Postharvest scouting for weeds in tree nut orchards presents an opportunity to evaluate this year’s orchard floor management program and see which weed species have invaded or spread, where weed populations are highest and severity of infestation. Field margins, ditch banks and irrigation canals should be included in the scouting.

Scouting allows you to adjust control practices for the following year and select the best options for the weed species of concern. It can also help with selection of the best cultivation method for the weed stage.

Correct identification of weeds at all stages of growth is especially valuable, said John Roncaroni, UCCE weed specialist emeritus. Grass species identification can be difficult, he said, but it is important to determine because some species like ryegrasses are more resistant to glyphosate while summer grasses are not. Knowing the weed can also help determine if mechanical or cultural control methods would be useful. Weeds can look similar but have very different management requirements.

The UCCE publication Sacramento Valley Orchard Source recommends scouting a couple of times during the winter to catch weeds when they are young and more easily controlled. Records of weed infestations should be kept and mapped to show where weeds are a persistent problem.

Keeping records of weed infestations from year to year can help show trends and which management tactics are working.

Another recommendation is to look for different weeds in different management zones. Tree row checking may show effectiveness of previous herbicide applications. The ground cover in row middles can show if perennial seedlings are increasing. Orchard borders and row ends are places to look for new weeds.

Almond Board of California reports that as of as of 2021, there were 13 weed species commonly found in almond orchards with confirmed cases of herbicide resistance biotypes in California. Due to the increasing numbers of herbicide resistant weed species, early control of escaped weeds can reduce cost of an annual orchard floor management program. An example in Sacramento Valley Orchard Source notes that spot treating two acres of glyphosate resistant palmer amaranth with a tank mix of Glufosinate and Gramoxone is more affordable than trying to control this weed in an entire 50-acre block.

Citrus tree growth, health and productivity are influenced by the rootstock chosen. In a Citrus Research Board webinar, Dr. Mike Roose, professor emeritus of genetics at UC Riverside, outlined the newest challenge for citrus rootstocks: heat and drought tolerance.

Choosing a rootstock involves balancing objectives, strengths and weaknesses. Roose advised choosing a rootstock with characteristics matched to the location and objectives. With an existing planting with a known rootstock, management should be done to avoid known stresses to the rootstock. For example, Phytophthora control may be necessary for many rootstocks. Soil fumigation may be needed if there is low tolerance to nematodes. Spray regimes for Asian citrus psyllid may control aphids that vector Tristeza.

Salinity, calcareous soils and drought pose serious challenges to rootstocks.

Roose noted that there are little comprehensive data on drought resistance of citrus rootstocks and the topic is complex because drought is location specific. Major citrus producing countries have diverse climate, soils vary in water holding capacity and cultural practices such as mulching, irrigation and berms vary widely.

Roose said drought tolerance is a complex trait that is important at different growth stages and involves multiple adaptations. Those can maximize extraction of water from soil while minimizing loss from leaves. Deep roots and altered leaf morphology, including thicker cuticle, narrow leaves and changes in stomata, are also mechanisms that aid in drought tolerance.

Iron chlorosis is a condition that occurs on more alkaline soils (pH 7.5 to 8.5) because availability of iron is reduced. Water with higher pH can also enhance soil problems. A typical iron chlorosis symptom in citrus is interveinal chlorosis. A trial of Tango rootstocks from 2010 to 2020 showed differences in tolerance among five different rootstocks. Pomeroy trifoliate and Rich 16-6 had high tree losses while trees on Swingle, Carrizo, c35 and Volk had few chlorosis symptoms.

Roose said citrus rootstocks differ in tolerance to salinity and to specific ions present. Citrus shows yield decline when soil salinity reaches 1.7 dS/m with about 16 percent yield loss per additional 1 dS/m. If soil salinity is 4 dS/m, then 2 .0 inches of water per foot of rootzone are required to reduce salinity to 1.5 dS/m.

High transpiration increases the uptake of chlorine and other ions from soil. Leaf chlorosis levels are inversely related to water use efficiency and membrane transporters are involved in Cl uptake. Overall tree tolerance is also influenced by the scion.

Rootstock choice can also affect tree vigor in the absence of diseases, soil and water stresses, climate, compatibility with scion and genetics when zygotic seedlings are used as rootstocks.

Mealybug management is an important part in controlling grapevine leafroll disease.

