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Mitigating Pesticides and Sediment in Tail Water using Polyacrylamide (PAM)

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Sprinkler and flood irrigation often generates runoff that transports sediment from agricultural fields to downstream rivers, lakes, and estuaries. Additionally, some classes of pesticides, such as pyrethroids, bind to the suspended sediments in tail water which can cause toxicity to aquatic organisms in these receiving waters. Currently numerous rivers and creeks in California are considered impaired by pesticides and sediment transported with drainage from agricultural land. As water quality regulations become stricter, growers will need to implement practices on their farms that treat potential pollutants in runoff.

Tail water can be a particularly challenging water quality problem on the central coast of California where overhead sprinklers are widely used for vegetable production. Sprinklers can contribute to high concentrations of suspended sediment in tail water because the force of the falling water droplets erode soil aggregates and allow fine particles to be carried with runoff water. Research that we have conducted on the central coast has shown that adding a low concentration of polyacrylamide to irrigation water can dramatically reduce sediment loads and sediment-bound pesticides in agricultural tail water (Fig. 1). Across a range of soil types, polyacrylamide treatment reduced sediment concentration in runoff by more than 90 percent on average. On some highly erosive soils polyacrylamide reduced sediment concentration in sprinkler induced runoff by more than 98 percent. Total phosphorus and nitrogen concentrations in sprinkler runoff were also reduced by 40 percent to 70 percent using polyacrylamide.

Despite dramatic improvements in water quality, polyacrylamide, also called PAM, has been slow to be adopted as a management practice on the central coast and in much of California. One reason may be because of misunderstandings about how to most effectively use PAM for treating runoff, especially for sprinkler irrigation. Another reason is that several of the physical properties of polyacrylamide make it challenging for handling and applying to fields.

Runoff from overhead sprinkler
Figure 1. Runoff from overhead sprinkler water untreated (left) and treated with 5 ppm PAM (right).

Brief background on PAM

Polyacrylamide (PAM) is a simple polymer molecule made of carbon, nitrogen, and oxygen. Various forms of PAM exist, but the type used to stabilize soil and prevent erosion is a very large, mostly negatively charged molecule (12-15 megagrams per mole). Agricultural PAM is commercially available in dry powder (granular), emulsified liquid, and dry tablet forms, and costs as little as $4 to $6 per pound of active ingredient depending on the formulation, supplier, and cost of the raw materials used for manufacturing PAM (i.e. natural gas). PAM is used for many nonagricultural purposes such as a flocculant for waste and potable water treatment, processing and washing of fruits and vegetables, clarification of juices, and paper production. It is also a component of makeup.

Use of PAM for Irrigation and Erosion Control

Because PAM is a very long, linear molecule it easily binds to soil aggregates, thereby preventing soil erosion during irrigation events. Beginning in the early 1990’s numerous studies demonstrated that low application rates of PAM (1 to 2 lb/acre) reduced runoff and improved water quality in furrow systems by stabilizing the aggregate structure of soil, improving infiltration, and flocculating out suspended sediments from irrigation tail water. Most of the research and demonstrations of PAM were conducted in furrow systems in Idaho and Washington states where soils are very erodible. By 1999, almost 1 million acres of land were annually treated with PAM in the northwest of the United States. Additionally, growers in the San Joaquin Valley and the Bakersfield areas of California used PAM to reduce soil erosion under furrow and flood irrigation.

Working with PAM

PAM can be very difficult to use if it is not handled correctly. Wet PAM is very slippery, and because it solubilizes slowly in water, PAM spills should be cleaned up with a dry absorbent rather than washing it with water. Although it is not toxic to humans, some precautions should be taken when handling PAM: Use gloves to avoid irritation to skin. Goggles will prevent eye exposure. Also, a dust mask is recommended when pouring or handling dry granular or powder forms of PAM to avoid inhalation.

One rule of thumb to keep in mind is that it is much easier to add water to PAM than to add PAM to water. Dry PAM rapidly absorbs water, increasing its original volume many times to become a slimy, gooey substance. Dissolving dry PAM in water can be challenging. Because PAM is a very large molecule it does not dissolve readily into water and requires many hours of agitation to dissolve. It will often stick to the side of a tank when being mixed. Also, mixing up concentrations greater than 0.15 percent in water is nearly impossible because the solution becomes very viscous and difficult to agitate. Some manufacturers sell effervescent PAM tablets which aids dissolution in water, but still only relatively dilute solutions can be mixed up.

For these reasons it easiest to use liquid PAM products that have been emulsified with a carrier such as mineral oil or humectants, or work with dry PAM products, such as granular PAM or PAM in a tablet form. The emulsified liquid products generally have active ingredient concentrations ranging from 25 to 50 percent.

Application Methods

For applications in furrow systems dry or liquid product can be added to water flowing in a head ditch or main line (if gated pipe is used) at a rate to achieve a 2.5 to 10 ppm (parts per million) concentration. Automated equipment can be used to spoon feed granular PAM into flowing canal water. The application can be made continuously during the irrigation or until the water advances almost to the end of the furrows. An alternate application method, called the “patch method” involves spreading granular PAM to the first 3 to 5 feet of each furrow. The granular PAM slowly dissolves as water flows down the furrows. A similar strategy is to add a PAM tablet at the beginning of each furrow. Applications into sprinkler systems require specialized equipment for injecting either liquid PAM or dry PAM into pressurized pipe which will be discussed in more detail later.

Environmental and Food Safety

Only PAM products labeled for application to food crops should be used in agriculture. Also, the buyer/processor/shipper of the produce should be informed that PAM is being applied to the crop, especially if the application is made near harvest.

Agricultural PAM used for soil erosion is not toxic to mammals. Environmental studies of anionic (negatively charged) PAM have not shown any detriment to fish, algae, and aquatic invertebrates such as Ceriodaphnia dubia, and Hyalella azteca. Polyacrylamide is sometimes confused with acrylamide monomer, a precursor in the manufacturing of PAM. Acrylamide monomer, a potent neurotoxin, has a high, acute toxicity in mammals. The Federal Environmental Protection Agency (EPA) requires that PAM sold for agricultural uses contain less than 0.05 percent acrylamide monomer. In soil, PAM degrades rapidly by physical, chemical, biological, and photochemical processes, but it does not decompose into the acrylamide monomer. A previous study of the movement of PAM from agricultural fields showed that less than three percent of the applied product remained in the runoff leaving the field. The remaining PAM in the tail water was almost completely removed through adsorption to suspended sediments as the water flowed a distance of 300 to 1000 ft in the tail water ditch.

One concern with using emulsified liquid PAM is that the mineral oil carrier can have toxicity to downstream aquatic invertebrates. However, toxicity from the mineral oil can be avoided by using PAM formulations with either high concentrations of PAM (>50 percent active ingredient) or with non-oil carriers such as humectants.

Optimizing PAM for Sprinklers

Although many research studies have evaluated the efficacy of PAM in furrow systems, fewer studies have evaluated the use of PAM with sprinklers. Applications of PAM made before irrigating with sprinklers, such as by spraying PAM solution or broadcasting dry product on the surface of the soil were far less effective than continuously injecting PAM at a low rate into the irrigation water. Injecting PAM only at the beginning of an irrigation was also less effective in controlling sediment in runoff than a continuous application at a low concentration during the entire irrigation. Our studies on the central coast show that injecting PAM to achieve a 5 ppm concentration in the irrigation water provided the highest reduction in sediment, nutrients, and pesticides in the tail water using the least amount of product. In some fields 2.5 ppm PAM provided equal efficacy for control of suspended sediments as 5 ppm PAM. Treatment with PAM should be started with the first irrigation after planting and continue during the following two to three irrigations. PAM should be reapplied when sprinkler irrigating after the field has been cultivated. Product can be saved in fields where very little runoff occurs during the first few hours of an irrigation by making an initial application for the first half hour and then applying product again when runoff becomes significant.

Injecting PAM into Pressurized Irrigation Systems  

The chemical characteristics of PAM that make it so effective as a flocculant, also make it difficult to inject into pressurized irrigation systems. Liquid PAM solutions are viscous and will clog chemigation equipment with valves such as diaphragm and piston pumps, as well as venturi injectors. Also, because the desired PAM concentration in the irrigation water is low, injection rates as low as one to three ounces per minute are needed to treat 10 to 15-acre fields. Most small, gas-powered centrifugal pumps usually cannot be easily calibrated to inject at these low rates. Peristaltic pumps can be adjusted to inject slowly but often are not designed to operate under high pressures that are common in sprinkler main lines. The best type of pump that we have identified for injecting liquid PAM is an auger pump (Fig. 2.) which has no valves and can inject viscous solutions at very low rates. These pumps also maintain a consistent injection rate at pressures as high as 100 psi (pounds per square inch).

Trailer outfitted for injecting liquid PAM
Figure  2. Trailer outfitted for injecting liquid PAM into the main line of an irrigation system using an auger metering pump.

Although liquid formulations of PAM are the best option for pressurized irrigation systems, we are currently exploring methods of injecting dry PAM into pressurized sprinkler systems. The advantage of dry PAM is that it is generally cheaper than liquid products and the possibility of introducing toxicity from the inactive emulsifying ingredients in the liquid products is eliminated. Another advantage of this approach is that it may require less labor since no pump must be calibrated and managed during an irrigation. The dry PAM applicator is loaded with cartridges containing either granular or tablet forms of PAM (Fig 3.). A portion of the water from the main line is diverted through the applicator chambers and then added back to the main water stream. Although the PAM concentration is lower than can be achieved with liquid PAM, preliminary tests have shown that as much as 90 percent of the suspended sediments can be eliminated in the runoff (Fig 4). Further studies during the upcoming season will evaluate the practicality of use this applicator in commercial fields.

Prototype dry PAM applicator
Figure 3. Prototype dry PAM applicator for pressurized irrigation systems (left) and PAM cartridge that inserts into the applicator (right).

