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Evaluation of Grafted Tomato Plants for California Fresh Market Production Systems


Why graft?
Grafting involves joining a fruit-producing shoot (called the ‘scion’) of a desirable cultivar onto the disease-resistant rootstock of another cultivar.  For example, let’s say you normally grow the cultivar ‘QualiT-47’ for fruit production, but that cultivar is susceptible to a soilborne disease problem in your fields, then you could graft the top part of a ‘QualiT-47’ seedling onto the root-portion of a more disease-resistant cultivar. In the case of tomato rootstocks, the majority of the cultivars are interspecific hybrids between cultivated tomato (Solanum lycopersicum) and wild tomato species (most commonly Solanum habrochaites, or less often S. peruvianum or S. cheesmaniae). Solanum habrochaites is known from other published research to be tolerant of salinity, drought, cold temperatures, and resistant to many soilborne diseases and many of these benefits have been demonstrated to be conferred to the grafted plant when an interspecific hybrid rootstock is used.

Most of us are familiar with grafting as a standard practice for California fruit and nut trees and grapevines, but it has experienced only limited commercial adoption among annual crops in California thus far. Grafted tomato transplants are commonly utilized in the commercial greenhouse industry, where tomatoes are produced under protected culture and are generally grown over a much longer production cycle, often a 10-month period. There are greenhouse producers in Southern California, but it is more common in British Columbia, Ontario, Mexico and other US states (Arizona and others).  In many countries in Latin America, Europe and Asia, grafted plants represent a large percentage of the tomato industry. For example, in Spain, 50 to 70 million grafted plants are grown annually for greenhouse production systems. There has also been some adoption of grafting by high-tunnel tomato growers in the eastern United States. In Southern California, the nursery Plug Connection is marketing grafted tomatoes to home gardeners, dubbing them as a “Mighty ‘Mato”. This allows a home gardener to grow an heirloom tomato variety, which often has little or no disease resistance, without worrying about rotation or other soilborne disease control measures.

Our goal was to evaluate the potential for grafting standard tomato cultivars onto rootstock cultivars that possess resistance to soilborne diseases and nematodes. Our primary objective was to evaluate the yield performance of grafted plants in replicated trials in commercial fresh market (“mature green”) production fields in the northern San Joaquin Valley. Our team consisted of myself, Scott Stoddard with University of California Cooperative Extension (UCCE) in Merced County, and Michael Grieneisen and Minghua Zhang in the Department of Land, Air and Water Resources at the University of California, Davis. This project produced the first publicly-available research results on grafted tomatoes for California production systems.

How is it done?
For each tray of grafted tomatoes to be produced, two trays of seed are sown; one tray of the rootstock seed and another tray of the scion seed. At approximately one month after sowing, the young seedlings are grafted. Both seedlings are cut at the hypocotyl, and the scion shoot is spliced onto the rootstock stump. The method we used is a commonly used splice-graft with small, soft, silicone clips to hold the scion and rootstock together during healing. Grafted transplants cost more than non-grafted transplants due to increased seed costs and the labor required to do the grafting. Thus far, grafted tomato plants are only available from a few sources in California, and we won’t know what the cost for grafted tomato transplants will be until they are being produced in larger volumes here. The use of fully- or semi-automated grafting robots is emerging as a way to reduce labor costs and improve the survival rate of grafted plants. This of course requires significant capital investment. About 20 seed companies offer tomato rootstock seeds (see list at http://www.vegetablegrafting.org/tomato-rootstock-table/). However, for a nursery facility or grower considering doing their own grafting, building a healing chamber may be a hurdle. There is research underway by others to look at conducting the one-week healing period inside the greenhouse. For more information on the logistics of grafting on a commercial scale, please see the Vegetable Grafting Manual, the link for which is provided at the end of this article under “More information”.

Figure 2. Production of splice-grafted tomato plants for our field trials.

Field Trials in the Northern San Joaquin Valley
The trials were conducted in commercial production fields at six locations over three years from 2016 to 2018; three locations in San Joaquin County and another three locations in Merced County. The treatments included all combinations of the scions and rootstocks listed in Table 2.

Table 2. Scion and rootstock cultivars used in our field trials. *Note: Galilea is a roma/saladette type, while the other seven cultivars are all round types; all but Dixie Red were developed for the Western U.S. mature green production system.

The plots were laid out in a randomized complete-block design with four replicate blocks, each block measuring approximately 80 by 40 feet. The cooperating growers managed the experimental plots similarly to the rest of their field with respect to pest control, fertilization, irrigation, and other management practices. Plants were mechanically transplanted into prepared beds at a 4- to 5-inch depth per normal practice; the graft union ended up well below the soil surface. In staked or trellised production systems in other regions, the graft union is typically kept above ground to realize the full benefit of the rootstock pathogen resistance. With graft union buried below the soil surface, soilborne pathogens may attack the scion crown tissues or adventitious roots arising from the scion.  Due to the lack of significant pathogen pressure in our fields, we believe this was not an issue for these trials.

