Vine mealybug (VMB), which is spreading throughout grape growing regions in California, has six to seven generations a year (photo courtesy Lodi Woodbridge Winegrape Commission.)
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.
Beet curly top virus affected tomato plant. A new strain of this disease has been confirmed in Colusa County. It is vectored by the beet leafhopper (photo by Amber Vinchesi-Vahl,)
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.
Symptoms of tomato spotted wilt virus include extensive distortion, yellowing and necrosis of leaves. Plant infections should be confirmed with molecular testing. Locations of infected plants should be documented to allow follow up next year (photos courtesy A. Vinchesi-Vahl.)
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.
Symptoms of tomato spotted wilt virus include extensive distortion, yellowing and necrosis of leaves. Plant infections should be confirmed with molecular testing. Locations of infected plants should be documented to allow follow up next year (photos courtesy A. Vinchesi-Vahl.)
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.
Adult leafminers cause no damage and live only one to two weeks. After mating, the female lays single eggs on the underside of leaves (left photo by David Rosen, right photo by Jack Kelley Clark, both courtesy UC Statewide IPM Program.)
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.
Adult leafminers cause no damage and live only one to two weeks. After mating, the female lays single eggs on the underside of leaves (left photo by David Rosen, right photo by Jack Kelley Clark, both courtesy UC Statewide IPM Program.)
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.
Accurate leaf analysis is difficult if the right parts of the plant are not submitted. Trees selected should be of similar age, variety, rootstock and vigor (photo courtesy Jenna Overmyer, Precision Agri Lab.)
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.
Adult threecornered alfalfa hopper can be found in vineyard cover crops in the spring (photo by Jack Kelly Clark, courtesy UC Statewide IPM Program.)
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.
Drought-induced Boron deficiency on a Thompson Seedless shoot (all photos courtesy University of California.)
The San Joaquin Valley (SJV) is in the midst of an ongoing drought. Total precipitation in Fresno from October to March was 7.3 inches which amounts to 60% of the historical average of 12 inches. With little or no rain in the forecast and in anticipation of another dry and hot summer, growers might reflect on some environment-related issues observed in 2021.
Figure 1. Precipitation, irrigation hours and soil moisture content (volumetric) at a soil depth of two feet from May 2021 to March 2022.
Maintaining Adequate Soil Moisture is Critical
Many growers who suffered greatly from delayed spring growth (DSG) in 2021 allowed the soil to become too dry the preceding fall or winter. Sufficient carbohydrate content of the vines and adequate soil moisture are the keys to dodge DSG. Healthy carbohydrate content in the vines can be attained with a balanced canopy vs yield and a pest-free canopy kept in good condition postharvest. Adequate soil moisture can be achieved through postharvest irrigation or even winter irrigation if precipitation is lacking. Postharvest irrigation helps prevent early defoliation, reduces the chance of winter freeze damage, helps leach any accumulated salts and helps rehydrate the vines after they emerge from dormancy.
Thus, a key lesson learned from 2021 should be the importance of maintaining soil moisture during the winter months. The beginning of last winter saw record precipitation, and many growers took proactive measures to prevent DSG in the dry second half of winter in early 2022. For example, in Figure 1, postharvest irrigation (end of September), winter irrigation (mid-November) and early spring irrigation (mid-February) have been noticed from this vineyard, which is one of the collaborative sites contributing to the UC IPM weather station networks (ipm.ucanr.edu/weather/grape-powdery-mildew-risk-assessment-index). As a pilot study site, we added a pressure switch and soil moisture probes at soil depth of 1, 2 and 3 feet to help growers improve their irrigation scheduling. We are glad to see growers are taking advantage of those data to manage vineyard water to reduce the risk of DSG.
Ideally, a regional soil moisture content network could provide critical winter soil moisture information to guide growers near the station to decide whether to irrigate the vines during the winter month.
Figure 2. Monthly max ambient temperature between 2021 and last 20 years’ average. Data are collected from CIMIS Station #80 at Fresno State.