Other Potential Benefits of PAM

In addition to water quality benefits, we have observed or measured agronomic benefits from the use of PAM. Because PAM stabilizes soil aggregates, soil is less likely to crust under the impact of sprinkler droplets, which improves infiltration and decreases the volume of runoff. In one field trial, PAM reduced runoff from four successive sprinkler irrigations from 4000 gallons per acre to less than 1500 gallons per acre. Less crusting of the soil may also improve germination of small seeded vegetables such as lettuce. In one of four replicated field trials conducted in commercial lettuce fields, we were able to measure an increase in yield and plant weight with the use of PAM. This yield increase may be a result of better infiltration of moisture or because the seed emerged earlier than in the non-treated plots. Although we have not conducted long-term studies, anecdotal reports from growers who applied PAM to their fields over successive years were that soil structure was improved by keeping the fine particles in place.

 

Samples of field runoff
Figure 4. Samples of field runoff from irrigation water treated using the dry PAM applicator (left) and untreated (right).

An additional benefit of PAM is saving costs associated with cleaning out ditches and retention ponds that become clogged with sediment during the irrigation season. Often once or twice per year growers on the east side of the Salinas valley who farm on soils prone to crusting and runoff must schedule a backhoe and crew to remove sediment from ditches and ponds and redistribute the material in their fields. To reduce costs with using PAM, growers also can receive cost-share payments under the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Environmental Quality Incentive Program) (EQIP).

 

In summary here are a few key concepts on using PAM for controlling sediment in runoff:

  • Polyacrylamide is a long linear molecule that binds to soil and can flocculate suspended sediments in water.
  • PAM does not readily solubilize in water and increases the viscosity of water (thickens).
  • Concentrations of 2.5 to 5 ppm PAM in irrigation water are ideal for optimizing erosion control benefits under sprinkler and furrow irrigation.
  • For agricultural purposes only use anionic PAM products for erosion control and labeled for use on food crops.
  • Small amounts of granular PAM can be applied to the beginning of furrows before flood irrigating (1 to 3 lbs/acre).
  • For sprinklers PAM needs to be injected into the irrigation water during the entire irrigation event.
  • Applying PAM to the soil before sprinkle irrigating or only at the beginning of an irrigation will not maximize control of sediment in runoff.
  • PAM should be applied during the first three to four irrigations after planting or transplanting and when irrigating after soil cultivation.
  • Auger pumps are ideal for metering liquid PAM products into pressurized sprinkler water.
  • PAM itself is not toxic, but the mineral oil in some liquid PAM products can be toxic to aquatic organisms.
  • A prototype applicator is being developed to inject dry PAM into pressurized sprinkler systems, although it is not yet commercially available.

 

Further information on using polyacrylamide is available on the UC Cooperative Extension Website for Monterey County  (http://cemonterey.ucanr.edu/Custom_Program567/Polyacrylamide_PAM/)

Evaluation of Nitrogen Stabilizers to Improve Corn Yield and Plant Nitrogen Status

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Figure 1. Nitrogen stabilizers applied at sidedress fertilizer application. Photo courtesy of Michelle Leinfelder-Miles.

Introduction

Nitrogen (N) is part of a balanced, natural cycle in the environment among the atmosphere, soil, plants, animals, and water. Nitrogen is the most important element needed by crops, and we often add nitrogen fertilizer to optimize crop productivity. Nitrogen use in agricultural systems must be reported for regulatory compliance under the Irrigated Lands Regulatory Program and the Dairy Order to help ensure that a greater fraction of the applied N is recovered in the harvested crop and not lost to the environment. Nitrogen management gives consideration to the four R’s:

  • Right source: selecting a fertilizer source that matches with crop need and minimizes losses.
  • Right rate: applying the right amount based on crop need and nutrient availability through other sources.
  • Right time: applying the nutrient when the crop can use it.
  • Right place: fertilizer placement that optimizes the crop’s ability to use it.

The four R’s address management considerations (e.g. fertilizer program, irrigation), but site characteristics (e.g. soil, cropping system, weather conditions) also influence N recovery in the crop. Also important to improving crop N recovery is understanding barriers to adopting best management practices, such as costs or risks to crop quality or yield.

While the four R’s articulate four principles for nitrogen management, the N cycle in cropping systems is complicated. Nitrogen can be introduced and lost by various paths. We generally add N with fertilizer or organic matter amendments—such as crop residues, compost, or manure. Fertilizers provide N in plant-available forms—ammonium (NH4+) and nitrate (NO3–). Organic matter amendments must be mineralized before the N is available for plant uptake. Mineralization is a process that involves soil biology converting organic N to NH4+. The timeline of this conversion will depend on the properties of the amendment, environmental conditions—such as soil temperature and moisture, and the activity and abundance of soil microbes.

In the soil, NH4+ has different fates. It can be immobilized by microorganisms, taken up by plants, fixed to soil particles due to its positive charge, volatilized to ammonia gas (i.e. lost from the system), or converted to NO3–—a process known as nitrification. Nitrification is a two-step process. The first step is the conversion of NH4+ to nitrite (NO2–) by Nitrosomonas bacteria. The second step is the conversion of NO2– to nitrate (NO3–) by Nitrobacter bacteria. These two steps generally occur in close succession to prevent the accumulation of NO2– in the soil. Conditions that affect nitrification include soil aeration, moisture, temperature, pH, clay and cation content, NH4+ concentration, among others. Just as NH4+ has different fates in the soil, so too does NO3–. Plants preferentially take up NO3–, but if NO3–  is present when plants are not in need of it, then NO3– may be immobilized by microorganisms, volatilized to nitrogen gas (i.e. lost from the system), or leached out of the root zone (i.e. lost from the system).

Technologies have been developed to mitigate N losses from the system. These technologies are collectively known as enhanced efficiency fertilizers (EFF) and include additives, physical barriers, and chemical formulations that stop, slow down, or decrease fertilizer losses. Nitrogen stabilizers, slow-release fertilizers, and polymerized fertilizers are examples of EEF. Nitrogen stabilizers are fertilizer additives intended to improve crop N use efficiency and reduce N losses to the environment by interrupting the microbial processes that change N to its plant-available forms. We developed a trial to evaluate two N stabilizer products with the objective of determining whether the treatments improved corn silage yield or plant N status compared to fertilizer alone. We did not attempt to measure N losses from the system (e.g. leaching, denitrification), as these are very challenging to quantify.

The products in our trial were Vindicate (Corteva Agriscience) and Agrotain Plus (Koch Agronomic Services). Vindicate delays the nitrification process by inhibiting the Nitrosomonas bacteria that converts NH4+ to NO2–. Vindicate has bactericidal activity, and the active ingredient is nitrapyrin. Agrotain Plus has two modes of action—reducing ammonia volatilization and delaying nitrification. Ammonia volatilization is the conversion of NH4+ in the soil to ammonia gas (NH3) in the atmosphere, and it is reduced by inhibiting the urease enzyme. Ammonia volatilization is most problematic when the N source is urea-based and not incorporated or watered into the soil. The active ingredients of Agrotain Plus are Dicyandiamide (DCD), which delays nitrification, and N-(n-butyl)-thiophosphoric triamide (NBPT), which reduces volatilization. DCD has bacteriostatic activity, which means it slows the metabolism of Nitrosomonas. We hypothesized that N stabilizers would improve yield and N uptake over the fertilizer-only treatment, providing growers with a tool for nutrient stewardship.

Methods

The trial took place in San Joaquin County on a DeVries sandy loam soil. The field had a winter wheat crop that was cut for forage in the late spring. Dry manure was applied to the field between wheat harvest and corn planting, which occurred on May 24, 2018. The variety was Golden Acres 7718. At-planting fertilizer provided approximately 12 lb N per acre (4-10-10). Sidedress fertilizer application occurred on June 21st and provided approximately 105 lbs N per acre (UAN 32). Four treatments were applied at sidedress, when plants were at V3-4 stage of development (Fig. 1). The N stabilizers were applied at the label rates, and the treatments were: 1) Vindicate at 35 fluid ounces per acre, 2) Agrotain Plus at 3 pounds per acre, 3) combination of Vindicate and Agrotain Plus at aforementioned rates, and 4) fertilizer-only, no stabilizer product (“untreated”). Plots were 35 feet across (i.e. fourteen 30-inch rows), in order to adapt to equipment of different widths, by 900 feet long. Treatments were randomly applied in three replicate blocks. Aside from the treatments, the trial was managed by the grower in the same manner as the field.

We evaluated soil N status, plant N status, and silage yield. Prior to planting, 20 soil cores were randomly collected from across the trial and aggregated by foot-increments, down to two feet. Mid-season leaf and soil samples were collected when the corn was in the R1 stage (i.e. silking). Soil was collected from 10 in-row locations in each treatment, and aggregated by foot-increments, down to two feet. Leaves were sampled from ten plants in each treatment, sampling the leaf one-below and opposite the earleaf. Harvest occurred on September 20th. All fourteen rows were harvested for weight, and samples were collected at the silage pit for aboveground biomass N analysis. The samples were dried at 60⁰C for 48 hours for calculating dry matter (DM). Post-harvest, 10 in-row soil cores were collected to one-foot depth and aggregated for each treatment. Laboratory analyses were conducted by Ward Laboratories (Kearney, NE; https://www.wardlab.com/). We used Analysis of Variance to detect differences in treatments and Tukey’s range test for means separation (JMP statistical software). Treatments were considered statistically different if the P value was less than 0.05.

Results and Discussion

There were no statistically significant differences among treatments for plant tissue N, yield, dry matter, or total N removed at harvest (Table 1). Mid-season leaf N averaged 2.88 percent across treatments, and aboveground biomass N at harvest averaged 1.12 percent. At mid-season, leaf N from 2.7 to 3.5 percent indicates that the plants had sufficient N to carry the crop to harvest, and at harvest, whole plant N from 1.0 to 1.2 percent indicates that the N fertilization program was adequate for maximizing yield [1]. Calculated to 30 percent dry matter, average yield across treatments was 38.8 tons/acre, and dry matter was 35 percent. There was a trend for the two treatments with Vindicate to have a higher N removed than the two treatments without it, but the difference was not statistically significant. The low coefficient of variation (CV), which is a measure of variability in relation to the mean, indicates low variability among replicates for all of these parameters.

