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Cover Crops in California Agriculture: An Overview of Current Research

By
Shulamit Shroder
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Almond Orchard Cover Crop study in Kern County, March 2019 (photo by S Shroder.)

Growers throughout the country and around the world plant a wide range of cover crops for a variety of reasons. Cover crops can reduce soil compaction, improve water infiltration, improve soil structure, and feed soil microbes: they encourage a healthier and more diverse soil ecosystem.

Researchers in California are analyzing the best ways to incorporate cover cropping into the state’s diverse agricultural systems, from high-value vegetable production on the central coast to the cotton, tomato, and almond fields of the central valley.

 

Cover Crops on the Central Coast

Researchers working with central coast vegetable growers have devised innovative ways to use cover crops to reduce nitrate leaching and agricultural runoff, thereby improving both local ecosystems and soil health.

Eric Brennan and his team at the USDA Agricultural Research Service started the Salinas Organic Cropping Systems trial in the Salinas Valley in 2003 to understand the long-term impacts of various cropping systems and soil amendments. This trial focuses on organic lettuce and broccoli, two of the high-value crops grown in the area known as the nation’s salad bowl.

To maintain soil organic matter and provide nutrients to their crops, organic vegetable growers in this area prefer applying compost instead of planting cover crops. The amount of time that cover crops require for incorporation and decomposition can shorten the growing season for these high-value crops (Brennan & Boyd, 2012.) To make this practice more feasible for growers in the area, this group of researchers has developed three strategies for integrating cover crops into the vegetable cropping systems of the Central Coast.

Option 1: Plant the cover crops only in furrow bottoms, not the entire field. After 50 to 60 days of growth, the grower can spray the cover crops and then do the usual tillage necessary to prepare the ground for planting the cash crops. By planting time, the cover crop residue has already decomposed. This method reduces runoff and erosion but does not reduce nitrate leaching, so this is best for fields with runoff problems but without high nitrate levels. However, this method makes controlling weeds during a wet winter difficult and costs more than simply leaving the field bare (Brennan, 2017.)

Option 2: Plant non-legume cover crops on the vegetable beds and mow the cover crops repeatedly throughout the growing season. This maximizes nitrate scavenging while minimizing the amount of residue that needs to decompose right before planting. The ideal cover crop for this practice would be a grass, like cereal rye. Repeated mowing would reduce the amount of water lost to evapotranspiration from the cover crop but still enable the rye to scavenge nutrients that could otherwise be lost to leaching (Brennan, 2017.)

Option 3: Turn the cover crop residues into a highly nutritious juice and compost. To do this practice, a grower would plant a non-leguminous cover crop in October and allow it to grow until mid-December, at which point it will have scavenged most of the nitrogen that it will use. The grower then harvests the cover crop, leaving as little residue behind as possible. They can then feed the residue into a screw press, which will separate the liquids and solids. The liquid component has a relatively low nitrogen concentration and can be applied to the vegetable crop to fulfill some of the crop’s nutrient needs. The solid residues can be composted and applied at a convenient time, to provide organic matter to the soil (Brennan, 2017.)

Researchers are still working on refining these strategies, but they could allow central coast vegetable growers to reap the rewards associated with cover crops while maintaining a profitable enterprise.

Field day at the West Side REC in 2010, discussing cover cropping and conservation tillage (photo courtesy Jeff Mitchell, UCCE.)

 

Annual Systems in the Central Valley

For the past 20 years, Jeff Mitchell and his team at UC Cooperative Extension have studied the effects of reduced tillage and cover crops on a tomato-cotton rotation at the UC’s West Side Research and Extension Center. This study measures the efficacy of these practices in reducing air pollution and increasing soil organic matter. Reduced tillage and cover cropping have resulted in less dust emissions compared to conventionally managed fields (Mitchell et al., 2017.) They found that cover cropping increased soil organic matter more than conservation tillage alone did (Veenstra et al., 2006.) Overall, these practices have improved soil health by increasing aggregate stability, water infiltration, and soil organic matter while maintaining similar yields to the conventional system (Mitchell et al., 2017.) This study has allowed researchers to see the long-term effects of conservation tillage and cover cropping on tomato and cotton systems in the San Joaquin Valley.

Another UC research team in the Central Valley, led by Kate Scow at the Russell Ranch near UC Davis, examined the long-term effects of cover cropping on organic tomatoes and corn. These researchers found that cover cropping encouraged the proliferation of diverse types of beneficial fungi known as arbuscular mycorrhizal fungi (Bender & Bowles, 2018). Under optimal environmental conditions, cover cropping was correlated with higher tomato yields. In contrast, corn did not enjoy the same benefits from organic management that the tomatoes did and had lower yields compared to fields without cover crops (Bender & Bowles, 2018). These studies have found important benefits to including cover crops in annual systems, but growers will need to further refine the practice to fit their needs.

 

Perennial Systems in the Central Valley

Amélie Gaudin and her team from UC Davis and UC Cooperative Extension are quantifying and communicating the benefits and tradeoffs of planting winter cover crops in almond orchards. They established trials throughout the Central Valley. Planting cover crops in almonds increases bee forage, improves soil health, and encourages resiliency. The researchers have found that cover crops resulted in increased water infiltration. Despite the common concern that cover crops would increase frost risk, they found that cover cropping did not affect ambient air temperatures 3 and 5 feet above the ground. Moreover, the ground cover worked as a buffer, keeping temperatures more stable than bare ground did (Gaudin, 2020.)

Other benefits included a decrease in sodicity, improved trafficability in the wintertime, and an increase in aggregation. The soil microbial ecosystem showed increased biomass. Bees enjoyed a more diverse, varied diet, contributing to better bee health. Finally, cover crops reduced weed diversity and growth. They did not reduce germination since both the cover crops and the weeds emerged at the same time. All these benefits start to outweigh the costs of implementation after about 10 years (Gaudin, 2020). Many of these soil and ecosystem benefits are not unique to almond orchards, and could also benefit other perennial cropping systems in the Central Valley.

