Grape vine trunk diseases affect productivity and shorten life of a vineyard.

Akif Eskalen, UCCE Specialist and plant pathologist in the Department of Plant Pathology at UC Davis, in a presentation for Sustainable Winegrowing, explained his research into naturally occurring microorganisms for use as biocontrol against fungal pathogens that cause grape vine trunk disease.

Grapevine trunk diseases are prevalent in mature vineyards. The disease complex includes Eutypa, Esca, Botryospaeria and Phomopsis diebacks, but Eskalen said that at least 60 different fungal species have been identified in grapevines.

Delayed pruning and pruning wound protectants are two prevention routes identified in research, but a survey revealed that the majority of growers use neither preventative practice.

Antagonistic microorganisms already live in the plant tissue, Eskalen said, but they may become depleted. He and his research team are studying how to deliver the beneficial microorganisms back to plants, both in the nursery and in established vineyards.

Eskalen said there is evidence that these beneficial microorganisms not only increase the host plant’s defense mechanism, but also improve the health of the plant and potentially increase yield. Inside their host plant, the naturally occurring beneficial microbes secrete secondary metabolites that inhibit the growth of the harmful fungal organisms. Eskalen said they have identified several of the beneficial microbes and are now focusing on methods of delivery back into host plants.

Biological control can be an important tool in controlling grapevine trunk diseases, Eskalen said. The fungal pathogens that cause diseases each have a different mode to enter a plant and cause disease. No single fungicide can prevent all of those pathogens.

He also pointed out that naturally occurring microbes might be lost over time in vineyards where no other plants exist. Cover crops in vineyards foster more diverse microbe populations. Those populations will differ due to climate, soil type and other environmental factors. Eskalen said his research team is sampling vineyards in different areas of California to get a ‘big picture’ of beneficial microbe populations.

With this information, cultural practices can be adopted to encourage growth of the beneficial microorganisms. Increasing their levels in a plant, Eskalen said, will not only help with disease defense, but also improve overall health and yields.

Mass production and delivery of beneficial organisms is the goal of this research. Trials are introducing the microbes into cuttings in the nursery prior to grafting. In mature vineyards, vine injection and soil application are under study.

UC Riverside scientists are working on breeding new varieties of citrus that will be resistant to citrus greening disease.

Citrus greening disease or Huanglongbing (HLB) is a bacterial disease that has killed citrus orchards worldwide. It has been detected in California citrus, but not in commercial citrus production. The disease is vectored by Asian citrus psyllid.

UCR researchers believe a sustainable solution to preserving the citrus industry is to develop varieties that carry natural resistance to HLB. The UCR research team, including Dr. Chandrika Ramadugu, has been awarded funding by the National Institute of Food and Agriculture to pursue breeding work. The research team includes collaborators from Texas A&M, University of Florida, Washington State University and USDA.

Resistant varieties of citrus have been identified, but the challenge is to use them to generate hybrids that will have the flavor consumers prefer along with resistance. The plan is to generate many hybrids and screen them for suitability for the citrus marketplace.

The Australian finger lime is one of six micro citrus varieties from Australia and is being used to create the hybrid varieties. It carries resistance to HLB, but few other attributes to fit the citrus market. The finger limes are about three inches long and roughly the size of an average person’s index finger, but fruit from juvenile trees can be less than one inch long.

The UCR team is currently studying the genetic makeup of the hybrids that have already been produced. Analyzing the new plants’ DNA will show if they carry enough disease resistance along with marketable qualities. Dr. Ramadugu said currently most of the hybrids have lime- or lemon-like fruits. The research team is still in the process of breeding other types of citrus.

One of the main challenges in this process is the length of time it takes before the hybrid citrus varieties bear fruit. With the help of UCR plant cell biology professor Sean Cutler, the team is hoping to accelerate the time it takes for the hybrid plants to bear fruit in the greenhouse. Clones of the best plants will be grown in Florida and Texas field trials.

Other approaches in the HLB fight at UCR include altering soil and root bacteria to improve plant immunity, and using an antibacterial peptide to clear HLB from an infected tree.

Work has begun to determine if whole-orchard recycling (WOR) can be as successful in walnut orchards as it has been in almonds.

In a UCCE Virtual Walnut Series, UCCE orchard systems advisor Luke Milliron detailed WOR efforts in two walnut orchard trials.