 

Treatment Midseason (R1) Leaf Total N

(%)

 Aboveground

Biomass Total N

(%)

 Yield at

30% DM (tons/acre)

 DM

(%)

 Total N Removed at Harvest

(lbs N/acre)
Vindicate 2.97 1.12 40.4 0.37 272
Agrotain Plus 2.97 1.11 37.7 0.34 250
Vindicate and Agrotain Plus 2.71 1.16 38.7 0.34 269
Untreated 2.87 1.09 38.3 0.35 251
Average 2.88 1.12 38.8 0.35 261
CV (%) 4 2 3 3 5
P value 0.32 0.18 0.48 0.20 0.39

Table 1. Plant N, yield, dry matter (DM), and N removed results for the 2018 N stabilizer efficacy trial. There were no significant differences among treatments.

 

The pre-plant (post-dry manure application) soil nitrogen status was 17 parts per million (ppm) NO3-N and 4 ppm NH4-N for the 0-12 inch depth, and 7 ppm NO3-N and 2 ppm NH4-N for the 12-24 inch depth. When soil NO3-N is below 25 ppm in the top foot of soil, it is recommended to apply N fertilizer in order to prevent yield reductions [2]. There were no differences among treatments in soil N status at the mid-season sampling, but there were differences at the post-harvest sampling (Table 2). Mid-season soil NO3-N averaged 32 ppm across treatments in the top foot of soil, and 10 ppm in the second foot of soil, which is an adequate concentration to carry the crop through to harvest. Soil NH4-N averaged 4 ppm and 2 ppm across treatments for the top foot and second foot, respectively. The CV was high for mid-season soil data, which indicates high variability among replicates. Post-harvest soil NH4-N averaged 2 ppm across treatments in the top foot of soil, but soil NO3-N was higher than at any other time during the season, averaging 46 ppm across treatments. These results may indicate that the dry manure mineralized later in the season, after the peak demand of the corn. Post-harvest soil NO3-N above 20 ppm is considered high and indicates that this crop was not deficient in N [2]. The low CV for NO3-N indicates low variability among replicates. The significant differences among treatments are not well-understood, particularly as the control (fertilizer-only) treatment had soil NO3-N that was not different from any of the treatments. Interestingly, Vindicate had the lowest post-harvest soil NO3-N and a trend toward higher N removed (though not statistically higher), which may indicate that product use made N available at a time that optimized N uptake.

 

Treatment Mid-season NO3-N (ppm) 0-12 inches Mid-season NO3-N (ppm) 12-24 inches Mid-season NH4-N (ppm) 0-12 inches Mid-season NH4-N (ppm) 12-24 inches Post-harvest NO3-N (ppm) 0-12 inches Post-harvest NH4-N (ppm) 0-12 inches
Vindicate 33 10 4 2 38  b 2
Agrotain Plus 32 10 4 2 44 ab 2
Vindicate and Agrotain Plus 32 9 3 2 57 a 2
Untreated 31 12 4 2 43 ab 2
Average 32 10 4 2 46 2
CV (%) 25 31 20 36 7 19
P value 0.99 0.75 0.58 0.97 0.04 0.89

Table 2. Soil N status (as NO3-N and NH4-N) at mid-season and post-harvest samplings for the 2018 N stabilizer efficacy trial.

Summary

N is part of a balanced, natural cycle in the environment and is the most important nutrient in cropping systems. Giving consideration to N management will help ensure that a greater fraction of the applied N is recovered in the harvested crop and not lost to the environment, and keeps growers in regulatory compliance. Enhanced Efficiency Fertilizers, such as N stabilizers, have been shown to improve crop yield in regions like the Midwest and the Northeast, and may help to mitigate N losses from the environment. In our trial, we evaluated the efficacy of N stabilizer products for improvements in corn silage yield or plant N status compared to fertilizer alone. Under the management and environmental conditions of this trial, we found no differences in yield or plant N status; however, plant and soil tests indicated that N was never limiting in the trial. If N was lost from the system, the loss was not large enough to result in N limitation in the control. Future study should test these products using different N sources and N rates (e.g. grower rate and grower rate minus 50 lbs N/acre). It may be possible to reduce the fertilizer N rate without sacrificing yield.

References

  1. Nutrient Management for Field Corn Silage and Grain in the Inland Pacific Northwest. https://www.cals.uidaho.edu/edcomm/pdf/PNW/PNW0615.pdf.
  2. Nutrient Management Guide – Silage Corn (Western Oregon). https://catalog.extension.oregonstate.edu/em8978.

 

 

 

Pierce’s Disease and Glassy-winged Sharpshooter: Still a Threat to California Viticulture

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First known as Anaheim grapevine disease or California vine disease, Pierce’s disease (PD) has impacted California’s grape production since the late 1880’s. Grape growers had been losing vineyards to an unidentifiable disease, which prompted the US Government to hire Newton B. Pierce, the first United States Department of Agriculture (USDA) plant pathologist and namesake of the disease. Pierce had spent considerable time walking vineyards in Los Angeles, San Bernardino and Orange Counties where Muscat of Alexandria used for raisins and Mission and other wine grape varieties were dying at alarming rates (2). At the time, approximately 25,000 acres had been infected and or lost to an unknown “malady”. Pierce also spent time traveling to France, Italy and other Mediterranean grape growing regions studying plant disease symptom expression and declining grapevines to compare with those found in California. After researching all aspects of California viticulture production, including pests, diseases and associated symptoms, Pierce could never correctly identify the cause of California vine disease. For many years, a plant virus was thought to be the culprit of PD. It wasn’t until the mid-1970’s when University of California (UC) researchers isolated a bacterium from diseased vines. Once isolated, the bacteria were reintroduced to healthy grapevine plants that developed Pierce’s disease symptoms within 2-4 months (1). The bacterium is known to move from vine to vine with the help of insect vectors representing the sharpshooter (Cicadellidae) and spittlebug (Cercopidae) families. Infected insects ease of movement into a vineyard can be devastating in a few seasons when a PD susceptible grape variety is planted.

 

Although Pierce’s disease outbreaks occurred in California vineyards from time to time since being ID’d it was not a primary issue for the grape industry. Major raisin, table and wine grape growing regions had moved north into the San Joaquin and Sacramento Valleys and the central and north coast. The mild, southern California weather was the perfect environment for the pathogen, vectors and disease development. In contrast, the seasonality of the interior valleys and coastal grape growing regions seemed to result in a lower PD incidence year to year. However, there were PD “hot spots” located near riparian areas (i.e. Napa and Kings Rivers) or alfalfa planting that experienced significant vine deaths. Those hot spots were costly to individual farming operations but was not a concern for the industry. That changed when the disease/vector dynamics shifted.

 

In 1999, the nonnative PD vector, glassy-winged sharpshooter (GWSS), arrived in Temecula Valley. At that time a once thriving southern California wine industry was experiencing rapid vine death. GWSS turned out to be an effective vector and superior flyer when compared to native sharpshooters. Additionally, vineyards planted next to citrus proved to be a deadly combination. GWSS used citrus groves to feed, breed and for protection from potential predators from late fall to early spring. California grape growers were concerned about their future as they watched Temecula Valley vineyards die. As GWSS spread to other parts of California, PD became a much greater concern and problem. According to K.P. Tumber et. al (3) California growers are paying $56+ million in lost production and vine replacement annually.

Photo courtesy of University of California

Pierce’s Disease: Cause, Symptoms and Management

Cause of Pierce’s Disease

A gram-negative bacterium was found to be the causal organism of Pierce’s Disease in 1975 (1). Prior to Xylella fastidiosa being identified, it was thought that a virus was responsible for the demise of southern California vineyards. Newton B. Pierce, the diseases namesake, began researching the cause in the late 1880’s but never properly identified it as a bacterium.

 

Living in the xylem of grapevines, X. fastidiosa blocks the movement of water and nutrients throughout the plant. Once infected, the bacterium moves systemically from the point of infection to other parts of the plant. Early season symptoms can be confused with nutrient deficiencies (i.e. Zinc), displaying interveinal leaf chlorosis and stunted growth. Late season symptoms have a scorched foliage appearance resulting from the plants inability to move water through the xylem vessels. Young vines are more susceptible than older vines to infection and may die by the end of the season, while older plants may display symptoms over several seasons. However, when bacteria populations increase to a level that restricts significant sap movement, foliage and fruit will dehydrate and die. Geographical location, time of year and variety (Table 1) will determine how severe the symptoms become. As temperatures increase, fruit will shrivel, and green shoots mature poorly and never cure prior to winter. At this point, financial losses are expected to impact vineyard viability.

 

Pierce’s Disease Symptoms

 

  • Springtime symptoms consist of grapevine leaves displaying interveinal chlorosis
  • Late-summer or fall symptoms consist of grapevine leaves displaying concentric rings of drying from the leaf margin towards the center. Leaf margins of red or black grape varieties turn red and then brown
  • Leaves that have turned brown will detach, leaving only the petioles attached to canes
  • A unique disease symptom is the irregular, patchy bark maturity, leaving half the shoot brown and half green, displayed as islands of green and mature brown coloration
  • Berries on clusters will shrivel and/or raisin

 

Pierce’s disease symptoms can often be confused with nutritional deficiencies, water related issues or other diseases. Multiple tissue samples should be shared with your pest control advisor (PCA), certified crop advisor (CCA), local farm advisor or university plant pathologist to correctly ID the symptoms. Once properly identified, a treatment plan can be devised to improve the vineyard’s health.