Mustard cover crops in a table grape vineyard, March 2020 (photo by S. Shroder.)

 

Funding Options

UC and USDA researchers have found benefits to cover cropping in diverse agricultural systems throughout California, from almond orchards to lettuce and tomato fields. These include reducing erosion, compaction, and nutrient leaching, along with improving soil aggregation and providing habitat for beneficial insects. Cover crops may improve the soils upon which your crops depend and increase your operation’s resiliency in the face of a changing climate.

The California Department of Food and Agriculture’s Healthy Soils Program and the USDA NRCS EQIP provide incentives for planting cover crops. Check out cdfa.ca.gov/oefi/healthysoils/IncentivesProgram to learn more about the CDFA’s program. There are 10 technical assistance providers working throughout the state who can help you select your cover crop species, apply for the program, and implement your practices. Go to ciwr.ucanr.edu/Programs/ClimateSmartAg to find your closest climate smart specialist.

Community Education Specialist Alli Fish and a daikon radish cover crop in December 2019 (photo by Rose Hayden-Smith.)

 

Works Cited

(2010). [Field day at West Side Research and Extension Center] [Photograph]. California Agriculture. http://calag.ucanr.edu/Archive/?article=ca.v070n02p53

Bender, S.F & Bowles, T.M. (2018). Effects of AMF diversity and community composition on nutrient cycling as shaped by long-term agricultural management. Russell Ranch 2018 Annual Report. https://asi.ucdavis.edu/sites/g/files/dgvnsk5751/files/inline-files/RRSAF%20Progress%20Report_2018.pdf

Brennan, E. B. (2017). Can we grow organic or conventional vegetables sustainably without cover crops? HortTechnology, 27(2), 151-161.

Brennan, E. B., & Boyd, N. S. (2012). Winter cover crop seeding rate and variety affects during eight years of organic vegetables: I. Cover crop biomass production. Agronomy Journal, 104(3), 684-698.

Gaudin, A. (2020, February 4). What do cover crops have to offer? [PowerPoint slides]. University of California Agriculture and Natural Resources. https://ucanr.edu/sites/calasa/files/319850.pdf

Mitchell, J. P., Shrestha, A., Mathesius, K., Scow, K. M., Southard, R. J., Haney, R. L., … & Horwath, W. R. (2017). Cover cropping and no-tillage improve soil health in an arid irrigated cropping system in California’s San Joaquin Valley, USA. Soil and Tillage Research, 165, 325-335.

Veenstra, J., Horwath, W., Mitchell, J., & Munk, D. (2006). Conservation tillage and cover cropping influence soil properties in San Joaquin Valley cotton-tomato crop. California Agriculture, 60(3), 146-153.

Lettuce Dieback: New Virus Found to be Associated with Soilborne Disease in Lettuce

By
William M. Wintermantel
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Figure 1. Romaine lettuce plant near maturity showing classic symptoms of outer leaf yellowing and necrosis. Symptoms may develop at any growth stage (all photos courtesy W.M. Wintermantel.)

Lettuce dieback is a soilborne virus disease known to cause significant losses for lettuce production throughout all western growing regions. The disease was originally described in the Salinas Valley in the late 1990s following severe flooding along the Salinas River but has now been found throughout coastal and inland lettuce production regions of California, the winter production region in southwestern Arizona and Imperial Valley, California.

The disease is most prevalent on romaine lettuce but is known to occur on all non-crisphead (iceberg) lettuce types. Most modern crisphead lettuces are resistant, and an increasing number of romaine cultivars now carry resistance as well. Symptoms of lettuce dieback include yellowing and necrosis of outer leaves, stunted growth and death of affected plants (Fig. 1). Plants infected young may fail to develop beyond the 8 to 10 leaf stage, but symptoms can develop at any point in the growing season, and fields often exhibit a range of plant sizes with some plants appearing healthy and maturing normally, while others become stunted and never fully develop (Fig 2).

Figure 2. Romaine lettuce plants in a field showing variation in severity typical of lettuce dieback including stunted growth, as well as yellowing and necrosis of outer leaves.

Initial symptoms begin with yellowing and necrosis (death) of small veins in outer leaves, with the necrosis expanding into larger areas within and between veins. Inner leaves of the head usually retain their color, but some romaine varieties may also exhibit bright chlorotic flecks within veins of leaves at the center of the head that resembles tiny stars. These are most visible when affected leaves are held up to a light source (Figure 3).

Figure 3. Romaine lettuce leaf from the inner portion of a head showing star-shaped chlorotic flecking in veins characteristic of lettuce dieback disease on romaine.

This vein-flecking symptom is not always present on infected romaine, but when observed it is an excellent diagnostic indicator. The vein flecking symptom is less common on other types of lettuce and is more difficult to observe on red lettuce. Losses resulting from lettuce dieback can range from a few plants to complete loss of crop. In most severely affected fields lettuce heads are not harvested because the plants will not meet quality standards. Symptoms of the disease are frequently found in low lying areas with poor drainage, in areas near rivers, on recently flooded land, and in areas where soil has been dredged from a river or ditch and spread onto adjacent fields.

Symptoms of lettuce dieback can be mistaken for those of other diseases, particularly lettuce drop, a disease caused by a fungus, and symptoms of two viruses transmitted by thrips. It is fairly easy to differentiate lettuce drop from lettuce dieback because lettuce drop, caused by fungi in the genus Sclerotinia, results in a soft rot, outer leaves often flatten against the ground, and heads easily separate from the root, whereas with lettuce dieback the root remains firmly attached to the head. The two thrips-transmitted viruses, impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV), also cause necrotic (dead) patches on leaves of infected lettuce plants that resemble symptoms of lettuce dieback, and therefore it can be difficult to differentiate the two diseases. Diagnostic tests can be used to differentiate lettuce plants infected with these viruses from those with lettuce dieback disease. Serological detection methods including commercially available immunostrips that can be used in the field to determine infection with INSV or TSWV, but immunostrips are not available for the viruses associated with lettuce dieback disease. Therefore, confirmation of lettuce dieback requires laboratory testing, which can include both molecular biology and serological methods. In some cases, lettuce plants may be infected by multiple pathogens simultaneously and this may complicate diagnosis.