At a Fresno State grapevine viruses symposium, Kent Daane, UC Berkeley extension specialist, said systemic insecticides are slow to kill mealybugs, while mealybugs’ ability to transmit the disease occurs at a fast rate.

A case study published in Frontiers in Microbiology noted that control of mealybug and grapevine leafroll disease is among the top priorities in the U.S. wine grape production industry. It is most prevalent in cool climate regions where fruit on infected vines has delayed maturity that results in lower brix, affecting value of the crop.

There are three biological components to grapevine leafroll disease: a complex of viruses, grapevine host plants and species of mealybugs and soft scales that transmit the virus. Symptoms appear in the fall when red grape cultivars display leaf reddening. In white cultivars, there is slight leaf chlorosis. Both red and white cultivars develop downward rolling of leaf margins and phloem disruption.

Vine mealybug (VMB), which is spreading throughout grape growing regions in California, has six to seven generations a year and all stages of overlapping generations are found on canes, clusters and leaves and under bark on trunks and cordons.

Daane said traps are effective in determining if vine mealybug is present in a vineyard. Grape growers can be surprised to find this pest in vineyards since infestations can be difficult to spot. The highest flight rates for VMB are later in the season close to harvest.

Traps can be a good starting point for a concerted regional effort in control of VMB. Asking neighbors to also trap and compare trap counts can help with control decisions.

Grape mealybug is a native pest that has a large complex of natural enemies and is often under good biological control.

Current management strategies for vine and grape mealybug include insecticides, mating disruption, biological control and management of some ant species. The Argentine ant in particular farms the mealybugs and is aggressive in California vineyards, causing a struggle in maintaining a low mealybug population.

Conclusions from studies of mealybug control in major grape-growing regions of the world show a combination of approaches are needed for grapevine leafroll disease. It must be managed on a large scale and a long-term management strategy is necessary. Infected vines or blocks will continue to be a source of infection and the disease will spread in the presence mealybug vectors.

Having a source of certified uninfected propagation materials is also a component of avoiding grapevine viruses.

An unusual strain of beet curly top virus (BCTV) has been confirmed in Colusa County this growing season. This new outbreak follows an outbreak last year, possibly due to the drier conditions which resulted in the vector of the disease, beet leafhopper, migrating into processing tomato fields earlier than usual.

UCCE Colusa County Farm Advisor Amber Vinchesi-Vahl said that BCTV is not normally a serious issue in processing tomatoes in the northern tomato growing regions. She noted that the confirmed virus strain was different than the strain found in Fresno and Kern counties.

Some damage to fruit and plants occurs when beet leafhopper nymphs and adults feed in tomato fields, but the main issue is vectoring BCTV.

Vinchesi-Vahl , in the Vegetable Crops newsletter, said that other insects cannot transmit the virus to tomatoes. The virus is not transmitted or spread by seed, touch or machinery.

Curly top-infected plants turn yellow and stop growing. Leaves roll upward and turn purplish. Leaves and stems become stiff. Spring plantings are the most susceptible as beet leafhopper migrates from overwintering host plants when they become dry. The west side of the San Joaquin Valley is where most infections occur.

Vinchesi-Vahl said that beet leafhoppers do not complete their life cycle in a tomato field. They migrate in, feed and then move on to preferred hosts, including sugarbeets. For this reason, it is not likely that beet leafhoppers will be visible in field inspections.

When beet leafhoppers migrate out of the field, that ends the virus transmission. The infected plants, particularly the earliest infected, will die. There will be a mix of plants with different stages of the virus resulting from multiple flights of beet leafhopper into the field.

Managing surrounding weed hosts around tomato fields may be helpful in reducing sources of the virus. The UC IPM Guidelines say that insecticides applied to infested fields to control beet leafhopper and reduce the spread of the curly top pathogen may prevent some infield spread, although infected plants will not recover. In areas that are at annual risk of beet leafhopper infestations, application of a systematic insecticide may have some impact. Beet leafhopper populations are greatest in years with rainfall that promotes growth of its weed hosts in the foothills.

Growers are asked to be on the lookout for BCTV in tomato fields and to contact Vinchesi-Vahl at acvinchesi@ucanr.edu if high incidence is found.

A resistance-breaking strain of tomato spotted wilt virus (TSWV) was detected in May in a northern Sutter County processing tomato field, according to the UCCE Vegetable Crops Newsletter.