Management

Management strategies will depend on several factors. Insect vector, grape variety and location will have a significant impact on the success of managing PD. The four main insects that transmit PD are the blue-green sharpshooter (Graphocephala atropunctata), native to coastal regions near riparian areas; the green sharpshooter (Draeculacephala minerva) and red-headed sharpshooter (Xyphon fulgida), native to interior valleys; and the glassy-winged sharpshooter (Homalodisca vitripennis), a non-native species to California and the most dominate vector of X. fastidiosa. It is important to monitor for sharpshooter insects if PD is to be managed. Sticky cards, sweep nets and visual observations of the vineyard and nearby properties will help in determining the population size and what control measures will be needed. Once identified, properly timed insecticide applications will help reduce the population. Vineyards located in areas where the PD bacterium is common, and temperatures are mild will be a challenge at keeping PD under control. In this case, identifying and managing the insect vector will be most important. If GWSS is the primary vector, insecticides will need to be timely to keep insects from moving into the vineyard. Citrus planted next to a vineyard will have to be sprayed as well to keep populations in check. Citrus should be visually checked for adults, nymphs and eggs. Visit the UC Pest Management Guidelines online for the most current insecticide management strategies (4,5).

 

Locations that have a history of PD should not be planted to highly susceptible varieties like Chardonnay if possible. Finding a more tolerant PD variety will improve the health of the vineyard. Newly developed PD resistant varieties have been released from UC Davis. These numbered wine grape selections (Table 1.) can be planted in areas with a high incidence of PD and used for blending with traditional varieties. Unfortunately, there are not any PD resistant varieties for raisin or table grape production, but research is ongoing.

 

Table 1. Variety Susceptibility to X. fastidosa
Highly susceptible Moderately susceptible PD resistant*
·         Chardonnay

·         Redglobe

·         Fiesta

 ·     Riesling

·     Chenin blanc

·     Cabernet Sauvignon

·     Ruby Cabernet

·     Muscat of Alexandria

·     Thompson Seedless

 

 ·         07355-075 (50% Petite Sirah, 25 % Cabernet Sauvignon)

·         09331-047 (50 % Zinfandel, 25 % Petite Sirah, 12.5 % Cabernet Sauvignon)

·         09356-235 (50 % Sylvaner, 12.5 % Cabernet Sauvignon, 12.5 % Carignane, 12.5 % Chardonnay)

·         09314-102 (62.5 % Cabernet Sauvignon, 12.5 % Carignane, 12.5 % Chardonnay)

·         09338-016 (62.5 % Cabernet Sauvignon, 12.5 % Chardonnay, 12.5 % Carignane)

*UC Davis PD resistant wine grape varieties released in 2017 from Dr. A. Walker

 

Grapevines showing unusual foliar symptoms should be taken to your local UC Cooperative Extension office for identification. Plant tissue suspected of having Pierce’s disease can be sent to a diagnostics lab for positive identification using molecular tools. Leaf blades and petioles sampled from green portions of the cane in the late summer to early fall will give the best results. Vineyard insects should be caught and identified, too. A sweep net or sticky cards strategically placed in the vineyard can be used to survey insect populations in areas displaying foliar symptoms. Unique insects can be taken to your local Agriculture Commissioners office or the California Department of Food and Agriculture—Plant Health and Pest Prevention Services Division for identification.  These first steps are paramount for developing a management plan.

 

 

References

Pierce’s Disease

  1. Davis, MJ, Purcell, AH, Thomson, SH, 1977. Pierce’s Disease of Grapevines: Isolation of the Causal Bacterium.
  2. Pierce, NB. 1892. The California vine disease: a preliminary report of investigations. U.S Dep. Agric. Div. Veg. Pathol. Bull. 2, 222.
  3. Tumber K, Alston J, Fuller K. 2014. Pierce’s disease costs California $104 million per year. Calif Agr 68(1):20-29. https://doi.org/10.3733/ca.v068n01p20
  4. UC Pest Management Guidelines – Pierce’s Disease: Xylella fastidiosa http://ipm.ucanr.edu/PMG/r302101211.html
  5. UC Pest Management guidelines – Sharpshooters http://ipm.ucanr.edu/PMG/r302301711.html

 

Grapevine Heat Stress and Sunburn Management

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Figure 1. Berry shrivel, raisining, and sunburn of Syrah during the heat wave

 

Heat waves with extreme daily temperatures are becoming more and more common in the San Joaquin Valley (SJV) during the middle of growing season, e.g., July and August. In 2017, grape growers in the SJV have experienced two to three weeks with maximum daily temperature ≥ 110 °F. Sunburn with the associated severe water stress have resulted in significant yield loss and poor berry quality at harvest. Berry sugar, organic acids, anthocyanins, and phenolics all can be impacted by extreme daily temperature. Sugar accumulation can be significantly affected since the leaf photosynthetic rate starts to decrease when the canopy temperature passes 30 °C. Under high berry temperature (≥ 30 °C), the degradation of organic acids start to accelerate as well as anthocyanins and phenolics.

 

Water Stress

When the heat wave occurs, it usually also causes grapevine water stress due to the need of evaporative cooling in order to lower the canopy temperature. High daily temperature coupled with severe water stress will eventually reduce the berry size and ultimately make the berry shrivel and raisin (Figure 1). Several vineyard practices can be adopted by growers to alleviate the potential damage from the heat wave and reduce the yield loss as well as the degradation of berry composition:

  • Row orientation
  • Trellis selection
  • Variety selection
  • Canopy management
  • Irrigation scheduling
  • Canopy shading
  • Canopy cooling
Mechanical leafing
Figure 2. Mechanical leafing at “morning” side of the canopy during bloom

Row Orientation

The optimum row orientation in the SJV is southwest to northeast with approximately 45° angle to have the equal sunlight exposure on both sides of the canopy. The traditional row orientation of raisin vineyard in the SJV of east to west is still good to minimize the direct light exposure on fruit-zone. North to south row orientation should be avoided for sunburn susceptible varieties, e.g., Muscat of Alexandria and Chardonnay.

 

Trellis Selection

Trellis selection is as important as row orientation. Vertical shoot positioning trellis is usually not suitable in the SJV due to the excessive light exposure on fruit-zone. Two-wire vertical trellis, or “California Sprawl”, is the most common and yet suitable for the SJV. Any trellis with a sprawling system is preferred under the hot climate.

 

Varieties

Variety evaluation has been on-going in University of California (UC) Kearney REC for a couple of years and the initial data has confirmed that certain varieties from southern Mediterranean regions can tolerate the heat and produce the decent yield and berry composition. Some varieties, e.g., Fiano, are under commercial test to further prove their suitability under the SJV’s hot climate. However, the adoption of alternative varieties might largely depend on marketing and consumers’ acceptance.

Shade cloth on fruit-zone
Figure 3. Shade cloth on fruit-zone at “afternoon” side of the canopy

Canopy Management

Canopy management, e.g., shoot thinning and leafing, is applied to provide enough light exposure and air circulation on fruit-zone without exposing the clusters to too much direct sunlight. Hand or mechanical leafing (Figure 2) can be applied only on the “morning” side of the canopy to avoid the afternoon sunlight exposure on fruit-zone.

 

Irrigation Management

Irrigation management might be the most critical and powerful tool for growers and the appropriate irrigation scheduling can be applied to avoid excessive heat damage/water stress as well as berry sunburn. Severe deficit irrigation should be avoided before the heat wave occurs to make sure vines with no or minimal water stress under the extreme daily temperature. Soil moisture sensor, pressure chamber, or basically by feel and appearance can help growers to assess soil moisture and vine water status, or growers can simply follow the grape evapotranspiration (ET) report (https://ucanr.edu/sites/viticulture-fresno/Irrigation_Scheduling/) to decide the amount of irrigation per week to avoid severe grapevine water stress during the heat wave.

 

Canopy Shading

Canopy shading including shade cloth (Figure 3) and sun protectant, e.g., Kaolin and CaCO3 (Figure 4), can be used to shade the canopy and fruit to avoid excessive light exposure and sunburn. Cost and timing might be the most important factors when growers decide to use shade cloth and sun protectant. Generally, the optimum timing to apply canopy shading is after berry set or several days before the heat wave.

Canopy cooling can also be applied by in-canopy misting. Studies in Australia have found by in-canopy misting it can cool canopy and clusters by several degrees, and ultimately improve yield and berry composition during the heat wave (https://www.wineaustralia.com/research/search/completed-projects/ua-1502).

Sun protectant
Figure 4. Sun protectant of CaCO3 foliar spray during veraison

Integrated Approach

Finally, it might require to take the integrated approach by using more than one mentioned strategies to maximize the production and berry quality during the heat wave. Growers should consult local farm advisors and conduct the small trials to evaluate the effectiveness of different approaches under the local condition.

ACP Control With Systemic Insecticides

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Tamarixia radiate male cisr

 

All photos courtesy of Citrus Disease and Pest Prevention Program

Last summer’s long hot spell may have contributed to the low trap counts of Asian Citrus Psyllid (ACP) in the Central Valley, but researchers remain adamant that keeping the numbers low is the best defense the state’s citrus belt has to keep out Huanglongbing (HLB).

Meanwhile, detection of HLB infected trees in residential areas of the southern California counties of Orange, Los Angeles and San Bernardino, continues to expand.

At the Kern County Spring Citrus meeting, Dr. Beth Grafton-Cardwell, director of the University of California (UC) Lindcove Research and Extension, said the threat to commercial citrus is real.

ACP tamarixia emergence hole

Best Techniques for Reducing Spread of ACP

Newly hatched ACP nymphs feeding on an infected tree quickly pick up the bacterium that causes the disease, and move on to infect other citrus trees. Removal of infected trees can help slow the spread, but detecting an HLB-infected tree can take time. Grafton-Cardwell said trees may initially only be infected on one quadrant and can be missed in a survey. It may take a year or two before the entire tree is infected, diagnosed and removed.

Coordinated spray treatments by growers when warranted by trap finds, treating with insecticides that have an extended residual and being vigilant about cultural practices are the important steps in keeping the state’s citrus industry viable in the face of HLB.

Movement of stem and leaf material, whether by harvest crews, hedging and topping equipment or on spray rigs can help prevent ACP from hitchhiking to new territory.