Lettuce dieback is probably a very old disease of crisphead (iceberg) lettuce that disappeared for many years before reemerging with a new name as a disease of other lettuce types. In the 1930s a disease known as brown blight devastated lettuce production in California with symptoms that closely resembled those of lettuce dieback based on descriptions and illustrations at the time.

Iceberg lettuce was the main type of lettuce grown in the 1930s, and it suffered severe losses from brown blight for many years until a source of resistance was identified by a USDA scientist, Ivan Jagger. This source of resistance was eventually bred into all subsequent iceberg lettuce types, beginning with the variety Imperial, and this eliminated the threat from brown blight. In the early 2000s, after the appearance of lettuce dieback, USDA scientists identified a source of resistance to lettuce dieback from the crisphead lettuce variety Salinas, and through genetic studies found that the source of resistance to lettuce dieback is also present in the brown blight-resistant lettuces developed by Jagger over 70 years earlier, but was not in earlier susceptible lettuce varieties. In other words, only crisphead lettuce varieties that predate the variety Imperial could develop symptoms of lettuce dieback. This suggests the two diseases may actually be the same. The resistance to lettuce dieback has been incorporated into several romaine lettuce varieties, as well as some leaf and butter lettuce varieties, but there remain many lettuces that are susceptible to lettuce dieback disease.

Since the late 1990s, lettuce dieback has been believed to be caused by infection of lettuce plants with either of two viruses from the genus Tombusvirus; tomato bushy stunt virus (TBSV) and Moroccan pepper virus (MPV). These viruses are absent from healthy lettuce but have been found regularly in association with lettuce dieback disease. However, there have been numerous situations in which neither virus was found in association with obvious disease symptoms. Furthermore, it has not been possible to consistently and easily reproduce disease symptoms when lettuce is inoculated with either virus in a laboratory setting, raising the possibility that an additional virus may contribute to causing lettuce dieback disease.

In an attempt to identify a possible additional virus contributing to lettuce dieback disease, high throughput sequencing (HTS) was used on several lettuce plants exhibiting dieback symptoms, which led to the identification of a new virus consistently associated with diseased plants but not with healthy lettuce plants. This novel virus was most closely related to a recently identified and poorly characterized virus from watermelon in China, watermelon crinkle leaf associated virus, which was found using the same HTS approach.

The newly identified lettuce virus, tentatively named lettuce dieback associated virus (LDaV) shares an extremely low genetic relationship with the watermelon virus, which suggests that although the two viruses are related, they are very distantly related to one another. Using a combination of HTS and traditional DNA sequencing the genome of the new virus, LDaV, was assembled and methods were developed to allow rapid detection of the virus from lettuce leaf extracts using RT-PCR, a routine laboratory diagnostic method. LDaV has now been found not only in lettuce showing dieback symptoms collected recently, but it has also been found in older archived samples of lettuce nucleic acid collected from plants showing dieback symptoms over the past 20 years, including many that also contained MPV or TBSV. To date, LDaV has not been found in healthy lettuce plants. Interestingly, genetic comparison showed that LDaV isolates collected from coastal California production regions are closely related to one another, and desert isolates from Arizona and Imperial Valley, California also are closely related to one another. However, coastal and desert isolates differ genetically from one another, suggesting perhaps some regional adaptation of the virus to plants grown under the different climatic conditions.

Further research will clarify the role of LDaV in lettuce dieback disease and how it relates to the two tombusviruses, MPV and TBSV, that have long been linked to the disease. Studies to date, however, strongly suggest a role for LDaV in lettuce dieback disease development, and research is in progress to clarify the ability of LDaV to produce lettuce dieback symptoms when inoculated to lettuce plants, as well as whether or not the new virus can infect lettuce plants carrying a gene for resistance to lettuce dieback.

Choosing Activator Spray Adjuvants for Permanent Crops

By
Franz Niederholzer and Rhonda Smith
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Choosing the right activator adjuvant can avoid phytotoxicity damage or losses from excess spreading and pesticide runoff from the target plant.

Agricultural spray adjuvants are materials added to the spray tank when loading the sprayer. They include products classified as activator adjuvants and marketed as wetters/spreaders, stickers, humectants, and/or penetrators. Activator adjuvants are marketed to improve the performance of pesticides and foliar fertilizers.

Activator adjuvants can have a place in tree (and vine) crop sprays, but matching the material to the job can be tricky. A bad match can lead to minor or major losses to the grower. Minor losses can result from excess spreading and pesticide runoff from the target plant. Phytotoxicity can cause major damage.

This article describes ingredients and functions of activator adjuvants commonly sprayed on tree and vine crops. Suggestions regarding activator adjuvant selection are offered. Growers must make their own activator adjuvant use decisions based on experience, particular needs, and risk tolerance.

 

Should You Use an Adjuvant?

Read and follow the specific instructions on the label. If the pesticide or foliar fertilizer label indicates the product should be used with certain types or brand of adjuvant(s), that’s what you need to use. For example, the Bravo Weather Stik® label cautions against using certain specific adjuvants and puts the responsibility in the PCA or grower court regarding adjuvant use. If the label includes phrases such as “use of an adjuvant may improve results” or “complete coverage is needed for best results” then you may want to look into selecting and using an appropriate activator adjuvant.

