Brown marmorated stink bug continues to spread in agricultural crops in California.

This large, invasive insect pest can cause significant damage to fruit crops, and peach orchards are one of this pest’s preferred feeding sites. First detected in the U.S. in the late 1990s, it has been causing crop losses in East Coast fruit orchards since 2010.

In his presentation for California Cling Peach Day, UCCE IPM advisor Jhalendra Rijal said BMSB is spreading slowly into the crop production areas of the northern San Joaquin Valley. As BMSB feed on 170 crop and non-crop host plants, the reproducing populations of BMSB have been established in residential areas of more than 16 California counties, with a majority of them being in the Central Valley. He emphasized the importance of identification of stink bug species when crop damage occurs.

Adult BMSB are about 0.75 inch in length, and larger than Consperse stink bug and red shouldered stink bug. BMSB can also be distinguished from other stink species by the two white bands on their antennae and legs.

In California, BMSB can have two generations per year. Adults will begin to emerge from overwintering sites in mid-March and may continue through May. Because of that, Rijal said the reproductive stages are staggered and both adults and nymphs can be found simultaneously in orchards. He also warned that populations can build quickly early in the season, especially when overwintering sites or non-crop hosts such as trees of heaven are present nearby.

BMSB feed on most of the plants out there with fruiting structures, Rijal said, but peach is one of the preferred hosts. BMSB can feed on all stages of the peach fruit development from early stage through ripening. Feeding causes both external and internal damage. Unlike almonds, early feeding does not cause fruit drop. Surface depression, gumming, necrotic lesions, cork-like lesions and whitish tissue beneath the surface are all signs of BMSB damage in peaches.

Damage is generally confined to the orchard edges. In a survey from 2017 to 2020, Rijal said that BMSB activity was cyclical. The severity of the damage to crops in 2020 was lower than previous years, but BMSB were found in more orchards

Monitoring for BMSB in an orchard can be done with sticky panel traps with BMSB lures. These panels are placed on a pole at the four-foot level from the ground in orchard edges. They attract both adults and nymphs. Beat trays and visual scouting can also confirm presence of this pest.

There are insecticides to reduce the BMSB population, mostly from the broad spectrum pyrethroid groups. These insecticides are often detrimental to natural enemies of other pests such as mites. Also, one spray is very likely not going to be effective if the population is present in the orchard throughout the season, similar to what has been seen on the east coast. The parasitic Samurai wasp is a specific natural enemy of BMSB and has been detected in many states including Washington, Oregon and in the Los Angeles area of California.

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.