impact on fruit set as well as on fruitlet abscission. In several fruit trees, it has been reported that even foliar spraying of one or both elements has improved productivity. In one experiment, Nonpareil trees were sprayed at full blossom with boric acid at 0.2%, Zn-50 at 0.3% and, with the combination of these micronutrients as a separate treatment, compared with a non-sprayed control. Fruit set of the B treatment was 27.7% and Zn was 22.2%. However, the B + Zn combination produced a significantly higher fruit set (38.1%) (Sotomayor et al., 2002).

 

Additional Benefits

As consultants, we must understand that at times only a small amount of a specific nutrient is required, and due to low soil mobility of the nutrient, it is actually more efficient to supply the small amount of needed nutrient as a foliar application. This can be the case for both macronutrients and micronutrients.
There can also be other conditions that cause a nutrient to be less available to crop roots. One example is cool, excessively wet conditions on an alkaline soil (e.g. pH >8.0) where iron is less available to certain crops (e.g. iron chlorosis of citrus trees) in heavy wet clay. Even if supplemental iron fertilizer has been applied to the soil before or at planting, it may not be available through the roots. This can also be true for numerous other crops. By monitoring the trees and observing patterns, we can offset the chlorosis in our crop with a supplemental iron foliar application. Spray applications of elements such as iron, zinc, copper and manganese may have to be repeated every three weeks when we get deficiencies of these nutrients.

Many studies include yield data collected after applying foliar sprays of various fertilizer solutions, not only to nutritionally deficient crops, but even to crops that have adequate levels of nutrients such as potassium. Crops including wheat, almonds, tomatoes, citrus, cucurbits, pome fruits and rice among many others react positively to certain nutrient sprays even when adequate soil nutrient levels are present.

A good reference to the benefits of foliar spraying is in citrus research. After years of trials addressing HLB or Huanglongbing citrus greening disease in Florida, it was determined that a foliar nutrition approach to the disease was a very viable option (www.citrusbr.com.br/download/Foliar_nutrition_forHLB.pdf). The nature of the disease restricts the ability of the plant to adequately take up nutrients though its damaged root system. Finding an alternative way to increase plant nutrition and prolong the life and productivity of the infected tree was crucial. Numerous combinations and timings were trialed.

Trials show that in the case of one nutrient combination, potassium nitrate feeding increased yields in citrus. Foliar applications with potassium nitrate have proven to be highly efficient in fulfilling the potassium requirements for many crops. The combination of potassium and nitrate in this fertilizer has been found to be beneficial in improving fruit size, dry matter, color, taste and integrity as well as resistance to biotic and abiotic stresses for citrus and tomato fruit. Moreover, the integration of potassium nitrate in routine management or in specific growth stages resulted in remarkably positive benefits to cost ratio.

 

Take Caution

In our management plan, caution should be used when foliar application might be more efficient but not practical. When a foliar application is relatively effective, but we cannot supply the needed nutrient in one application, multiple applications would be needed, spaced out sufficiently and timed as often as once a week. Multiple applications in field crops can be expensive due to fuel, equipment and labor costs, or where there may not be sufficient time to apply enough of the needed nutrient. We must measure the economics and limitations when determining our fertilizer plan.

In the case of a severe P deficiency, there may not be time to apply sufficient applications of low rates of foliar P to be effective. The application costs can become excessive based on the return in production. In this situation, it may be better to realize there is not much that can be done in the current season to correct the issue. Recognizing the deficiency, the preferred course of action is to apply sufficient P fertilizer to the soil prior to the planting of subsequent crops to correct the P deficiency.

If we continue to use phosphorus P as an example, we also need to understand foliar P efficiency and when it does have a fit in our fertilizer program. Foliar-applied nutrients have the benefit of being 4 to 30 times more efficient, and there is no risk of groundwater contamination. Studies using labeled P on apple, cherry, corn, tomato, potato and bean crops have shown that as much as 12 to 14% of the total P can be supplied by multiple foliar sprays. Since P can be very immobile in the soil, foliar applications can be up to 20 times more effective than soil applications.

Recognizing another nutrient use as a foliar, we can see that some foliar N sprays compared to soil applications of N include lower application rates and the ease of obtaining timely, uniform applications. With attention to best-use guidelines, the efficiency of foliar-applied N may be optimized at nearly 95 to 100%. Based on the foregoing information, if the recovery of soil-applied N can be impaired to as low as 15 to 62%, it can then be concluded by the method of estimation that foliar-applied N has an efficiency of 1.3 to 1.6 times soil-applied N at the low end and seven times at the upper end. If foliar-applied N can be up to seven times more efficient than soil-applied N, then on a pound-for-pound basis, it makes sense that we could use this information to prevent N loading to ground water. In crops such as citrus, cherries and wine grapes, we may be able to achieve 100% of the needed N by foliar application. This is not always the case, but understanding your N efficiency in your soils may prompt the need for a supplemental foliar N application. Mature, low-N-requiring crops could acquire 40 to 50 pounds N with foliar application alone. This in no way warrants replacing all of our soil applied N in all crops by using foliar.