Before proceeding with use of an activator adjuvant, first look at your existing spray program. Are you already doing the best spray job you can? Good spray coverage begins with proper sprayer calibration and set up. Is your sprayer calibration dialed in for different stages of canopy development? Optimum sprayer set up—gallons of spray per acre, ground speed, fan output, and nozzle selection/arrangement—changes from dormant to bloom to early growing season to preharvest sprays. Adjusting your sprayer to best match orchard and vineyard conditions at each general stage in canopy development is the foundation of an effective, efficient spray program. An activator adjuvant will not make up for excessive tractor speed, poor nozzle arrangement and/or worn nozzles. Your money is best spent first dialing in your sprayer(s) for the whole season, before considering an extra material in the tank (that is not required on the label).

If you have your sprayer(s) dialed in for each orchard and stage of growth, now is the time to say “OK, I want to think about a little extra boost to my spray job.”

 

Which Activator Adjuvant to Choose?

First, know the properties of the pesticide you will use. Does it work on the plant surface or inside the plant? This is a key point in selecting adjuvants. Here is a quick review of the main classifications and characteristics of activator adjuvants as they currently appear in the field. Note: Certain products can provide more than one adjuvant property that can be beneficial in the field. For example, non-ionic surfactants can work as surfactants and penetrators, depending on use rate.

Wetters/spreaders: These materials contain surfactants that decrease the contact angle and increase the spreading of the spray droplet on the target. High rates of wetters/spreaders may also increase penetration of pesticide into the target tissue (leaves or fruit), potentially causing phytotoxicity. Excessive spreading of pesticide spray solution and runoff from the target may result when using a new or higher rate of spreader—especially when using silicon “super-spreaders”. Test new combinations of spreader material(s) and spray volume before regular use. Spray volume per acre or adjuvant use rate will probably have to be reduced if a labeled rate of adjuvant provides excessive spreading.

To check for excessive spreading, place a length of black plastic sheeting under several trees or vines in a row. Secure the plastic with spikes, wire staples, and/or weights. Spray the new adjuvant and pesticide combination using your current sprayer set up. Reenter the field right after spraying, wearing appropriate PPE, and evaluate coverage. If material is pooling at the lower portion of leaves and/or fruit, excessive spreading is occurring. Check to see if pooling is occurring only in a certain area(s) of the canopy or throughout the canopy. If more spray solution is landing on the black plastic tarp under the trees/vines than between them, then runoff is occurring. [Some ground deposit should be expected from standard airblast sprayer use.]

Compare the results of your adjuvant test with a similar application of your current pesticide/adjuvant combination on another portion of the row. If there is no pooling or runoff with the new adjuvant in the tank, you can use the adjuvant with confidence. A lack of pooling or run off with the new adjuvant also might mean that your old sprayer setup and tank mix didn’t deliver adequate coverage.

If the test with the new adjuvant showed pooling on leaves and/or runoff on the ground, you have several choices: 1) You can reduce spray volume per acre by replacing some or all nozzles with smaller nozzle sizes on the sprayer in an effort to reduce overspreading. If you saw overspreading on some portions of the canopy, but not others, reduce nozzle size only on the part of the spray boom that targets the over-sprayed part of the canopy. Recheck spray coverage if nozzling changes were made. 2) Reduce the adjuvant rate and recheck coverage/spreading. 3) You can just go back to your established program without the new adjuvant.

What’s the “best” course of action? That depends on your farming operation. Reducing spray volume per acre means more ground covered per full spray tank – a potential time and cost savings. If spraying is done during the heat of the day in hot, dry climate, spray water evaporation is a major issue and it may be best to keep the higher spray volume and reduce the spreader rate or eliminate it entirely. Checking coverage and overspreading allows you to make the best decision possible, avoid damage and, hopefully, save money. All farming operations are different. Make the choice that best fits your farm.

Stickers: These adjuvants can increase the retention time of the pesticide on the leaf and reduce rain wash off. They may limit movement of systemic pesticides into the plant, and are probably most beneficial when used with protectant materials (cover sprays). Do you overhead irrigate? Is there rain on the horizon? If you answer yes to either one of these questions, you may benefit from using a sticker.

Humectants: Under low humidity conditions humectants can help reduce spray droplet evaporation before and after deposition on the plant. This is especially valuable when small droplets and/or materials that must be absorbed into the plant (systemic pesticides, PGRs, nutrients, etc.) are used in the summer under high temperature and low relative humidity conditions.

Penetrators: Frequently used with herbicides, these products include oils (petroleum, vegetable, or modified vegetable oils) and non-ionic surfactants used at higher rates. In crop sprays, penetrators can be used to increase absorption of systemic pesticides (for example, oil with Agri-Mek) as well as translaminar materials. Penetrator adjuvants should be used with caution or avoided entirely with surface active pesticides such as cover sprays or else phyto may result. Finally, some penetrators can increase the rain-fastness of some pesticides.

 

What Adjuvant Material to Choose?

Use a product intended for crop spraying. Many activator adjuvants were developed and intended for use with herbicides. Products that are advertised for use with plant growth regulators should have a higher chance of crop safety compared with those that don’t. This is still no guarantee of a phyto-free application.

Ask for help from the adjuvant manufacturer’s sales rep if needed. How much do they know about the particular activator adjuvant in the spray mix you are planning?

 

Will the Adjuvant Work?

If you choose to use an adjuvant that is not specifically listed on the pesticide or foliar fertilizer label, jar test the planned spray solution first. Use the same spray water source. Include all leaf feeds, other adjuvants, and pesticide(s) that you plan to put in the spray tank. Do this before tank mixing these materials.

A lot of time and money rides on effective pesticide application. Do your homework before the spray tank is filled and you will be well on your way to solid results.

Soil Solarization and other Weed Control Options in Strawberries

By
Cecilia Parsons
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strawberries
Mulches provide natural weed control in most strawberry growing regions.