Whole-orchard recycling involves tree removal and chipping, then spreading the wood chips over the orchard footprint and incorporating them into the soil. Burning restrictions and loss of cogeneration plants that would pay for wood chips spurred research in WOR over the past 10 years. UCCE farm advisor Brent Holtz has been studying WOR in almonds, and his research and field trials show both soil and tree benefits in replanted orchards.

Documented benefits include increased soil organic matter and carbon, increased soil nutrients and increased soil microbial diversity. There was no evidence of increased replant disease and no interference in pre-plant fumigation. There were also water use related improvements and increased orchard productivity.

Milliron shared that while his first trial, which begun in 2018, had some challenges with chip spreading, results of soil and leaf analysis were encouraging. Root lesion nematode levels in the soil were low, indicating successful fumigation. Leaf analysis showed no differences in nitrogen. Potassium and boron levels were slightly higher in trees grown on chipped ground. There were no growth differences between second leaf trees in chipped and non-chipped ground.

Due to the challenges of this trial, Milliron said the total tonnage of chips incorporated could not be determined.

A second WOR walnut trial in collaboration with Cliff Beumel at Agrimillora California is underway.

In this recent trial, Milliron said the dry chip tonnage came out to 91 tons per acre or 136 tons wet. In almond, chips are typically spread about two inches thick. In the walnut trial, the chips were spread at closer to three to four inches, and Milliron said they tried to put chips back at the same rate that trees were removed.

One of the “carrots” for growers is inclusion of WOR in CDFA’s Healthy Soils program designed to promote carbon sequestration and reduce greenhouse gas.

Some of the qualifications in the CDFA program include: trees must be at least ten years of age; orchards must be chipped and incorporated in place; chips must be evenly distributed throughout the orchard and incorporated into the soil to at least six inches in depth.

A list of whole orchard recycling providers can be found at orchardrecycling.ucdavis.edu/california-orchard-recycling-resources.

Keeping orchard floors bare is standard practice in California citrus production, but a Placer and Nevada County study on mulching mandarin trees is breaking ground for alternative management.

As the study enters its fifth year, UCCE farm advisor Cindy Fake noted some of the key findings. Mulched orchards have lower soil temperatures during the hot summer months and soil moisture is rarely depleted. Excessive fruit drop is mitigated and herbicide use has declined.

Parts of this foothill mandarin growing region east of Sacramento have much poorer soils than other growing regions. There are also hillside plantings where erosion is an issue, Fake said. Summer heat can be intense, and the trend toward longer dry conditions adds stress to the trees. Frost damage at the elevation of this growing region is rare and use of microsprinklers provides frost protection.

The mulching trials were initiated to help mandarin growers maintain and improve tree health and resilience in the face of climate change.

A downside of the mulch applications is the time and labor it takes to mix and apply.

Growers participating in the trials applied mulch composed of 50% horse manure and 50% wood chips, as both materials were readily available in the area. Mulch was applied under the canopies of the trees at depths ranging from four to six inches. The depth was reduced to four inches after two very wet years. Mulch was applied in the spring, Fake said, and it is critical to apply to moist soil. If it is applied to dry soil, it will take three to four major irrigations to move water through the mulch into the soil.

The trials also showed that when applying mulch to an existing orchard, soil moisture must be monitored. Growers used to applying the normal amount of water to their trees will have to adjust their scheduling, Fake said, as about 30% less water is needed.

The trial showed that moisture levels under the mulched tree were consistent and the profile is able to retain moisture with minimal depletion at a 6-inch to 12-inch depth. Most moisture depletion is occurring in the top six inches of soil and is lower than the control. Moisture levels in the control depleted at a more rapid rate throughout the soil to a depth of 12 inches.

By applying mulch in the spring, Fake said it would decompose over the next six months and not be a food safety issue at harvest. She also recommends mulching new trees to help maintain moisture for root growth.

More information on the mulch trial can be found at

Placer/Nevada Foothill Farming website. There is also an information sheet on how to use mulch on the UC ANR website.

All walnut varieties are susceptible to damage from walnut husk fly (WHF). WHF are distinguished from other fruit fly varieties by their yellow coloring at the base of their wings and by the dark triangular band on their wing tips. Size is similar.