Bio control

Although ACP finds in the San Joaquin Valley have been spotty, Grafton-Cardwell said the coordinated treatments are a tremendous tool.

“We are still in the eradicative mode here,” she stressed.

Besides the San Joaquin Valley, growers in the desert and Coachella areas have also been keeping the lid on ACP populations with coordinated treatments. The Ventura coastal growing region and Riverside-San Bernardino citrus have more ACP pressure. A summary of 224 scouted sites in California from June 2017 to September 2018 showed that at Ventura’s 47 sites, 87 percent had ACP nymphs present. In the Riverside San Bernardino region of the 47 sites, 88 percent were infested. The 50 sites in the San Joaquin Valley had zero percent while Coachella’s 45 sites had 8 percent.

Foliar application

Samples

The hot, dry weather in the desert and San Joaquin Valley growing areas help harden new flush depriving ACP as they need soft flush to lay eggs and as food for the nymphs. Growers or farm managers are asked to sample for ACP whenever young flush is present. The protocol is to sample one flush on ten trees on each border of a block. If ACP is found, the grower liaison should be notified to confirm a find and make plans for a coordinated treatment. Grafton-Cardwell said not to rely on empty yellow sticky traps to make determine if ACP has invaded an orchard, as they prefer the new flush.

Workshops on sampling for ACP will be held again this year, Grafton-Cardwell said.

When growers are asked to participate in a coordinated treatment they should respond quickly and use the most effective product possible. These treatments are another reason why ACP levels have been lower in the San Joaquin Valley, plus growers are also using pyrethroids to control glassy winged sharpshooter.

It is important to note that ACP tend to be found on the border trees of the blocks. For all insecticide applications, the borders should be treated before treating the interior. Research has shown, Grafton-Cardwell said, that 80 percent of the ACP in a block are on the border trees. This does not hold true for young citrus.

Inspecting leaves

Residual Toxicity

In addition to the coordinated treatments, the residual toxicity of the pesticide used is important. Broad spectrum products that have a four plus week residual include Baythroid, Danitol, Actara, Admire, Leverage and Agri-flex. These products come with a warning that use may cause flare ups of scale or mites. Insecticides that are selective with a two to four week residual are Delegate, Exirel, fujimite, Movento and Surround. Materials allowed in organic production have a residual of less than two weeks. They include Pyganic, Entrust, oils and Celite. These need to make direct contact to be effective and Grafton-Cardwell recommends two spray applications to increase chances of control.

The longer the residual, the more effective the product will be in controlling ACP as eggs and nymphs are difficult to reach with a spray and adult ACP can fly in from untreated areas and not be affected. The goal is to keep ACP nymphs below 0.5 per flush. Admire and Platinum gave the best results.

Lab research

Biological Control

Biological control, release of the parasite Tamarixia by California Department of Food and Agriculture (CDFA) throughout ACP infested residential sites in southern California, will continue, Grafton-Cardwell said. Releases in commercial citrus are not feasible due to use of spray applications for other insect pests and timing.

Tamarixia populations build and move into citrus October-November, after fall flush.

Control measures buy time for research and horticultural advances including early detection, using genetic engineering to create a protected tree, and HLB resistance. Other strategies include higher density orchards planned for shorter tree life span, using interference RNAs to prevent ACP from picking up the disease and growing citrus under protective cover.

Foliar application

Pest Control Districts

Judy Zaninovich, Kern County ACP/HLB grower liaison said residential finds of ACP were very high 2015-16. The county pest control district’s pilot program for residential citrus has taken out 2,000 trees near sites where ACP was detected. There are similar pilot programs in southern California counties.

In southern California a total of 1,127 HLB positive trees have been removed. Last year at this time the number was 501 trees. This shows the disease is spreading, but also that CDFA is improving their detection.

Last year, Zaninovich said, the potential for a late summer spike in ACP populations was recognized and coordinated treatments were done. Knowing there is the potential for an upswing in ACP at that time, she said the plan would be repeated this year. She said there is also evidence that nighttime applications may be more effective.

Tamarixia

Irrigation Injection

Best practices for application of systemic pesticide imidacloprid delivered via irrigation was discussed by both Sarge Green, director of Center for Irrigation Technology at Fresno State and Rick Leonard of Bayer.

Distribution optimization is the key. The goal there is to make sure the water is in the right place at the right time. Green said the soil type controls movement of the material and pore size dictates movement. Matching water delivery to the soil type will improve efficacy of the material applied. Green noted that regular maintenance and auditing of the water delivery system is important in micro and drip systems.

Leonard supplied some of the basics for efficient use of imidacloprid delivered via irrigation. Admire systemic can be tank mixed with fertilizer, but needs agitation. In a 12 hour set, the product should be injected in a one to two hours period after the first three to four hours of the set to achieve the best distribution.

It will take two to three weeks for the material to move up from the roots into the trees. The cooler the weather during that time, the longer it will take to move throughout the tree. The best strategy of use is to target the fall flush.

Ventura coastal area growers have a more difficult time achieving success with this systemic application, Leonard said, due to the high clay and organic matter soils. If the material only reaches the sub lethal levels for ACP, it invites resistance.

Do Liquid Digestates, By-Products of Bio-energy Production, Have Nematode-Suppressive Potential?

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Experimental Tank Wagon
Experimental tank wagon for band application of liquid digestate in a walnut orchard. The digestate is pumped via a custom nozzle underneath the tree row. Food hygiene guidelines need to be observed.

Large amounts of organic wastes of food or animal origin accrue in cropping systems and in the food industry. Traditionally, many of these byproducts could remain in the agricultural production chain. For example, almond hulls may be used as dairy feed. Others ended up in landfills. With the continually increasing amounts, and for other market changes, alternative uses are urgently needed. When converting these energy-rich materials to biogas, organic matter from food waste or animal manure are processed through anaerobic digestion by microorganisms in specialized biodigesters. The resulting biogas is then used as fuel for electricity and heat generation or put into cars and other vehicles as transportation fuel. The anaerobic digestion process has been favored to reduce the emissions of methane and other gases from organic waste materials during natural decomposition. Although animal manure is probably the most widely used substrate for anaerobic digestion worldwide, food waste is another organic substrate due to its high methane production potential. Besides biogas, a liquid effluent called anaerobic digestate is also produced from digestion processes. The disposal of such residues represents an environmental and economic challenge. A meaningful use of this material would favorably impact environmental stewardship by reducing waste disposal issues, and could benefit agriculture by recycling the nutrients in the digestate for plant growth benefits.

Experiment with pepper in microplots.
Experiment with pepper in microplots. Microplots are contained areas of two foot diameter and five feet long culvers perpendicularly inserted in the ground, and filled with test soils. Each of these plots allow for precise application amounts of digestates or other treatments.

Plant-parasitic Nematodes

Plant-parasitic nematodes are a constraint in crop production, especially in perennial crops in California. Long cropping cycles, soils that favor high nematode densities, and favorable climate conditions, increase nematode reproduction. In the past, nematode-infested fields have been effectively treated with soil fumigants before planting or with various post-plant nematicides. The use of fumigants and non-fumigant nematicides is challenged by human and environmental health concerns. For example, regulation limits the use of 1,3-dichloropropene materials under a so-called township cap—so quantity restriction based on the entire amount used in an area. Clearly, more environmentally friendly alternatives to the use of these chemicals are urgently needed.

Environmentally Friendly Alternatives

A number of studies have investigated the potential of these digestates as bio-fertilizers. Because these wastes originate from plant material they are nutrient rich and their use fits into a cyclic production of returning byproducts to the primary field production. Such cycling has positive environmental effects. In some studies, the potential of these digestate for managing pests and diseases in different crops were explored. In a study in Germany, anaerobically digested maize silage suppressed the sugar beet cyst nematode, a major pest of sugar beet production in Central Europe. Using organic materials as nematode management tool is challenging because such materials can vary greatly in their physico-chemical composition. This composition likely will impact the nematode-suppressive potential of digestates. It probably depends not only on the substrate but also on the conditions during anaerobic digestion.

Watermelon experiment
Watermelon experiment for testing for efficacy of digestates to suppress nematode population densities. Watermelon seeds are grown in root-knot nematode-infested soil after at-plant application of digestates for one month. Then roots are harvested and examined for nematode-induced galling.

In a project supported by the Department of Pesticide Regulation (DPR), digestates from different sources of different processing conditions and substrate base as well as varying chemical constitution showed differences in nematode suppressive potential. This illustrated the challenge of working with organic materials, and the need to quickly and easily characterize the nematode suppressive potential of digestate. For this purpose, a robust fast turn-around bioassay was tested in three different incubation environments, two different growing containers, and with two different nematode life stages as inoculum. In this test, a single radish seed is planted into nematode-infested soil in small containers after a small amount of digestate has been added. After four to five days, a staining procedure is used to visualize the nematode that have penetrated the young radish roots. Low numbers compared to roots that did not receive the digestate suggest some suppression of nematode infection. In this project, results were similar in the different contexts, and the digestate tested was able to suppress nematodes in all contexts. Based on these results, this bioassay may be useful as a quality control tool for measuring nematode suppressivenesss of organic liquids that could possibly be implemented by commercial laboratories.

Temperature

Temperature is one of the most significant parameters influencing anaerobic digestion. Biogas generation through the anaerobic digestion process can take place over a wide range of temperatures, from as low as 50 F (10 °C) to 135 F (55 °C), corresponding to psychrophilic <68 F (< 20°C ), mesophilic 68 to 104 F (20-40°C ), and thermophilic >104 F (>40°C ) conditions. Because of an increased biogas yield, in most cases, digesters are operated under mesophilic or thermophilic conditions. Temperature does influence the activity and composition of microorganism groups. This influences the methane yield and likely the constitution of the resulting digestate possibly influencing the nematode suppressive potential. Of course the substrate, which can vary between different organic wastes will impact this constitution as well. The substrate and the process may therefore impact what secondary metabolites are produced during digestion, and thus nematode suppressive potential. Therefore, liquid manure and food waste both processed either mesophilically or thermophilically were used in a number of experiments to study the influence of these two factors.