Weed competition can have a serious impact on growth and productivity in commercial strawberry production. The diversity of climates among California’s growing regions requires different weed management options. The high-value of this crop, long-season and the fact that it is hand harvested twice weekly requires an effective weed control program.

Steve Fennimore, UC Cooperative Extension weed specialist, based in Salinas, said by far the most common tools used to suppress weeds in conventionally grown strawberry fields post planting are opaque mulches, pre-plant fumigation, hand weeding and herbicides.

Mulches are dark color films that restrict light and shade out weeds. In southern coastal areas, clear plastic is used to warm the ground and achieve an earlier harvest, but clear films can also promote weed growth. Fumigants and herbicides are used in conjunction with the clear film to control weeds.

In addition to pre-plant fumigation, placing clear film on the strawberry bed tops promote plant growth and opaque film on the sides of the beds is another weed control option. UC IPM Guidelines recommend securing opaque mulches to the soil prior to transplanting strawberry plants. Slits are made in the mulch at desired spacing. The smallest hole possible to insert plants will help minimize weed growth and seed deposition.

Of the common weeds in strawberry production, yellow nutsedge is one that cannot be controlled by using an opaque mulch as the shoots can puncture the mulch and grow through it. This is a perennial weed that grows from tubers capable of survival in the soil for three years.

Soil solarization is another technique for weed suppression in organic strawberry production. This strategy works where warm daytime temperatures are sustained, but is not effective in cooler coastal regions. This strategy, pre-plant, involves covering the soil with clear plastic and wetting the soil to field capacity. University of California research showed that temperatures of 108 to 131 degrees F could be reached in the top two inches of the soil. The plastic is left on four to six weeks. Solarization to kill weed seeds is most effective in the two 12 inches of soil. While solarization works for weed control, the heat generally does not penetrate deep enough to kill pathogens deeper in the soil.

Soil treatment with steam has also been effective in killing weeds, but lethal temperatures have to be reached in soil to kill weeds with propagules are present and it requires specialized equipment.

Anaerobic soil disinfestation can reduce the numbers of many annual weeds but it has limited efficacy on perennial weeds.

Preventing Herbicide Damage in Young Almond Trees

By
Cecilia Parsons
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Applying a mix of latex paint and water to young almond tree trunks has been the standard protection practice against herbicide damage. Research done at Nickels Soil Lab in Arbuckle last year showed that a hardening off period prior to herbicide applications provided better protection.

Former UCCE orchards advisor Dani Lightle and UC Davis graduate student Drew Wolter initiated the trial in 2019 and Wolter is continuing the study this year. Wolter said he was surprised to see the level of protection that a hardening off period provided compared to the paint application.

Weeds competing with young almond trees for water and nutrients can impact the long-term production of the orchard. Standard practice has been to apply white latex paint to the lower two to three feet of the trunk prior to applying herbicides. The young, tender bark of the trees that have been covered by a carton since planting is especially vulnerable to systemic and contact herbicides.

The trial used low and high rates of glyphosate and glufosinate or a mix of the two for weed control. The herbicides were applied in second leaf almond orchards with different levels of trunk protection including unpainted trunks which had been hardened-off for nine weeks, fresh paint, old paint and cartons.

Results from the trial showed that tree stress caused by trunk applied systemic and contact herbicides was lowest in the cartoned and unpainted treatments where tree trunks were allowed to harden off for nine weeks. Wolter observed that hardening of the green and tender tree bark after the protective carton is removed provided the best protection from herbicides. When the newly exposed bark was sprayed with an herbicide, Wolter said, the result was tree stress with wilted canopies and tissue damage. In the trial, trees treated with top of label rate tank mix of glyphosate and glufosinate had 4 to 22 percent greater trunk damage than trees that were not painted.

Wolter said the most effective trunk protection option for young almonds is to use a carton. When it is removed, waiting nine weeks before applying an herbicide is recommended. A practical strategy for controlling weeds and protecting young trees is to apply the herbicide early in the spring before the protective carton is removed. If necessary, a second application of herbicide can be made nine weeks after carton removal. Hardening off is crucial, Wolter said, and is best done in late spring or early summer.

Monitor for New Fungal Pathogen in Citrus

By
Cecilia Parsons
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(figures courtesy Greg Douhan, UCCE.)

A new fungal pathogen that causes twig and shoot dieback and woody cankers in citrus can be found in Central California orchards and citrus orchardists should be on the lookout for symptoms in early summer months. Symptoms are caused by the fungal species Colletotrichum, which is a well-known pathogen in citrus. This new species of Colletotrichum, C. karsti, is distinct from C. gloeosporioides, which causes the post harvest disease anthracnose.  Recent field surveys in the Central Valley found both species in citrus.

According to UC researchers Dr. Akif Eskalen and Dr. Florent Trouillas, the most characteristic symptoms of this disease are the gum pockets which appear on young shoots either alone or in clusters and the dieback of twigs and shoots.

Figure 2. Pathogenicity of Colletotrichum spp. on ‘4B’ clementine after 15 months. Vertical lines represent standard error of the mean.

These symptoms were reported in clementine, mandarin and navel orange varieties. Pathogenicity tests on clementine mandarin also confirmed that C. karstii is a more aggressive pathogen of citrus in California compared to C. gloeosporioides. Research continues to focus on understanding the biology of the fungal pathogens and factors that influence the disease expression with hopes of developing management strategies. There is no chemical control as the pathogen survives in the cambium layer. Pruning out infected wood and disposing of it outside the orchard can reduce the inoculum level in an orchard.

Greg Douhan, UCCE area citrus advisor for Tulare, Fresno and Madera counties, said the infections, first noticed in the valley in 2013, has been seen in many citrus varieties. Infections can be minor, with a dieback of a single shoot on a tree or significant with woody cankers and multiple shoots affected. All age trees can become infected.

Figure 1. Symptoms of Colletotrichum Dieback. A) Shoot dieback symptoms on Clementine, B) Gumming symptoms on an infested shoot. C) Branch dieback symptoms on Clementine. D) Wood discoloration and canker on the wood.