Damage to developing walnuts occurs after the female husk fly lays eggs inside the walnut husk. Feeding by larvae results in a soft and dark husk inside.

UCCE IPM advisor Jhalendra Rijal presented information on the biology, monitoring and control of this walnut pest for the West Coast Nut walnut conference.

Rijal said that WHF damage appears different than sunburn damage on walnut husks. Sunburn will cause a dry appearance, typically in the sun-exposed side of the nut, while nuts infested by WHF will appear soft and moist at the feeding site. Larval feeding inside the hull causes staining on shells and makes husks difficult to remove after harvest. Early season infestation can cause shriveled kernels.

WHF produce one generation per year. Adults emerge from soil in the summer months. Female WHF will feed on some sugar and nitrogen sources out in the orchard including aphids or bird feces for a week or so prior to laying eggs. Larvae hatch from eggs, feed on the husk and drop to the ground to pupate under the soil over winter.

Rijal said monitoring of adult WHF emergence is important to time insecticide applications for control. Yellow sticky traps with the attractant ammonium carbonate should be hung on the north side of the tree as high as possible and shaded by foliage. Check these traps two to three times per week to help to determine if WHF is emerging and laying eggs.

Rijal advised using a lens to identify female WHF in the traps. They are larger than the males and have light-colored segments on legs. Crushing their abdomen will reveal if they are carrying ‘rice-grain’ looking eggs.

Insecticide applications may be needed as soon as the first female WHF with eggs is found, depending on the orchard damage history, Rijal said.

The control materials target adult WHF and spray applications should be made before large numbers of egg-carrying females are found. The most effective control insecticides can be found at on the UC IPM web site.

Multiple applications may be necessary to cover the entire susceptible period as adult flies emerge over two to three months in the summer.

Rijal said research is needed to determine if cultural control (disking orchard soils to break life cycle) is possible or use of potential biocontrol agents such as insect pathogenic fungi or nematodes is needed to kill the larvae or pupae when they are in the soil. There is not natural biological control known for WHF.

Mechanical pruning operations continue to increase in San Joaquin Valley wine grape vineyards as a way to save on labor costs. Dormant pruning, suckering and leaf removal in vineyards can all be done mechanically.

George Zhuang, UCCE viticulture farm advisor in Fresno County, in a presentation at the San Joaquin Valley virtual Grape Symposium, said existing trellis systems in vineyards can be converted to mechanical pruning systems and retain production and fruit quality.

The most common trellis systems in San Joaquin Valley wine grape vineyards have been two-wire bilateral cordon, “California sprawl,” and quadrilateral, and both can be adapted to mechanized pruning, Zhuang said. The single high wire system is the standard system for mechanization.

Recent trials have looked at differences in cordon height and how that affects vineyard production. Transitioning existing vineyards to a single high wire system has been most successful for mechanized pruning operations. This trellis system has a single high wire at 62 to 66 inches in height, is single canopy, non-shoot positioned and has around 35% exposed leaf area. Production is at 18 to 24 months and yields are 11 to 24 tons per acre with 7×10-foot spacing.

Zhuang said the other trellis system in use in vineyards where mechanized pruning is done is the quadrilateral. This system has a divided canopy and a higher percentage of exposed leaf area. Production depends on variety and spacing of vines.

Setting up the box size is important in mechanized pruning. The spur height sets the height of the bearing surface. Precision pruning is four inches, while 6- and 8-inch set ups may require some hand pruning to keep from overloading the vines as the bearing surface increases.

Two Fresno county trials were done to compare single wire height in trellis systems for winegrapes. Vines were planted in 2017 and hand pruned the first year of production in 2019. In 2020, vines were mechanically pruned. Heights tested were 68-inch cordon and 52-inch, the classic “California Sprawl” height. First yields were comparable in both systems. Sugar (measured by Brix) was increased in the high wire system due to more leaf area, Zhuang said. Water use was lower in the higher wire system.

He said the trial would continue to determine if there are significant differences as the vines mature.

Availability of mechanical pruning custom operators may be a limiting factor going forward. The machinery is a significant investment more suitable for larger acreages. Smaller growers would need to hire a custom operator. Zhuang said he has seen efforts by winegrape growers to design and build their own pruning machinery.