 

Conclusion

In summary, it should be pointed out that we have barely scratched the surface of all the features and benefits of foliar nutrition sprays. Get to know your crop and soils as well as the correlation between total nutrients and available nutrients. Understand if your crop has shown positive response to foliar treatment even when adequate nutrient levels are in the soil. Ask yourself: when do soil conditions prevent adequate nutrient uptake? What conditions should I be aware of to optimize foliar applications (rain, temperature, wind, sprayer capability, pH of my solution, humidity, physiological growth stage, activity of plant parts like stomate, additives such as pesticides or adjuvants and antagonist as well as synergistic reactions with other nutrients.) Study absorption rates and what affects them. Like all other things in agriculture, foliar nutrition is a tool.

 

References

Sotomayor, C., Silva, H. and Castro, J. (2002). Effectiveness of boron and zinc foliar sprays on fruit setting of two almond cultivars. Acta Hortic. 591, 437-440, https://doi.org/10.17660/ActaHortic.2002.591.67.

Impact of Some Biostimulants in Improving Strawberry Yields

By
Surendra K. Dara
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The study included six treatments applied to 300-ft-long beds with single drip tape in the center and two rows of strawberry plants (all photos by S. Dara.)

Microbial and botanical biostimulants have been used for promoting plant growth and to help plants withstand various pests, diseases or environmental stressors. While macro and micronutrients are necessary for plant growth and optimal yields, biostimulants play multiple roles by increasing the bioavailability of nutrients, improving nutrient and water absorption, and protecting plants from pestiferous organisms, either through direct antagonism or by triggering plants’ defense mechanisms (Berg, 2009). In addition to improving health and yields, biostimulants are also known to increase nutritional quality (Parađiković et al., 2011; Fierentino et al., 2018).

Multiple field studies in California demonstrated the potential of biostimulants and soil amendments in improving yields in tomato (Dara, 2019a; Dara and Lewis, 2019) and strawberry (Dara and Peck, 2018; Dara, 2019b). As the knowledge of biostimulants and their potential for sustainable agriculture is expanding, there has been a steady introduction of biostimulant products in the market warranting additional studies. A study was conducted to evaluate the potential of different biostimulant materials on strawberry growth, health and fruit yields.

The study measured the impact on growth, health and yield parameters for each treatment.

 

Methodology

This study was conducted in an experimental strawberry field at the Shafter Research Station during 2019-2020. Cultivar San Andreas was planted on October 29, 2019. No preplant fertilizer application was made in this non-fumigated field, which had both Fusarium oxysporum and Macrophomina phaseolina infections in the previous year’s strawberry planting.

Each treatment was applied to a 300-foot-long bed with single drip tape in the center and two rows of strawberry plant on either side. 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 18-foot buffers in between. This study included both biostimulant and nutrient supplements, but this article presents data from the biostimulant treatments only. Treatments were applied either as fertigation through the drip system using a Dosatron, or sprayed over the plants with a handheld garden sprayer.

 

Treatments

The following treatments were evaluated in this study:
Grower Standard (GS): 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 were applied 20 times at weekly intervals through fertigation. This fertilizer program was used as the standard for all treatments except for the addition of biostimulant materials.

GS + Abound: Transplants were dipped in 7 fl oz of Abound (azoxystrobin) fungicide in 100 gal of water for four minutes immediately before planting. Transplant dip in a fungicide is practiced by several growers to protect from fungal diseases and is considered as another standard in this study.

GS + Str10 + Rhizolizer: Str10 (Wickerhamomyces sp.) was applied at 5 fl oz/ac with molasses at 10 fl oz/ac immediately after planting and Rhizolizer (Trichoderma harzianum and Bacillus amyloliquefaciens) at 3 fl oz with a food source blend at 10 fl oz two weeks after Str10 application through the drip system. The same pattern was repeated starting from mid-February 2020. From February to May, 6 fl oz/ac of Rhizolizer was applied with 20 fl oz/ac of food source once a month. Str10 is an unregistered product with yeast that is expected to help with nutrient uptake and phosphorous mobilization for improved plant vigor and yield. Rhizolizer is expected to solubilize soil nutrients and improve crop growth and yield.

GS + ON-Gard: 32 fl oz of ON-Gard was applied every two weeks through the drip system from planting until canopy development and then sprayed in 50 gpa. ON-Gard is expected to increase the nutrient use efficiency and decrease abiotic stress to the plants.

GS + ON-Gard + RootShield Plus: 32 fl oz of ON-Gard (soy protein-based) was applied every two weeks through the drip system from planting until canopy develops and then sprayed in 50 gpa. RootShield Plus WP (T. harzianum and T. virens) was also applied at 2 lb/ac through drip immediately after planting with 1 lb/ac at the end of November and again at the end of December 2019. RootShield is a biofungicide expected to protect strawberry from phytopathogens and improve water and nutrient uptake.