Outbreaks of this disease in citrus trees is likely due to environmental factors, Douhan said. Outbreaks may be noticeable one year and not seen the next. Relative humidity and precipitation in California citrus growing regions are known to play a role in Colletotrichum infections. Conidia are dispersed by rain and humidity is conducive to spread of the pathogen. A spore trap study of the Colletotrichum species showed higher frequency during wet months, but there are additional avenues to infections. Wounding is also known to be an opening for infection. Wind and sand damage also give the species an opportunity to colonize citrus. Douhan said the Central Valley’s drier climate is likely a limiting factor to spread of the disease.

Time PGRs Right to Enhance Grape Color

By
Cecilia Parsons
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Grapes
PGRs provide color enhancement and other benefits to ripening grape crops.

Uniform and timely red table color development is a crucial component of grape quality. California’s table grape growing regions have climate conditions that favor productivity and other aspects of fruit quality, but hot summer temperatures can inhibit coloring. For this reason, many growers use certain plant growth regulators that can promote faster and more uniform color.

Different plant growth regulators are used in table grape production. Those used to increase berry size may also suppress coloration. Other PGRs are applied to bring color.

Matthew Fidelibus, UC Davis viticulture specialist, said the most common active ingredients in products sold to promote color in table grapes are ethephon and S-abscisic acid. Most table grape growers use some type of PGR to improve crop quality.

Degradation of ethephon releases ethylene, a plant hormone that promotes pigment accumulation in grapes. Timing and application rate for ethephon are important for optimizing quality. Late or excessive applications can result in soft berries, which decreases shelf life and opportunity to export.

Ethrel® is a trademarked ethephon product that has been marketed by Bayer to promote early uniform color development in red table grape varieties. Treatment is advised when 5 to 30 percent of berries show color. Rates will vary depending on site temperatures. Higher than recommended rates can cause fruit to soften. There is also a 14-day harvest interval after an application and MRLs (maximum residue levels) apply.

Vineyards under stress due to insect damage or under irrigation should not be treated with ethephon. Absorption by plant tissues is influenced by temperature, humidity and pH of plant surfaces. This PGR must be applied at a rate sufficient to wet vines and fruit clusters uniformly.

Following label instructions for PGRs is important, Fidelibus added, to achieve efficacy and also due to post harvest intervals with ethephon.

S-abscisic acid (S-ABA) is the active ingredient in ProTone®, a trademarked product marketed by Valent. S-ABA is a naturally occurring PGR that is involved in many processes of plant maturation including senescence. This product is exempt from post-harvest interval restrictions.

S-ABA is involved in many major processes during plant growth and development including dormancy, germination, bud break, flowering, fruit set, general growth and development, stress tolerance, ripening, maturation, and senescence.

While grapes can normally produce enough of this PGR for color development, high temperatures can hinder production. The ProTone product can be used to supplement the naturally occurring S-abscisic acid to achieve faster and more complete coloring.

New Adjuvant Technology Can Double the Potential of Insecticides This Summer

By
Cecilia Parsons
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Growers looking for different ways to maximize the effectiveness of each insecticide spray in the upcoming months have a new tool in the form of an adjuvant. Using technology that reduces evaporation and keeps tank mix droplets on the target in a liquid state twice as long as a typical surfactant, OMRI listed Ampersand® adjuvant gets more of your tank mix to the leaf and keeps it there longer, giving your insecticide time to perform its function. Learn more at www.attuneag.com.

Benefits of Being a Certified Crop Advisor in the Western United States

By
Jerome Pier
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CCAs have a deep knowledge of soil and water science and crop nutrition on which growers in California and Arizona have come to rely (photo by Marni Katz.)

California and Arizona grow produce that feeds the United States and the world. The diversity and value of the products grown in the West require a high level of technical expertise. The intensity of specialty agriculture must be balanced with concern for the environment to ensure sustainable crop production for generations to come. For these reasons, and more, it has never been a better time to add the Certified Crop Advisor (CCA) credential to the Pest Control Advisor license. The two credentials are complimentary.

The Pest Control Advisor (PCA) program consists of thousands of individuals who are licensed to make recommendations of restricted use pesticides in California and Arizona. The PCA program is a great career choice for individuals who want to make a living in agriculture. PCAs who want to provide the highest level of service to their growers should consider becoming a Certified Crop Advisor. There is nothing more important to a crop advisor than their reputation for making their growers successful. The deep knowledge of soil and water science and crop nutrition required to become a CCA means a grower is working with the most well-rounded crop advisor in the industry.

PCAs must study and display knowledge of integrated pest management (IPM) to receive their license. Integrated pest management emphasizes a holistic approach to controlling insects, weeds and diseases, relying on pesticides only when good farming practices can no longer contain the pest. However, the overall performance objectives for the PCA license are focused on laws and regulations pertaining to pesticides and don’t capture the breadth and complexity of agronomic practices outside of pesticide use. CCAs have the knowledge and experience to put IPM into practice. Growers know that good pesticide recommendations prevent loss of productivity but adding a balanced irrigation and nutrition program can result in gains in yield, quality and return on investment.

 

Keeping Up with Changing Times

CCAs must pass two challenging exams to obtain certification. The international exam tests the applicant’s general knowledge of soil and water science, nutrient management, crop production, and pest management. The state exam is more specific to management of irrigated specialty crops common across California and Arizona. Students in agricultural colleges who are interested in becoming CCAs should speak with their advisors about developing an appropriate curriculum that will help the candidate pass the exams. The West Region CCA Board has a program to subsidize the registration fees for the CCA exams for students. Check the West Region Certified Crop Advisors (WRCCA.org/exams) web site for more information. Candidates must have a Bachelor of Science in an agronomic field of study and two years of experience before they obtain certification. CCAs must stay up to date on current best agronomic practices by obtaining 40 continuing education hours every two years. When you are working with a PCA who also carries a CCA, you are working with the best.