Lesions on garlic cloves, which affect crop quality, have been associated with several species of Fusarium, said Tom Turini, UCCE vegetable crops farm advisor. Similar garlic diseases caused by this soilborne fungus were reported in other production areas, but recent studies investigating this issue in Central California until now have not been conducted.

The Fresno County 2019 crop report noted that 24,180 acres of garlic were harvested that year, making garlic one of the higher acreage vegetable crops in the county. In addition, the 2018-19 CDFA Agricultural Statistics Review documents that more than 80% of the garlic produced in the state was in Fresno County based on gross crop value.

Turini is working with UC Davis plant pathology specialist Cassandra Swett, who is receiving samples collected from fields and those submitted by growers and processors. In 2020, Swett ran tests to identify the Fusarium species associated with the clove lesions. The work planned for this growing season should provide a robust set of detailed information regarding the identity of the fungus responsible for the crop damage.

Swett reported that at least four species of garlic bulb rots have been recovered from samples submitted. Fusarium proliferatum has been the most common, being recovered from all storage facilities and farms. F. oxysporum and F. falciforme are also common, present in about 50% of farms or facilities. These three species have been confirmed to be capable of causing a clove rot in preliminary trials.

1. proliferatum is the only species found occurring alone, suggesting it might be the primary driver of bulb rot. A fourth species, F. brachygibbosum, was also found and is being tested for pathogenicity. Neither this species nor F. falciforme have previously been described as garlic pathogens. Swett said that with multiple species causing garlic bulb rot, it will be critical to identify strategies that can co-manage all species. Understanding when and how infections are occurring can help with identifying critical target periods for control.

Garlic bulb rots are not a new issue in garlic production, Turini said. Fusarium was previously associated with garlic rot in the Fresno production area and the fungus was considered ubiquitous. Current observations are that the issue is more common, and the character of the symptoms is not identical to those described decades ago. The work in progress will provide insight into what fungi are involved in the disease and other details critical in taking reasonable steps to mitigate the damage.

Walnut varieties with higher chill requirements during winter dormancy may be facing production challenges in the future. Climate prediction models are showing the amount of winter chill needed by trees may not be achievable every year.

UCCE Orchard Systems Advisor Katherine Jarvis-Shean said walnut varieties, including Chandler, might need some help to achieve a robust leaf-out in the spring. Research is ongoing to find the best strategies for material applications that help walnut trees overcome warmer winters.

Growers need to become familiar with the UC Davis Fruits and Nuts research on chill portions in preparation for warmer winters in the future, Jarvis-Shean said. Chill hour calculations differ from the newer chill portions models. Information on chill hours and portions can be found here.

Climate prediction models show that the amount of chill needed by walnut trees may not be achievable every year. A regional look at climate prediction models show more warming than cooling. There will still be variability, Jarvis-Shean said, but there will be more low-chill winters and more winters with lower chill than what is now experienced. By mid-century, the models are predicting less chill in the southern San Joaquin Valley than is necessary for the Chandler variety.

Research done over the last 30 years shows that dormancy is influenced by a number of factors, including hormones, transport capacity, oxidative stress and metabolism. Budbreak in the spring is normally preceded by a big upswing in starch production. Warm winters cause trees to lower their starch production and keep sugar levels stable. After trees make this adjustment, Jarvis-Shean said, they need more heat than normal in the spring to achieve high starch pre-budbreak.

Effects of a low-chill winter on walnut trees become apparent at leaf-out. Some buds don’t break, resulting in fewer flowers. A wider maturity window means there will be size variability in nuts. Early setting nuts are larger, taking up the available carbohydrates, and leaving later-maturing nuts much smaller.

Dormancy-breaking chemicals that can stimulate the hormone, transport capacity, oxidative stress and metabolism systems are under study to determine if their use can compensate for lower winter chill. Joint UC ANR/UC Davis studies are looking at applications of hydrogen cyanamide, also known as Dormex, a nitrogen cocktail and hormone analogue, over a couple of winters to understand how trees respond to treatments and to achieve consistent results.

Invasive roof rats are causing damage in citrus orchards throughout California.

Kern and Tulare counties are sites of roof rat infestations in citrus as well as Southern California citrus growing regions. Once a population of roof rats becomes established, they can be found throughout an orchard, girdling tree branches and damaging fruit.

Roger Baldwin, UCCE wildlife specialist, said a multi-year study is underway to develop management strategies for roof rats, which are causing significant damage in a number of tree nut and fruit crops.