In our trials, grafted plants were more vigorous and had better foliage cover of fruit at harvest than non-grafted plants of the same cultivar. We also measured NDVI (Normalized Difference Vegetation Index, a measure of the “greenness” or how much of the bed is covered with actively photosynthesizing foliage) and it was also slightly higher in grafted plots. Averaged across all six trials, marketable yield increased only 12 percent when grafting with ‘Maxifort’ or ‘DRO138TX’ as the rootstock, although the results were better in some individual trials. At the San Joaquin County sites, yields of non-grafted vines were similar to the statewide average yield and grafting increased yield significantly (25 to 40 percent depending on the year). Some scion-rootstock combinations were as much as 68 percent higher than the non-grafted plants of the same scion (e.g. ‘QualiT-27’ on ‘Maxifort’ at the San Joaquin site in 2018). At the Merced sites, yields of non-grafted vines were well above-average and grafting was much less beneficial. Many published field trials indicate that the yield advantages of grafted plants are greatest under sub-optimal growing conditions. Field sites with heavy soilborne disease pressure, or abiotic stresses may be the best candidates to see improvements with grafting.

Fruit Size and Quality
Many published studies have found that grafted plants produce a higher percentage of fruit in larger size classes than those produced by the non-grafted scion varieties. Averaged over all our trials, the differences in fruit size distribution between grafted and non-grafted were fairly small. In some trials, however, plants on vigorous rootstocks did have larger fruit. Some published studies provide measures of fruit quality, such as dissolved sugars, pH, total dissolved solids, vitamin C, lycopene, or even “taste-test” data. Those studies indicate that the quality of fruit from grafted plants seems to be slightly inferior to fruit from the non-grafted plants, though still commercially acceptable. Our field trials focused on yields, and we did not measure any fruit quality data. However, we did not notice any fruit defect problems in grafted vines. Also, in 2018 we did cut open both red and mature green fruit at harvest to make sure that there were no problems inside the fruit.

Variability From Trial to Trial or Field to Field
A study in Florida with determinant type cultivars has shown yield increases of 25 to 42 percent using certain rootstocks, but year-to-year variability also increased as compared to non-grafted plants. This variation underscores the importance of considering variable outcomes to determine the feasibility of grafted tomatoes here. Some fields will likely benefit more from grafting than others, and this may not always be predictable in advance.

Costs of field establishment are increased significantly with grafting. Materials costs for transplanting (seed plus nursery costs) alone might be $2,000 per acre or higher or more than with conventional transplants. However, we don’t yet really know what the costs might be if this were adopted commercially in California, so our plant costs are based on small volume sales prices. If we assume a cost of $0.40 per grafted plant, then a yield increase of 19 percent at a market price of $6.55 per 25-pound box would pay for the increased plant cost.

On-going and Future Work
Other research projects looking at grafting tomatoes are being conducted in California. A United States Department of Agriculture (USDA)-funded project with processing tomatoes is underway with collaboration of Gene Miyao, UCCE Yolo, Solano and Sacramento counties, Zheng Wang, UCCE Stanislaus County and myself, in addition to proprietary research being conducted by the industry. Rootstocks for heirloom tomato production are being evaluated by Margaret Lloyd, small farms advisor with UCCE in Yolo, Solano and Sacramento counties.

The California Department of Pesticide Regulation provided partial funding for this project but does not necessarily agree with any opinions expressed, nor endorse any commercial product or trade name mentioned. In addition, this project was supported by the Specialty Crop Block Grant Program at the U.S. Department of Agriculture through Grant 14-SCBGP-CA-0006. The contents of this report are solely the responsibility of the authors and do not necessarily represent the official views of the USDA. We also thank our grower-cooperators (Live Oak Farms and Pacific Triple E), Growers Transplanting Inc. for producing grafted plants, and the following companies that supplied the seeds: Monsanto/De Ruiter Seeds, Gowan Seed Company, Harris Moran Seed Company, and Syngenta Vegetable Seeds.

For more information:
Additional information on our field trials:

Detailed information on how to undertake vegetable grafting is available at: http://www.vegetablegrafting.org/resources/grafting-manual/

List of tomato rootstocks including disease resistances and where to order seed: http://www.vegetablegrafting.org/resources/rootstock-tables/solanaceous-rootstock-table/

Walnut Husk Fly Management


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.