Farming practices are constantly changing to meet new challenges. Over the 20 years, I have worked as an agronomist in California, trees and vines have replaced field crop acres and drip and micro-sprinklers have replaced flood and furrow irrigation. Growers switched from applying heavy doses of nitrogen fertilizer alone to balanced blends with lower total nitrogen applications, and they realized higher yields. Ironically, as growers’ efficiency has improved, so has increased scrutiny of nitrate pollution of ground and surface waters. In order to sustain the rich bounty of California agriculture into the future, documentation was needed to demonstrate that growers’ nitrogen fertilizer management practices were not contributing to ground water pollution.

The California State Water Resources Control Board (SWRCB) added ground water to the Irrigated Lands Regulatory Program (ILRP) in 2012. Soon thereafter, farmer coalitions formed within watersheds to begin the process of collecting data on nitrogen management practices. It was apparent that many technically qualified agronomists were needed to accurately complete the nitrogen plans the coalitions would present to the SWRCB. Clearly, CCAs were the most qualified service providers when it came to nutrient management. Nitrogen management plans require soil testing, knowledge of a grower’s fertility plan, yield forecasts and final harvest totals. CCAs have been proven to be one of the most trusted sources of information by growers in surveys across the United States. CCAs were a perfect fit to help build the data required to manage nitrogen on a watershed scale. Thanks to language in the ILRP, the West Region CCA program has grown since 2014 to have the largest number of CCAs.

A CCA understands how to use soil, water and plant tissue analysis to develop a balanced plant nutrition program that meets crop needs (photo by M. Katz.)

But being a CCA has benefits well beyond completing nitrogen management plans. Retail sales companies know that their business depends on strong fertilizer sales. A CCA understands how to use soil, water and plant tissue analysis to develop a balanced plant nutrition program that meets crop needs. Custom nutrient programs benefit the customer as they only spend money on necessary nutrients and maximize return on investment. Ag businesses benefit, in turn, as profitable farmers can pay their bills. Custom nutrition benefits the environment by applying the right fertilizer at the right time, place and rate to reduce waste.

Optimizing Soil, Water and Nutrients

Pesticide recommendations are made within a narrow regulatory framework and don’t allow for much creativity; one must follow the label. Fertilizer programs, on the other hand, can be very satisfying to create as there are many options and challenges to consider. In addition, technical expertise in managing water quality and soil salinity will be critical for the future as marginal lands and water reuse become more important for food production. Watching a healthy crop yield a bountiful harvest while knowing you played a key role is a very satisfying experience. Many of the most successful salespeople carry both the PCA and CCA as they can provide whole farm solutions that improve a grower’s bottom line.

CCAS provide whole farm solutions that improve a grower’s bottom line

Farmers in the western states face many challenges. As their operations increase in size and complexity, they have come to rely on the expertise that a PCA/CCA offers. They can trust that pests are being dealt with and their fertility programs are based on sound agronomic principles that will bring the most profitable production at the end of the season. Farmers also know CCAs are collecting data to help them demonstrate to regulators they are using the most conservative practices possible to prevent ground and surface water pollution. The sustainable future of the West’s farming depends on CCAs.

If you are already a CCA, we thank you for your membership. If you would like to become more involved in the WRCCA, there are regional committees that are looking for new members. Check wrcca.org for more information on regional committees. If you are not a CCA but are interested in the program, refer to both certifiedcropadvisor.org and wrcca.org for information on exams, performance objectives and many other topics related to the program. The Agronomy Society is making it easier to take exams by switching to remote proctoring. There are also many more opportunities to get continuing education hours on-line. Stay tuned for more articles from WRCCA Board members in the months ahead.

Early Season Vineyard Management

By
George Zhuang
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Handleafing on Petite Sirah (all photos courtesy G. Zhuang.)

Early season vineyard management is critical for several reasons. First, the grapevine microclimate and fungal disease severity at early season largely affect the yield, e.g., yield loss and fruit unmarketability due to fungal disease, and fruit quality, e.g., Brix and color for red varieties; second, some missteps in early season vineyard practices might hinder the following year’s success if they affect bud fruitfulness; finally, optimal early management might save you money on pest/disease management.

The season starts at budbreak and once buds start to push, the clock begins clicking. Although several vineyard practices during grapevine dormancy might also be important, such as pruning and fungicide application (e.g., lime sulfur), I will focus on post-budbreak vineyard management in this article.

The most important steps during early season vineyard management include:

  1. Irrigation
  2. Nutrition
  3. Pest/disease
  4. Canopy management
  5. Crop management

The objectives of early season vineyard management are simple and straightforward: To sustain yield with desired fruit quality at harvest with low disease/pest pressure. Irrigation, grapevine nutrition, pest/disease pressure, canopy management, and crop level all play in the formula to decide the timing and severity of vineyard practices at the early growing stage.

 

Irrigation

According to retired UCCE Viticulture Specialist Dr. Larry Williams, in a year with normal winter rainfall, a typical San Joaquin Valley (SJV) vineyard might not require irrigation until bloom, which might be approximately late April or early May in the SJV depending on weather, site, variety, and other farming practices. However, the timing of first irrigation can vary dramatically based on the winter and spring precipitation. Too much irrigation or precipitation at an early canopy development stage can promote rapid shoot growth and create a large and dense canopy which increases shading and relative humidity (RH) inside the canopy favoring fungal diseases, such as phomopsis, botrytis, and powdery mildew.