Roof rats are active year-round, building nests in citrus trees or burrows near the base of the trees. They forage away from their burrows, and signs of activity are sometimes difficult to see. Their burrows are about 1.5 to 2 inches in diameter, distinguishing them from ground squirrels.

Baldwin said roof rat activity may vary depending on the variety of citrus. In Meyer lemon, for example, he said rats have been found to eat the rind and leave the fruit. Extensive girdling of tree branches has also been observed in Lisbon lemon, but little branch girdling and more extensive fruit damage in navel orange. In navels, rats chew holes in the rind, eat the fruit and leave a shell behind. Another sign of a rat infestation, in orchards with snail populations, is piles of empty snail shells.

Baldwin said the UC study would be focused on cost-effective methods of control, including rodenticides and traps. He said the new A24 trap on the market, which use lures and CO2 cartridges triggered by rat activity, may save labor costs. However, these traps have not been tested in ag fields, so their efficacy is currently unknown. Rodenticides are another alternative, but restrictions may limit use. Diphacinone grain can be used, but bait stations are more effective when placed in trees rather than on the ground. Bait is also less likely to be eaten by non-target animals when placed up in the tree.

UC researchers plan to study rat movement patterns in orchards, which will help them test efficacy of management tools. Growers interested in participating in the study should contact UC research associate Ryan Meinerz at rmeinerz@ucdavis.edu for more information. More information on controlling roof rats can be found at UC ANR publication 8513, Managing Roof Rats and Deer Mice in Nut and Fruit Orchards.

The ‘delicate’ issue of reporting branched broomrape infested tomato fields continues in California.

Infestations of this parasitic weed in tomato fields can severely decrease yields because the weed attaches to tomato plant roots and extracts water and nutrients.

Gene Miyao, retired UCCE farm advisor from Yolo, Solano and Sacramento counties, said control options for branched broomrape are limited and expensive. The potential exists for spread of branched broomrape from field to field via equipment and field workers. Severe infestations of this weed occurred in Sacramento County in 1959. The California processing tomato industry responded with funding to begin an eradication effort which lasted for two decades. The program ended with successful control of the broomrape weed. Eradication was possible with the use of the fumigant methyl bromide.

Branched broomrape is classified in California as an “A” pest, an organism of known economic importance,and is subject to enforced action, including eradication, quarantine, regulation, containment, rejection or other holding action.

Extent of the branched broomrape infestation in tomato fields is not fully known, Miyao said. It has been reported in several tomato fields north of Stockton in recent years. Consequences of reporting an infestation to a county agricultural commissioner is complete loss of the crop without harvest within the infested area of the field. Miyao said not reporting an infestation also has consequences. At risk is the likelihood of increased spread of this weed and further crop loss in the future.

Research by UCCE weed specialist Brad Hanson found that emergence of broomrape occurs during early tomato fruit set and continues over a 10-week period. Attempts to hand weed broomrape can reduce the problem as it prevents seed production, but the weeding crew must carefully bag all shoots and stay out of uninfested parts of the field to contain the spread via seed. The hand removal must continue almost weekly for an extended period. Branched broomrape produces large amounts of tiny seeds, and seed set occurs within a week of emergence. Shoots of this weed are often hidden by the tomato plants and can go undetected.

Miyao said a grower who chooses to knowingly harvest through a broomrape-infested area will spread the seed down the row and will risk spreading seed across the field including into other fields. A grower who unknowingly harvests through an infestation is equally at risk.

The California Tomato Research Institute sponsored an herbicide trial to control branched broomrape in tomatoes. The trial in an infested field used two herbicides not currently registered for use in California. Compared to a successful Israeli program, it has not been as effective in the California trial. The herbicide rates and timing will be modified in the 2021 season. Miyao said this control program would be helpful, but if even a small percentage of the weed escapes, the seed bank in the soil will increase.

Miyao said an effort to eradicate this weed while the number of impacted acres is limited would be more cost effective than waiting until the infestation is widespread. The issue for the California processing tomato industry is that CDFA rates branched broomrape as a class “A” pest, which creates the ‘quarantine-like’ conditions of a no-harvest. Allowing the weed to not be under CDFA regulation could potentially cause substantial crop loss from this parasitic weed without an adequate control program.