Radish seedling
Radish seedling four days after seeding into nematode-infested soil and digestate amendment. This seedling has sufficient roots to allow for examination of nematode infection.

Food Waste Versus Manure

In the radish bioassay with the sugar beet cyst nematode, no difference in root penetration was found between the two substrates (food waste vs manure) but a significant difference was found between the two processes. The thermophilic digestates were able to reduce nematode root penetration by more than 50 percent compared to the mesophilic digestates. In greenhouse experiments, the digestates of different substrates and processes were used to treat watermelon in soil infested with Meloidogyne incognita (root-knot nematode, RKN) to test the versatility of nematode suppression. After five-weeks incubation, plants were harvested and roots evaluated for nematode damage (root galling, and number of egg masses). Nematode-induced galling was similar or higher in plants from the digestate treatments than for plants from the control. A numerically small but significant reduction in root galling was found in food waste compared to manure. None of the digestates resulted in better plant growth when compared to the control.

Small Field Experiments

Microplot and small field experiments were conducted to implement the findings of controlled conditions into practical field contexts. Application strategies included drench application of the digestates as pre- or post-planting treatments. In a bell pepper microplot trial in RKN-infested soil, five different digestates were applied at planting. Three months later, plants were harvested and roots assessed for nematode suppression. The digestates did not result in improved plant growth compared to the control treatments. Nematode damage in roots was not reduced after treatment with digestates. Although, populations for RKN after harvest, were lower in plots treated with mesophilic manure and similar to the nematicide control. Similar studies were conducted with almond and walnut and ring nematode, root-knot and root lesion nematodes but results were somewhat variable indicating the need for improved application strategies.

Root-knot
Root-knot nematodes are known for their root changing effects. Galls or the name-giving knots are visible on young seedlings, and older plants. Water and nutrient uptake are impeded by such unusual roots.

In summary, some beneficial effects of thermophilic digestates were observed on plant growth and nematode suppression compared to mesophilic digestates under controlled conditions. In preliminary tests in the greenhouse, nematode suppression was observed but under field conditions with different nematode pests of different crops, inconsistent results were obtained. Further experimentation is needed to elucidate the chemical nature of compounds conferring nematode suppression, and how to make use of this beneficial capacity of the waste product digestate. The environmental and economic benefits of cycling plant nutrients and concomitantly suppressing soil pests make this a valuable endeavor.

Iron Deficiency in Fruit and Nut Crops in California

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Figure 1. Advanced iron deficiency on almond tree in Kern County. Interveinal chlorosis are a typical symptoms of iron deficiency.

Micronutrients play a very important role in fruit and nut tree growth and development. Iron (Fe), which is an immobile micronutrient in the plant, is associated with chloroplasts and plays a role in chlorophyll synthesis. While Fe is considered the fourth most abundant element in the Earth’s crust, approximately five percent by weight, iron deficiency is a worldwide problem, and a common micronutrient deficiency in fruit and nut crops (Figure. 1) though it is uncommon in California.

Many orchards in the Central Valley are on semi-arid soils in areas where the evapotranspiration exceeds precipitation. Arid and semi-arid soils can also be found in the southwestern USA and the Mediterranean areas. In this article we will be focusing more on calcareous soils with free calcium carbonate (CaCO3) and soil solution pH in the alkaline range (i.e. above 7.5).

Before we get into the specifics of iron in the soil solution, we’ll give a brief description of pH. It indicates the concentration of H+ ions (protons) in a solution. Soils with low pH have more H+ ions than soils with a high pH. Because the equation is actually a logarithm (Equation 1), the amount of H+ ions does not increase linearly as pH decreases, it increases by a factor of 10. Thus, water with a pH of 5 has 10 times the amount of H+ protons than water with a pH of 6. Therefore, it is progressively harder to correct soil pH the farther it is from 7.

pH = -log[H+]
Equation 1: equation for conversion of the concentration of H+ ions in solution to pH. Since the equation is logarithmic, there is a 10x difference between consecutive values.

Soil pH is important as the different soil minerals that contain and release iron (Fe) into the soil-water solution decrease in solubility as pH increases, which results in only a tiny fraction of the total Fe that is in the soil to be available. In general, iron is more soluble and more available as pH decreases (acidic soils). Plants absorb some iron by diffusion at the root tips from the soil solution, and iron deficiency in California is mainly due to plants’ inability to take up iron due to soil factors such as poor soil aeration and/or high concentration of HCO3- in the soil.

Figure 2. Iron deficiency on newly growing leaves on an almond tree in Kern County with leaves showing
interveinal chlorosis.

Under low iron availability in the soil, the ability of trees and plants to mobilize iron immediately around the root is due to differences in genes between species. Scientists have categorized plants either as “Strategy I’ or ‘Strategy II’ based on their ability to mobilize Fe in the soil and make it available for uptake. Strategy I plants include all plants except grasses and include fruit and nut trees, while Strategy II plants comprise grasses such as wheat and corn. Under Fe deficient soil conditions, Strategy I plants excrete H+ into the soil, which acidifies it and makes iron more available for uptake.

In poorly aerated calcareous or saturated soils, carbon dioxide will become trapped in the soil due to poor gas exchange with the atmosphere. This will cause the production and accumulation of bicarbonates as a result of the interaction between CO2 and calcium carbonates in the soil. Bicarbonates react with the H+ released by roots and interfere with their ability to increase iron availability.

Symptoms of Iron Deficiency
The development of Fe deficiency symptoms is most prominent on young, newly developing leaves (Figure 2) because this element is immobile in the plant. The symptoms are characterized by interveinal chlorosis, (Figure 3). Under severe conditions, leaves have a white coloration due to the disappearance of chlorophyll, and leaves can turn necrotic and abscise. Leaf chlorosis due to iron deficiency reduces photosynthesis and will result in reduced fruit yields and fruit quality. These attributes are only for iron deficient plants; overfertilizing with iron will not increase these functions in the plant.

Pre-Plant Management of Iron Deficiency
The first step in assessing an orchard is site selection, followed by collection of representative soil samples for analysis based on the United States Department of Agriculture (USDA)/National Resources Conservation Service (NRCS) soil survey map. Send these soil samples to a commercial laboratory you trust to look at soil pH and the presence of free lime. It’s also helpful to get a water analysis to look for water pH and bicarbonates. When choosing a site, try to plant in a well-drained soil. Adequate root aeration will reduce the likelihood of iron chlorosis occurring. If the irrigation water contains more than 2 meq/L of bicarbonates, you may consider acidifying the water to a pH of 6.5 to reduce bicarbonate levels by 50 percent and prevent lime buildup in the soil and in your irrigation system. An agricultural laboratory can do a titration curve, which will tell you how much acid to add to decrease the water pH. We do not recommend decreasing irrigation water pH below 5.0. Alternatively, 133 pounds of 100 percent sulfuric acid will neutralize 1 meq/l per acre-foot of bicarbonate in irrigation water. Water acidification can be achieved by using acids such as sulfuric or phosphoric acid. Make sure you tell your laboratory what acid you intend to use, as substituting one acid for another can result in incomplete or over-acidification. Urea sulfuric acids, such as N-PHuric 10/55 and US-10, will also acidify the soil and are safer to handle, however, application rates should not exceed nitrogen (N) crop requirements, which limits its use for acidification. Some growers use a “sulfur burner”, which will convert elemental sulfur into sulfurous acid (H2SO3) by burning elemental sulfur in a small furnace producing sulfur dioxide (SO2). Combination of SO2 and water in the machine will form sulfurous acid that is injected in the irrigation system. Sulfurous acid is safer than sulfuric acid injection. Sulfur burners have a minimum design and production capacity potentially making the capital investment too expensive for smaller farms to consider.

Figure 3. Iron deficiency on newly growing prune leaves.

Acidification can be expensive and in extreme cases may not be viable to reduce pH in soils with a lot of free lime, as it will require large quantities of acid forming amendments to react with soil lime before the bulk soil pH begins to decrease. It takes approximately half a ton of soil sulfur to break down one  percent calcium carbonate in one acre-inch of soil. To manage these costs, soil amendments such as elemental sulfur or sulfuric acid can be banded or shanked in the tree row before planting. However, warm soil temperatures and soil bacteria are needed to convert the elemental sulfur to sulfuric acid and depending on the source of sulfur and its influence on particle size, structure, and solubility of the sulfur this may take several weeks to years to break down. Acids work much faster but are more expensive. It is important to remember that any acidification will break down free lime in the soil before the bulk soil pH is changed.

Table 1. Amount of soil sulfur needed to modify a loam soil. Adapted from the Western Fertilizer
Handbook, 9th Edition.

Rootstock choice is one of the most important choices you or your client will make before planting an orchard. This choice should be based on the site challenges such as pH, salinity, nematodes, and risk of bacterial canker, for example. If high soil pH and concern about iron deficiency is the most important factor to resolve, then use of Fe deficiency tolerant rootstocks is a good solution. Some of the rootstocks that are considered tolerant include some of the (peach X almond) hybrids such as Hansen 536, Bright’s, Titan, and Paramount (GF 677). However, these rootstocks are very susceptible to other soil issues such as poor drainage and root diseases, so pick your rootstock carefully. Other rootstocks tolerant to Fe deficiency are Krymsk86 which is a peach/plum hybrid used for almonds and Gisela 5 used for cherry trees.

Post-Planting Management of Iron Deficiency
After planting the trees, if your soils do not have a large amount of free lime, the best management practice is acidifying the soil around the root zone. This can be done using elemental sulfur or the injection of acids as described before, however you can easily damage your trees through acid injection so follow directions carefully. Do not apply sulfuric acid in established orchards at more than 1500 lbs per treated acre to prevent tree damage. Elemental sulfur takes longer but is safer for the trees. It is often more economical to acidify a band of soil rather than attempting to acidify the entire root zone.