Canopy management such as shoot thinning, leafing and cane trimming can alleviate some negative effects from excessive vigor or dense canopy. However, canopy management alone might not offer the complete solution if the excessive growth resulted from too much precipitation or irrigation. Understanding when and how much to irrigate is beyond the scope of this article. However, some basic tools, such as visual assessment and soil moisture meter, can generally serve the purpose of deciding when to irrigate. Other irrigation scheduling tools, e.g., crop evapotranspiration (ETc), can also offer information on how much to irrigate on a daily or weekly basis. Managing irrigation and adjusting the canopy accordingly can optimize yield and fruit quality, along with the effectiveness of fungicide/pesticide programs, and that will improve your profit and reduce your fungicide/pesticide costs.

 

Nutrition

Managing grapevine nutrition serves two purposes: First, to make sure there are no nutrient deficiencies that could limit yield level or fruit quality; second, to make sure there are no excessive or even detrimental levels of nutrients that could also lead to reduced yield potential and fruit quality, unnecessary expense, and unwanted effects on the environment. Among grape nutrients, N has the most impact on vine vigor and canopy growth. Excessive N either from fertilizer application or irrigation water can promote excessive canopy growth causing shading and high RH inside of the canopy. As a result, excessive shading can impact the fruit-zone microclimate, and create high RH, favoring fungal disease, which will reduce fruit quality/marketability and basal bud fruitfulness for the following year’s crop.

Opening the canopy by shoot thinning, leafing and cane trimming can increase the exposure of basal buds and clusters and improve spray coverage and air circulation. However, like irrigation, canopy management might not be enough to correct the negative impact resulting from excessive N status, if N is left unchecked, an oversupply of N will promote the canopy growth to diminish any benefit from canopy management.

Growers should conduct a visual assessment and consider laboratory results, and rely on historical records, like yield and pest/disease conditions, to adjust the grapevine nutrition program. Among all the measures, bloom petiole or leaf blade tests are recommended to take a snapshot of early vine nutrient status that will give growers enough time to adjust the fertilizer program accordingly. Grapevine bloom petiole critical values are published in Table 1, above. Be cautious with N critical value. The N critical value was solely established on data based on Thompson Seedless with own root. Growers should judge the grapevine N status with additional information, e.g., vine general health, vigor and yield.

 

Canopy Management

As I discussed previously, typical grapevine canopy management includes shoot thinning, shoot positioning, leafing and cane trimming. Based on the trellis type, growers might not need to apply all of them. Most common practices in the SJV are shoot thinning, leafing, and canopy trimming and all of them can be performed mechanically.

Shoot thinning is typically conducted when shoots are 8 to 10 inches; the objectives are to reduce shoot density and improve light exposure inside of the canopy as well as reduce the crop level. In the SJV, few growers adopted this practice due to the potential yield loss. However, shoot thinning can be beneficial when the vines are young with excessive crop or vines that have an excessive number of fruitful buds following mechanical pruning (see Figure 1, above). Shoot thinning regulates the crop load to avoid the negative impact of overcropping on berry ripening and potential carryover effect on the following year’s crop. Many researchers have shown the benefits of shoot thinning and a few have demonstrated the feasibility of mechanical shoot thinning (Geller and Kurtural 2012). However, the benefit of shoot thinning might gradually diminish during the season if the irrigation is unchecked since the canopy could recover and refill the gaps when water is abundant.

Leafing aims to increase light exposure on clusters and basal buds to improve the fruit quality and bud fruitfulness as well as improve spray coverage and lower disease pressure (Figure 2, above). Both timing and severity of leafing are critical to achieve success. Leafing after veraison typically has no or negative effect on fruit quality, especially in the SJV. Leafing around berry set is commonly recommended to improve the color of red grape varieties, and studies show better results from mechanical leafing in comparison to hand leafing. Recently, several studies including a couple in Fresno and Madera, have proven pre-bloom or bloom mechanical leafing might offer the most benefits in comparison to classical berry set leafing. Compared to berry set leafing, bloom leafing offers more or similar fruit quality benefits with less cost by eliminating the need for shoot positioning prior to leafing, since most shoots are vertically positioned at bloom (Figure 3 and 4, below).

In cool climates and less productive vine systems, pre-bloom or bloom leafing might reduce berry set and ultimately decrease yield (Achimovic. et al. 2016). However, in our study and other studies in the SJV (Cook et al. 2015), no effect on berry set and yield has been observed, and the effect on berry set and yield from leafing prior to bloom may largely depend on growing conditions and severity of leaf area reduction from leafing.

Cane trimming is used to open the canopy for light exposure and increase air circulation in order to reduce RH and fungal disease pressure when the canopy is excessive and dense. However, severe canopy trimming might result in significant loss of leaf area that can delay the berry ripening by reducing the photosynthetic productivity (Figure 5, below). Severe canopy trimming might also over-expose the cluster and cause sunburn before harvest. The goal of canopy trimming is to effectively open the canopy without severely reducing functional leaves and over-exposing the fruit.

Figure 5. Canopy trimming too close to the cordon damages the canes and leaves which delays ripening and over-exposes clusters.

In conclusion, canopy management should be integrated with water and nutrient management as part of early season vineyard practices paying attention to pest/disease management, growing conditions (e.g., climate, soil condition, and irrigation water availability and quality) and production goals to achieve the maximum production efficiency with low disease and pest pressure.

 

Reference:

Geller, J. and Kurtural, K. 2012. Mechanical Canopy and Crop-Load Management of Pinot gris in a Warm Climate. American Journal of Enology and Viticuture. 64: 65-73.

Acimovic, D., Tozzini, L., Green, A., Sivilotti, P., and Sabbatini, P. 2016. Identification of a defoliation severity threshold for changing fruitset, bunch morphology and fruit composition in Pinot Noir. Austalian Journal of Grape and Wine Research. https://doi.org/10.1111/ajgw.12235.

Cook, M., Zhang, Y., Nelson, C., Gambetta, G., Kennedy, J., and Kurtural, K. 2015. Anthocyanin Composition of Merlot is Ameliorated by Light Microclimate and Irrigation in Central California. American Journal of Enology and Viticulture. 66: 266-278

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