UCCE Vegetable Crops Advisor Amber Vinchesi-Vahl recommends monitoring fields for TSWV, particularly those planted with resistant cultivars. Fields where more than 3% of plants are showing symptoms are suggestive of the presence of the resistance-breaking strain. Symptoms include extensive distortion, yellowing and necrosis of leaves. Plant infections should be confirmed with molecular testing. Locations should be documented to allow follow-up next year.

Growers who suspect TSWV in resistant tomato varieties should contact Vinchesi-Vahl or deliver samples to the Colusa or Yuba-Sutter UCCE offices.

Incidence of tomato spotted wilt virus has been increasing in California processing tomatoes, Vinchesi-Vahl reports. It is spread from plant to plant by thrips, mainly western flower thrips. Tomato spotted wilt virus can only be acquired by immature thrips. Infected adults can transmit TSWV throughout their 30- to 45-day lifespan.

Symptoms for TSWV depend on stage of growth. In early vegetative stages, bronzing and wilting occur, and the entire plant may look off-color and have a crumpled appearance. At later stages, purpling and leaf curling are evident. Bumpy fruit with ring spots is seen in bearing plants. Symptoms can be similar to those caused by curly top and alfalfa mosaic, but if necrotic spots and rings develop on leaves, followed by necrosis and dieback of entire leaves and shoots, TSWV is indicated.

Effective management targets thrips and the virus. Growers should use transplants from greenhouses that monitor for thrips and inspect plants for signs of TSWV. Growers should avoid planting near established crops with confirmed TSWV infection. Thrips should be managed early in the season with chemical control as monitoring or degree day predictions indicate. Tomato fields with high populations of thrips and resistance-breaking strains of TSWV should be sprayed for thrips one to two weeks prior to harvest to reduce the spread of the adults to nearby fields. Weed control in and around the field is also recommended.

Best practices after the growing season include removal of old tomato plants and other host crops on a regional level. Weed control in adjacent fallow fields can reduce sources of infection.

For additional information on testing for TSWV or developing thrips management strategies, contact UC Davis’ Robert Gilbertson, rlgilbertson@ucdavis.edu.

Protection of Tango trees from citrus leafminer for the first three to four years after planting is warranted, UC research has found, and monitoring for larval mining activity should
be done to determine the timing and frequency of insecticide treatments.

Matt Daugherty, UCCE entomology specialist at UC Riverside, and Beth Grafton-Cardwell, UC emeritus entomology specialist, noted in their citrus leafminer study that monitoring is critical as the citrus leafminer populations vary during the season and tend to decline with tree age.

Citrus leafminer, Phyllocnistis citrella, is a tiny moth that was detected in the U.S. in 1990 and found in California’s Central Valley citrus growing regions in 2006. It was not considered a serious pest in mature citrus, but studies found that infestations in the rapidly growing Tango acreage could affect tree growth and, later, crop yields.

Daugherty said that anything that impacts growth of new flush in citrus will also impact citrus leafminer populations. Optimal temperatures for citrus leafminer development are between 70 and 85 degrees F with greater than 60% humidity.

Visual surveys for this pest in young trees should take place in the spring and summer months. UC IPM guidelines note that citrus leafminer has four life stages: egg, larva, pupa and the adult moth. Adults cause no damage and live only one to two weeks. After mating, the female lays single eggs on the underside of leaves.

Eggs hatch about one week after being laid. The larvae begin feeding in the leaf and produce tiny, nearly invisible mines. When the larva emerges from the mine, it rolls the edge of the leaf over, causing a curling of the leaf. Inside that curled leaf edge, the leafminer becomes a pupa.

Daugherty and Grafton-Cardwell evaluated three insecticide treatment regimens to reduce citrus leafminer densities compared to untreated trees to determine if growth and development of Tango mandarin trees were affected during the first four
years after planting.

They found that the number of leaves that were suitable for egg laying by citrus leafminer fluctuated, with the lowest numbers occurring during the summer heat. Both the amount of tender leaf flush per shoot and the citrus leafminer populations per leaf declined during the three years of the study, reducing the number of applications of insecticides needed as trees matured.

Systemic imidacloprid combined with multiple foliar insecticides significantly improved the yield of trees in years three and four when they first came into bearing.

Individual insecticide applications reduced leafminer density for two to three weeks, including the systemic Admire Pro® applied at seven fluid ounces per acre.