Another way to correct iron deficiency after planting is to apply foliar and soil chelated Fe which will result in a faster response. However, it is short-lived, expensive, and can be leached below the root zone under heavy irrigation. Chelated Fe most likely will need to be applied multiple times in the orchard’s lifetime. Applications of ferrous sulfate to the soil or the tree is a cheaper option compared to chelated Fe. However, in calcareous soils it will very quickly become unavailable for uptake and is not an appropriate option in these soils.

Sources:
Elkins, R., and Fichtner, E. (2012). Causes and control of lime-induced Fe deficiency in California fruit and nut crops. CAPCA (California Association of Pest Control Advisers) Advisor. August 2012.
Lauchli, A., and Grattan, S., R. (2012). Soil pH Extremes in: Plant Stress Physiology. CAB International, Editors: S Shabala, pp.194-209.
Sanden, B., L., Prichard, T., L., and Fulton, A., E. 2016. Assessing and Improving Water Penetration in: Pistachio Production Manual. UC ANR publication 3545, Editors Louise Ferguson and David Haviland, pp. 141-152.
Tagliavini, M., and Rombola, A., D. 2001. Iron deficiency and chlorosis in orchard and vineyard ecosystems. European Journal of Agronomy 15: 71-92.

Walnut Husk Fly Management

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Adult Walnut Husk Fly
Jack Kelly Clark, Courtesy University of California Statewide IPM Program.
WHF Life Cycle
Figure 1. Life cycle of walnut husk fly. | Michael Poe, Courtesy University of California Statewide IPM Program.

Walnut husk fly poses particular challenges for developing a truly integrated pest management (IPM) program due to the nature of its life cycle (one generation per year with a long emergence period) and lack of natural enemies. As a result, best practices for management rely heavily on monitoring and insecticide treatments. Precise timing based on monitoring method and rotation of chemistries to minimize resistance risk are keys to successful long-term control of this pest.

Susceptible Varieties

Damaged Husks
Photo 1. Husks damaged by walnut husk fly. | Jack Kelly Clark, Courtesy University of California Statewide IPM Program.

Walnut husk fly (WHF) damage earlier in the season causes shriveled and darkened kernels, increased mold growth, and lower yields. Later season infestations result in little kernel damage, but may stain the shells and make husk removal difficult. All commercial English walnut cultivars are susceptible to WHF infestation, although they differ in their relative degrees of susceptibility and thus damage potential. In general, Hartley, Tulare, Franquette, Payne, and Serr are considered more susceptible, with Howard, Ashley, Chico, and Chandler exhibiting less susceptibility (in order as listed). However, even less susceptible varieties can be damaged by high populations of WHF. Black walnut is also a preferred host, therefore proximity to black walnut can significantly increase WHF pressure in commercial Englishwalnut orchards.

The varietal differences in susceptibility have been correlated to fruit characteristics including husk color, husk hardness, fruit size, trichome density, and plant volatile profiles, in addition to temporal factors (i.e., more severe earlier season damage may be more evident in earlier-leafing cultivars). Current research led by Dr. Steven Seybold (United States Department of Agriculture (USDA) Chemical Ecology Entomologist) is characterizing the plant volatile profiles associated with differences in varietal susceptibility, which may lead to improvements in monitoring and control products, as well as inform plant breeding approaches for genetic resistance or tolerance to WHF infestation.

WHF Life Cycle

The life cycle and basic biology of walnut husk fly is fairly well understood (Figure 1). There is a single generation per year, with adult emergence historically beginning in early to mid-June and lasting through September in the Central Valley. In coastal areas, and recently some inland valley locations, emergence can be detected earlier, in mid- to late-May. Peak emergence is generally observed July through mid-August in most locations. Females must mate and develop eggs prior to the initiation of oviposition into the walnut husk, a period which averages approximately two weeks after emergence. Once eggs are laid, maggots emerge within approximately four to seven days, and feed on the husk for a typical period of three to five weeks. After this period, mature maggots drop to the ground and pupate in the soil. Most adults emerge the following year, but a portion of the population may remain in the soil as pupae for two or more years before emerging as adults.

WHF Maggot
Photo 2. Walnut husk fly maggot | Jack Kelly Clark, Courtesy University of California Statewide IPM Program.

Extended Emergence Period

The extended emergence period of the single generation of WHF, and significant differences in the timing of initial emergence, peak emergence, and end of the flight based on location, year, and other factors, have been the subject of much research. As opposed to some other key pests (e.g., codling moth), there is not yet a validated phenology or degree-day model available for growers and pest control advisors (PCA) to readily adopt to predict key WHF development and adult activity timings. Two recent publications out of University of California (UC) Berkeley (Emery and Mills 2019a, 2019b) investigated the effects of temperature and other environmental parameters on walnut husk fly development and timing. One study evaluated 18 years of historical trap catch data from 49 walnut orchards spanning the Central Valley to determine which factors most influence emergence timing and thermal requirements for development (degree days to emergence), with the goal of developing a phenology model that can be used to predict initial and peak emergence. Some of the factors evaluated included latitude, walnut cultivar, orchard age, winter precipitation, winter chill, and degree-day accumulation. While this model requires refinement for adoption by orchard practitioners (growers and PCAs), it represents a great step forward in improving our understanding of WHF developmental requirements to aid in our IPM program development.

Biological Control Agents

Biological control agents for walnut husk fly in California walnuts are virtually non-existent. The pest in general appears to have few natural enemies. Some reports from the state of Washington indicate that a predatory mite and anthocorid bug species have been observed feeding on WHF eggs, and some spiders and ants may feed on larvae and adults. In addition, chickens and other birds are said to be among the natural enemies of WHF. However, any naturally-occurring WHF biological control agents that may be found in walnut orchards are not known to provide any significant level of population reduction. Other mortali

Male and Female WHF
Photo 3. Male (left) and female (right) walnut husk fly adults. | Michael Poe, Courtesy University of California Statewide IPM Program.

ty factors, particularly those that may impact the overwintering pupal stage in the soil (e.g., intentionally augmenting soil moisture, various cultivation practices, effects or augmentation of insect-parasitic nematodes or other microorganism populations, soil insecticide applications) have been explored to some degree with no specific recommendations or guidelines emerging as a result.

WHF Management Guidelines

In spite of some of these challenges for WHF management, guidelines regarding treatmenttiming and options are available, and when employed properly tend to provide adequate control of WHF in many situations (albeit with more insecticide intervention than may be desirable or sustainable in the long-term). Because WHF activity and population abundance can vary significantly from orchard to orchard (even those in very close proximity), site-specific monitoring is necessary to get the most effective results from insecticide applications.

Monitoring

Monitoring should begin earlier than the June 15 historical guideline (no later than June 1 in the Central Valley is the more recent recommendation). Some reports of late May catches in 2016 further support the “earlier-is-better” practice—there is little harm in counting zeroes for a few weeks. Yellow sticky card traps baited with ammonium carbonate lures should be hun

Female WHF with eggs
Photo 4. Female walnut husk fly with eggs. Larry L. Strand, Courtesy University of California Statewide IPM Program.

g high in the canopy (minimum 2 per 10 acres) in dense foliage on the north side of trees and checked two to three times per week. Each orchard should be monitored individually for WHF activity to best determine if and when to treat. A summary article regarding the efficacy of available traps/lures for WHF monitoring was published in 2014 (http://www.sacvalleyorchards.com/walnuts/insects-miteswalnuts/walnut-husk-fly-trap-and-low-volume-spray-study/).

Treatment Timing

Treatment timing can be based on one of three monitoring methods (the first two have typically been most effective).

  1. Detection of eggs in trapped females. This is a simple process that requires slightly more time than counting overall trap catches and can increase the efficacy of treatments by timing applications to specifically target female oviposition activity. Females can be distinguished from males by the shape of the abdomen (pointier in females) and color of the front leg (female leg is entirely yellow, male leg is black close to the body, Photo 3). After females are identified, gently squishing the female abdomen will squeeze out eggs if they are present (Photo 4). Eggs resemble small grains of rice. Previous guidelines indicated that the treatment window is one week after egg detection. However, recent modifications suggest that treatments should be considered as soon as the first female with eggs is found because in practice there is often a lag time in getting the treatments out, and trap checks (even two to three checks per week) may not be frequent enough to represent initial egg development in the female population. Therefore, planning to treat as soon as possible after eggs are detected may be the best option to minimize infestation and damage. [Note that this is the preferred method for timing treatments unless using GF-120® alone; see below].
  2. Overall trap catches. For low to moderate populations, consider treatment when a sharp increase occurs in trap counts. In high pressure orchards or if using GF-120® alone, treatment should be considered when any flies are detected rather than waiting for a sharp increase in catches.
  3. Stings on nuts. This is the least preferred method, as damage has already occurred. However, examining nuts for stings (Photo 5) can provide indication of efficacy of your management program when using one of the first two methods. If using this method to time treatments, consider treating when the first sting is observed using full cover neonicotinoid materials that have some ovicidal activity mixed with an adulticide.

Continued monitoring throughout the season is crucial. Short-residual insecticides plus bait will generally kill WHF for seven to ten days. Target subsequent applications at two- to four-week intervals based on the efficacy of the previous spray and trap catches. Clean traps the day after application and check three to four days later. If the number of flies drops to near zero, the spraywas highly effective and a longer treatment interval may be used. If post-treatment catches increase or eggs are detected in trapped females, and the residual period of the previous treatment has elapsed, additional treatments may be required if harvest is more than three weeks away.

WHF Sting
Photo 5. Walnut husk fly sting. | Jack Kelly Clark, Courtesy University of California Statewide IPM Program.

There are several materials effective against WHF, both for conventional and organic orchards. All materials aside from GF-120® (which contains its own bait) should be applied with a bait (e.g., Nu-Lure®, molasses, etc.). However, very high population orchards with extensive previous damage may require full coverage sprays (no bait needed) to achieve adequate suppression. Keep in mind that rotation of chemistries (based on the Insecticide Resistance Action Committee (IRAC) mode of action classification) is critical to minimize resistance development for pests that are treated multiple times each season. Proper aphid management can also help limit movement of WHF within and between orchards by reducing honeydew accumulation (a food source for adult WHF).