GS + CropSignal: CropSignal was applied at 10 gpa six days before planting and at 5 gpa 30 days after transplanting through the drip system. CropSignal is a carbon-based nutrient formula containing botanical extracts along with cobalt, copper, manganese and zinc, and is expected to support the growth and diversity of beneficial aerobic soil microbes for improved soil structure, water retention, nutrient cycling and plant protection.

Parameters observed during the study included canopy growth (area of the canopy) in January, February and March; first flower and fruit count in January; leaf chlorophyll and leaf nitrogen (with chlorophyll meter) in January, February and May, fruit sugar (with refractometer) in March and May; fruit firmness (with penetrometer) in March, April and May; severity of gray mold (caused by Botrytis cinereae) and other fruit diseases (mucor fruit rot caused by Mucor spp. and Rhizopus fruit rot caused by Rhizopus spp.) 3 and 5 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) in March and May; sensitivity to heat stress (expressed as the number of dead and dying plants) in May; and fruit yield per plant from 11 weekly harvests between March 11 and May 14, 2020. Data were analyzed using analysis of variance in Statistix software and significant means were separated using the Least Significant Difference test.

 

Results and Discussion

The impact of treatments varied with different measured parameters. The interactions among plants, beneficial and pathogenic microorganisms in the crop environment, the influence of environmental factors and how all these biotic and abiotic factors respond to various biostimulant inputs can be very complex. The scope of this study was only to measure the impact on growth, health and yield parameters, not to investigate those complex interactions.

The canopy size does not always correspond with yields but could be indicative of stresses and how the plant is responding to them in the presence of treatment materials. Plants in some treatments had significantly larger canopy size in January and February, but the grower standard plants were significantly larger than the rest by March. Leaf chlorophyll and nitrogen contents were significantly different among treatments only in January where the grower standard plants had the lowest and the plants that received CropSignal had the highest. When the counts of the first onset of flowers and developing fruits were taken in January, plants that received ON-Gard alone had the highest number followed by the CropSignal and Abound treatments.

A similar trend was also seen for the average fruit sugar content. There was no statistically significant difference in the average fruit firmness among the treatments, although the value was numerically higher for the fruits in the CropSignal treatment. Severity of the gray mold, which occurred at low levels during the observation period, did not statistically differ among the treatments, but it was numerically higher in fruits from the grower standard plots. However, the severity of other diseases was significantly different among various treatments with the highest level in fruits from the grower standard and the lowest in fruits from plants that were treated with Abound. Temperatures were unusually high during the last week of May, and several plants exhibited heat stress and started to die. The number of dead or dying plants on May 28 was the highest grower standard and the lowest in Str10 + Rhizolizer and Abound treatments, although the differences were not statistically significant.

Table 1: The trial showed significant differences in marketable and unmarketable fruit yields among treatments.
There were significant differences in marketable and unmarketable fruit yields among treatments (see Figure 1.) Highest marketable yields were seen in ON-Gard and CropSignal treatments followed by the grower standard, ON-Gard + RootShield and Str10 + Rhizolizer. Transplant dip in a fungicide seems to have a negative impact on fruit yields as observed in the current study or earlier studies (Dara and Peck, 2017 and 2018; Peck unpublished data). While the grower standard had the highest amount of unmarketable fruits, the Str10 + Rhizolizer treatment had the lowest in this study. Fruit yield and some of the observed parameters appeared to be better in the grower standard with no fungicide or biostimulant

treatments, which has also been seen in some earlier strawberry studies.

Compared to the grower standard, marketable fruit yield improvement was seen only in ON-Gard and CropSignal treatments. However, marketable fruit yield was higher in all treatments compared to the grower standard with Abound. Sometimes, natural balance of the nutrients, organic matter and microbial community in the soil might result in optimal yields in the absence of pathogens or other stressors. However, it is very common to use fungicidal treatments or add biological or synthetic amendments to protect from potential threats and improving yields. These results help understand the impact of various biostimulant materials and warrant the need to continue such studies under various environmental, crop and soil conditions.

Field assistant Tamas Zold measures fruit weight to further determine impacts on treatments.

 

References

Berg, G. 2009. Plant-microbe interactions promoting plant growth and health: perspectives for controlled use of microorganisms in agriculture. Appl. Microbiol. Biotechnol. 84: 11-18.

Dara, S. K. 2019a. Effect of microbial and botanical biostimulants with nutrients on tomato yield. CAPCA Adviser, 22(5): 40-45.

Dara, S. K. 2019b. Improving strawberry yields with biostimulants: a 2018-2019 study. UCANR eJournal of Entomology and Biologicals. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=31096.

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.

Dara, S. K. and E. Lewis. 2019. Evaluating biostimulant and nutrient inputs to improve tomato yields and crop health. Progressive Crop Consultant 4(5): 38-42.

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(The author would like to thank BioWorks, Inc., Fauna Soil Production and Locus Agricultural Solutions for the financial support of the study and Marjan Heidarian Dehkordi and Tamas Zold for their technical assistance.)

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