Foliar tissue analysis determines essential or toxic levels of nutrients in plants. The analysis is used to detect tree response to a fertilizer program and to determine if there are nutrient deficiencies or toxicities that need to be corrected.

When taking leaf samples to determine your orchard’s nutritional needs, it is important that the sample sent to the laboratory not only be a good representation of the orchard, but also be the right part of the plant.

Scott Fichtner of Precision Agri Lab in Madera said that analysis of leaves from a tree nut orchard can let you know where to focus nutritional efforts and adjust for nutritional deficiencies or toxicities. Leaf sample analysis may also be necessary to justify application of nitrogen.

Accurate analysis is difficult if the right parts of the plant are not submitted. Trees selected should be of similar age, variety, rootstock and vigor. Do not take leaf samples from a tree that appears weak in comparison to others in the orchard, Fichtner said. For example, leaves that are water deficient or have been damaged by spider mites should not be included in the sample. Their nutrient levels will be lower compared to healthy leaves.

Leaf tissue samples can be collected throughout the growing season; however, the least change in concentration occurs from late June to July. The UC guidelines are generally correlated to July leaf tissue samples.

Samples from almond trees taken March through April should be the most recently matured leaves from the base of the spur. Samples in May should be the most recently matured leaf from the tip of the spur. Samples taken from June through October should be the terminal leaf on the spur. Pistachio and walnut samples taken April to October should be terminal leaflets.

Fichtner said that to achieve a representative sample from a block of trees, leaves should be pulled from each of the four quadrants of an individual tree. Sample 20 to 25 trees in a block to achieve a composite sample of 80 to 100 leaves. Samples should be placed in a paper bag for delivery or shipment to the lab and protected from temperature extremes.

Proper sampling is an integral and vital part of foliar analysis. A common issue with leaf sampling, Fichtner said, is that the person tasked with pulling the samples has not received training. Precision Agri Lab has training videos available for pulling leaf samples.

As researchers began to study Grapevine red blotch virus in grape vineyards and what causes this virus to spread, they began looking at possible transmission by pest insects.

Houston Wilson, UCCE specialist (Dept. of Entomology, UC Riverside) at the Kearney Agricultural Research and Extension Center, said that at this point, the only known insect vector was the threecornered alfalfa hopper (TCAH). This green, robust, wedge-shaped insect has piercing sucking mouthparts. While it can be found in vineyards, it strongly prefers legumes and is commonly reported as a pest of soybean, peanuts and alfalfa.

While conducting a study of the ecology and phenology of the threecornered alfalfa hopper, Wilson and his research team found that even though it can be found in vineyards and is capable of feeding on vines, this insect cannot actually complete its life cycle on vines. Instead, leguminous ground covers in the vineyard can support populations.

It is possible that TCAH can transmit the virus that causes Grapevine red blotch, a disease that affects grape quality, but Wilson said management of vineyard cover crops could provide cultural control. Adult TCAH can be found in vineyard cover crops in the spring. That is where they mate and lay eggs, and the immature nymphs then complete their development on leguminous ground covers. Nymphs typically mature into the adult stage just around the same time that the vegetation dries down, he said, which then triggers the adults to move up into the grapevine canopies where they potentially can feed and spread the virus if they have previously fed on red blotch-infected vines. By mowing and discing vineyard ground covers before the immature TCAH complete their development, Wilson said that fewer TCAH adults will make it up into vineyard canopies to feed on vines.

“Adult TCAH appear in ground covers in the early spring (March) and can be sampled with sweep nets. As those adult populations decline, it is likely that TCAH are mostly in the egg or nymph stage through April. It is during this time that the elimination of ground covers could have a negative impact on their populations since the nymphs are unable to migrate up to the vine canopy” Wilson said.

Wilson and his team are studying other possible insect vectors of Grapevine red blotch as well. One of these is the sharp-nosed leaf hopper, which appears to reproduce in vineyards and can pick up the virus, studies have shown, but Wilson said there is no definitive proof yet that this insect spreads the virus. Those studies are currently underway.

Grapevine red blotch has been in California vineyards since at least the 1950s, but grapevines were not tested for the disease prior to 2012. That was when growers grew concerned about transmission routes. Grapevine red blotch virus is limited to cultivated and wild grapevines. It is possible that in addition to insect vectors, the virus was also introduced in vineyards through grapevine propagation, Wilson said.