Figure 1. Life cycle of walnut husk fly.
Photos courtesy of University of California Statewide Integrated Pest Management Program

Susceptible Varieties
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 English walnut 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.

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.

Photo 1. Male (left) and female (right) walnut husk fly adults.

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 mortality 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.

Photo 2. Female walnut husk fly with eggs.

WHF Management Guidelines
In spite of some of these challenges for WHF management, guidelines regarding treatment timing 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.

Photo 3. Walnut husk fly sting.

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 hung 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 1). After females are identified, gently squishing the female abdomen will squeeze out eggs if they are present (Photo 2). 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 3) 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 spray was 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.

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.

ACP Control With Systemic Insecticides


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 holes.

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.

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.

1) Bio control. 2) Foliar application. 3) Inspecting leaves.

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.

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.

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.

4) Lab research. 5) Foliar application. 6) Tamarixia.

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.

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

Grapevine Heat Stress and Sunburn Management


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.

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.

Grapevine Trunk Diseases: Current Management Strategies


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).

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.

Managing Citrus Thrips is Especially Difficult During Drought Years


(originally March/April 2017)

Citrus thrips is a common pest of California citrus, attacking leaves and the calyx end of newly forming fruit, when the epidermal cells are quite sensitive.  In leaves, this causes distortion of the leaves and light lines of scarring

Thermal Infrared Sensors for Postharvest Deficit Irrigation of Peach


(originally published January/February 2017)

California has been in a historic drought and the lack of water has been a major problem for agriculture especially for crops that depend on irrigation. Deficit irrigation may be used in some cropping systems as a potential water saving strategy (Goldhamer et al., 1999). The term “Deficit Irrigation” simply means irrigating at less than the full amount required by crop evapotranspiration needs. For fruiting trees such as peaches, because fruit yield and quality at harvest may not be sensitive to water stress at some developmental stages such as during the non-fruit bearing postharvest season, there is an interest in applying deficit irrigation strategies. Deficit irrigation has not been widely used due partially to the lack of effective and fast methods of monitoring plant water stress in near real-time and determining associated risks of applying deficit irrigation. When crops are managed under deficit irrigation, the margin of error in timing and amount of water application becomes smaller before causing yield losses. Monitoring the soil and plant water status is more critical for reducing risks of a crop failure or permanent damage to the trees.  However, current established techniques of monitoring the soil and plant water status such as neutron probe readings of soil water profile and pressure chamber measurements of stem water potential are labor intensive, and lack the timeliness needed for irrigation scheduling purposes.

New Efforts on Fusarium Wilt of Lettuce Brings Disease Front and Center


(originally published September/October 2016)

Shortly after hiring on as the inaugural executive director of the University of Arizona’s new Yuma Center of Excellence for Desert Agriculture, a public-private partnership devoted to applied agricultural research needed by the desert agriculture industry, Paul Brierley asked his stakeholders what they would like the Center to address first.  The answer was resounding:  Help us mitigate plant diseases!  And not just any plant disease – help us with the seemingly impossible-to-eradicate Fusarium wilt of lettuce.  And thus began the Center’s odyssey against the insidious disease that is wiping out entire fields during warm-season production of iceberg lettuce – costing the industry millions of dollars.

Tomato Spotted Wilt Virus Management in Tomatoes


By: Thomas A. Turini, University of California Agriculture and Natural Resources, Vegetable  

Tomato spotted wilt virus is a thrips-transmitted virus that can infect many crops and weeds. In California’s Central Valley, in an important processing tomato production area, this virus disease may cause substantial economic damage. The most recognizable symptoms include fruit with oval protruding oval deformities or irregular concentric ring color patterns and this virus can kill shoots and plants, so both quality and yield are affected. The host range of this virus includes many common crops and weeds and likely survives the winter on a few weed or crop plants, but quickly amplifies on tomatoes in spring. Therefore, risk increases during the season. The virus is transmitted by thrips; primarily Western Flower Thrips, Frankliniella occidentalis in the Central San Joaquin Valley. The vector must feed on an infected plant as a nymph to be capable of transmitting the virus as an adult. Risk of loss due to TSWV can be reduced but management in high risk situations is going to depend upon several tactics.

Grapevine Red Blotch or Leafroll Disease


By: Larry Bettiga, Viticulture Farm Advisor University of California Cooperative Extension Monterey, San Benito and Santa Cruz Counties

(originally published May/June 2018)

Grapevine leafroll and red blotch disease are two virus-associated diseases that should be on the radar of all grape growers. The following article will hopefully provide you an update on these virus diseases based on our current knowledge. Summer surveying of vineyards for visual leaf symptoms is a great time to assess vineyard blocks for the presence of disease.