The UC IPM Pest Management Guidelines (ipm.ucanr.edu/PMG/r881301211.html) lists insecticides, baits, and rates for WHF. A summary of efficacy data for selected materials (updated September 2016) are summarized at (www.sacvalleyorchards.com/walnuts/insects-mites-walnuts/walnut-husk-fly-biology-monitoring-and-spray-timing/).

 

Referenced articles:

Emery, S. A. and N. J. Mills. 2019a. Effects of temperature and other environmental factors on the post-diapause development of walnut husk fly, Rhagoletis completa (Diptera: Tephritidae). Physiological Entomology 44: 33-42.

 

Emery, S. A. and N. J. Mills. 2019b. Sources of variation in the adult flight of walnut husk fly (Diptera: Tephritidae): a phenology model for California walnut orchards. Environmental Entomology 48: 234-244.

Grapevine Trunk Diseases: Current Management Strategies

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Background:
Grapevine trunk diseases (GTD) are currently considered one of the most important challenges for viticulture worldwide. These destructive diseases are caused by a broad range of wood-colonizing fungal pathogens, which primarily infect grapevines through pruning wounds. In most occasions, a single vine can be infected by more than one of these pathogens. The economic impact of GTD can be significant in both young and mature vineyards. Characteristic symptoms include poor vigor, distorted leaves and shoots, shoot and tendril dieback and berry specks caused by fungal toxins produced by some of these pathogens. Perennial cankers produced by canker-causing fungi on grapevine cause spur, cordon and trunk dieback and the eventual death of the entire vine.

Figure 1. Leaf (tiger stripes) (A), fruit (black measles) (B) and vascular (C) symptoms caused by esca disease complex. Esca (black measles) and petri disease are primarily caused by the vascular pathogens Phaeomoniella chlamydospora and Phaeoacremonium minimum, which are also involved in Petri disease in young plants (D).

Epidemiology
Most of the fungal pathogens responsible for GTD produce overwintering fruiting structures containing the spores of the fungus. When environmental conditions are favorable, these fruiting bodies release the spores into the environment. Spores will land on susceptible pruning wounds and will initiate infection completing their life cycle. In California, research suggests that the majority of GTD spores are released during winter (December to February) following primarily though not always precipitation events. GTD fungal pathogens have a broad host range and in California are known to cause dieback in many different native or introduced tree species and also in other woody perennial crops, including tree fruits and nut trees. Therefore, the source of GTD inoculum (spores) can come into a vineyard from multiple sources.

Figure 2. In mature plants, several basidiomycetes fungi (primarily in the genera Fomitiporia, Fomitiporella, Inocutis, Inonotus, and Phellinus) play also a role in disease and symptoms development. Characteristic symptoms are a white rot in the vascular system in many occasions observed as a yellowish-spongy wood.

Management in Nursery:

  • Treat pruning wounds on mother plants to prevent new infections
  • Sanitation in mother fields and during the entire nursery process
  • Disinfect grafting machines regularly
  • Reduction of the cutting hydration period
  • Apply control products (chemicals or biologicals) as a dip after grafting, before storage and/or before dispatch
  • Hot water treatment of dormant nursery plants prior to dispatch
Figure 3. Botryosphaeria dieback, commonly known in California as ‘Bot canker’ is caused by multiple species in the Botryosphaeriaceae family. Characteristic symptoms are the lack of spring growth of infected areas, including cordons (A) or spurs (B). Cross sections of infected parts reveal a wedge-shape canker (C). The GTD disease known as Phomopsis dieback and primarily caused by the fungus Phomopsis viticola shows very similar symptoms as Botryosphaeria dieback.

Management in Vineyards:

  • Use the cleanest plant material available when establishing new vineyards.
  • Minimize stress conditions on young vines after planting.
  • In California, delayed pruning has been shown to minimize infection of pruning wounds as wounds are done passed the high disease pressure period of winter months
  • In vertical shoot position (VSP) systems, double pruning has shown to facilitate late pruning of large acreage vineyards and thus, reduce infection.
  • Prune dead shoots, spurs and cordons below the symptomatic tissue (at least a few inches past the last symptomatic wood).
  • Make a clean and smooth pruning cut to speed up the callusing process at the pruning wound.
  • Sanitation is very important in the vineyard. Remove pruned and infected plant materials away to prevent the development and increase of GTD fungi overwintering structures in the vineyard.
  • Protection of pruning wounds with effective registered chemicals and/or biological control agents is the most effective way to prevent new infections from air-borne spores of GTD fungal pathogens. More than one application may be necessary to protect the pruning wound during its susceptible time period.
  • Remedial surgery, where visible infected parts of the vine (spurs, cordons and/or trunk) are removed, can be an effective strategy to eradicate the pathogen from the vine (primarily when cuts are done lower down on the trunk about 20 to 30 cm above ground) and thus, prolong the lifespan of vineyards.
Figure 4. Symptoms of Eutypa dieback, caused by the fungal pathogen Eutypa lata and several other Diatrypaceae species, are characterized by distorted and chlorotic leaves and short internodes (A) and by wedge-shape cankers (B)

Free access literature:
Gramaje, D., Úrbez-Torres, J. R., and Sosnowski, M. R. 2018. Managing grapevine trunk diseases with respect to etiology and epidemiology: current strategies and future prospects. Plant Disease 102:12-39.
https://doi.org/10.1094/PDIS-04-17-0512-FE
https://ucanr.edu/sites/eskalenlab/

Grapevine Heat Stress and Sunburn Management

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Heat waves with extreme daily temperatures are becoming more and more common in the San Joaquin Valley (SJV) during the middle of growing season, e.g., July and August. In 2017, grape growers in the SJV have experienced two to three weeks with maximum daily temperature ≥ 110 °F. Sunburn with the associated severe water stress have resulted in significant yield loss and poor berry quality at harvest. Berry sugar, organic acids, anthocyanins, and phenolics all can be impacted by extreme daily temperature. Sugar accumulation can be significantly affected since the leaf photosynthetic rate starts to decrease when the canopy temperature passes 30 °C. Under high berry temperature (≥ 30 °C), the degradation of organic acids start to accelerate as well as anthocyanins and phenolics.

Figure 1. Berry shrivel, raisining, and sunburn of Syrah during the heat wave. All photos courtesy of George Zhuang.

Water Stress
When the heat wave occurs, it usually also causes grapevine water stress due to the need of evaporative cooling in order to lower the canopy temperature. High daily temperature coupled with severe water stress will eventually reduce the berry size and ultimately make the berry shrivel and raisin (Figure 1). Several vineyard practices can be adopted by growers to alleviate the potential damage from the heat wave and reduce the yield loss as well as the degradation of berry composition:

  • Row orientation
  • Trellis selection
  • Variety selection
  • Canopy management
  • Irrigation scheduling
  • Canopy shading
  • Canopy cooling

Row Orientation
The optimum row orientation in the SJV is southwest to northeast with approximately 45° angle to have the equal sunlight exposure on both sides of the canopy. The traditional row orientation of raisin vineyard in the SJV of east to west is still good to minimize the direct light exposure on fruit-zone. North to south row orientation should be avoided for sunburn susceptible varieties, e.g., Muscat of Alexandria and Chardonnay.

Trellis Selection
Trellis selection is as important as row orientation. Vertical shoot positioning trellis is usually not suitable in the SJV due to the excessive light exposure on fruit-zone. Two-wire vertical trellis, or “California Sprawl”, is the most common and yet suitable for the SJV. Any trellis with a sprawling system is preferred under the hot climate.

Varieties
Variety evaluation has been on-going in University of California (UC) Kearney REC for a couple of years and the initial data has confirmed that certain varieties from southern Mediterranean regions can tolerate the heat and produce the decent yield and berry composition. Some varieties, e.g., Fiano, are under commercial test to further prove their suitability under the SJV’s hot climate. However, the adoption of alternative varieties might largely depend on marketing and consumers’ acceptance.

Canopy Management
Canopy management, e.g., shoot thinning and leafing, is applied to provide enough light exposure and air circulation on fruit-zone without exposing the clusters to too much direct sunlight. Hand or mechanical leafing (Figure 2) can be applied only on the “morning” side of the canopy to avoid the afternoon sunlight exposure on fruit-zone.

Figure 2. Mechanical leafing at “morning” side of the canopy during bloom

Irrigation Management
Irrigation management might be the most critical and powerful tool for growers and the appropriate irrigation scheduling can be applied to avoid excessive heat damage/water stress as well as berry sunburn. Severe deficit irrigation should be avoided before the heat wave occurs to make sure vines with no or minimal water stress under the extreme daily temperature. Soil moisture sensor, pressure chamber, or basically by feel and appearance can help growers to assess soil moisture and vine water status, or growers can simply follow the grape evapotranspiration (ET) report (https://ucanr.edu/sites/viticulture-fresno/Irrigation_Scheduling/) to decide the amount of irrigation per week to avoid severe grapevine water stress during the heat wave.

Figure 3. Shade cloth on fruit-zone at “afternoon” side of the canopy

Canopy Shading
Canopy shading including shade cloth (Figure 3) and sun protectant, e.g., Kaolin and CaCO3 (Figure 4), can be used to shade the canopy and fruit to avoid excessive light exposure and sunburn. Cost and timing might be the most important factors when growers decide to use shade cloth and sun protectant. Generally, the optimum timing to apply canopy shading is after berry set or several days before the heat wave.

Figure 4. Sun protectant of CaCO3 foliar spray during veraison

Canopy cooling can also be applied by in-canopy misting. Studies in Australia have found by in-canopy misting it can cool canopy and clusters by several degrees, and ultimately improve yield and berry composition during the heat wave (https://www.wineaustralia.com/research/search/completed-projects/ua-1502).

Integrated Approach
Finally, it might require to take the integrated approach by using more than one mentioned strategies to maximize the production and berry quality during the heat wave. Growers should consult local farm advisors and conduct the small trials to evaluate the effectiveness of different approaches under the local condition.

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