Cultural Practices Can Help Protect Against Heatwaves
Besides the severe winter drought, growers also experienced record summer heat in 2021. Max monthly temperature in 2021 was much higher than last 20 years’ average, especially in June and July (Figure 2), and monthly reference evapotranspiration (ETo) in 2021 was also higher than the last 20 years’ average (Figure 3). Sun-exposed clusters and berries under the extreme summer heat will develop sunburn, which will reduce fruit yield and quality, including Brix and raisin B&B grade. I have written a previous article about grapevine heat stress and sunburn management (Progressive Crop Consultant May/June 2019), and many tools are available for growers:
When developing a new vineyard, row orientation and trellis design can help minimize direct sun exposure to fruit.
Canopy management practices, such as shoot tucking, can help minimize the direct sun exposure to fruits.
Sunblock sprays, such as Kaolin and CaCO3, increase reflectance and thereby reduce solar heating of fruit and leaves.
Evaporative cooling, such as in-canopy micromisting, can be effective, but water use and disease pressure could be increased.
Last but not least, adequate irrigation to develop the canopy which can help shade the fruit and protect fruits.
Among all the above options, irrigation might be the most important to prevent fruit sunburn during heat waves, since the sufficient canopy promoted by irrigation does not only serve as the photosynthetic machinery to produce carbohydrate to ripen fruits, but also provides the shade for fruits to reduce excessive sun exposure. There are many irrigation tools which growers can use to watch out for potential water deficit in their vineyards:
Soil moisture-based irrigation.
Plant water-based irrigation.
Weather-based irrigation.
Figure 3. Monthly ETo between 2021 and last 20 years’ average. Data are collected from CIMIS Station #80 at Fresno State.
Most growers I have talked to have soil moisture sensors on-site, and currently, CDFA offers various grants (cdfa.ca.gov/oefi/sweep/) to help growers install soil moisture sensors.
No matter which soil moisture sensor you have, the key is to identify the soil moisture benchmark which you can target the irrigation to. So, how can soil moisture sensors help you to target the benchmark and manage the water in your vineyard? For example, based on Figure 1 (see page 26), from May 2021 to August 2021, growers utilized the soil moisture sensor (soil water volumetric sensor, Campbell Scientific CS655) to schedule the irrigation to maintain the soil moisture content range from 10% to 15% at soil depth of two feet.
On this site, three soil water volumetric sensors were installed at approximately one foot from the vine trunk and six inches from the emitter. Sensors were set at the depth of one, two and three feet beneath the soil surface. We assume growers are satisfied with the canopy and crop development during the season, and a field study done by Dr. Larry Williams at UC Kearney REC also shows 10% to 15% soil moisture content correlates to -1.2 to -0.9 MPa midday leaf water potential (Williams 2012). The range from -1.2 to -0.9 MPa of midday leaf water potential is regarded as mild water stress or no water stress for the SJV vines and can be used to maximize the crop production (Figure 4).
Given the soil type on this site is similar to the Hanford fine sandy loam soil in UC Kearney REC, the same range of soil moisture content can serve as a good benchmark for any future irrigation scheduling on this site. Please note that no water applied after mid-August aimed to prepare the soil for raisin drying. Unsurprisingly, in 2022, growers started the early spring irrigation in mid-February to target the same range of soil moisture content from 10% to 15% to prepare the budbreak and early shoot development. Therefore, growers can irrigate the grapevines by selecting the desired soil moisture benchmark based on the preferred canopy and crop development on your specific soil type.
Figure 4. The relationship between midday leaf water potential measured on Thompson Seedless grapevines and soil water content (SWC, measured as the percent of volume). At the study site of UC Kearney REC, the soil type is Hanford fine sandy loam and 10% to 15% SWC correlates to -1.2 to -0.9 MPa midday leaf water potential, which is regarded as mild water stress or no water stress for the SJV grapevines.
Nutrient Management During Drought
Low rainfall in autumn to midwinter can cause drought-induced boron deficiency. The symptoms are erratic budbreak, stunted and distorted shoots, misshapen and chlorotic leaves. The most classic symptoms after budbreak are dwarfed shoots that grow in a zigzag manner with numerous lateral shoots, and the tip of the primary shoot may die. Most shoots begin to elongate normally by late spring, but cluster size may be reduced. The cause is believed to be a late-season drought-induced boron deficiency that affects development of shoots within dormant buds.
The key to reduce drought-induced boron deficiency is postharvest irrigation. Traditionally, Thompson raisin vineyards go through the harvest and raisin-drying processes for a nearly two-month period without irrigation (Figure 1, see page 26). The postharvest irrigation can relieve the water stress and maintain a healthy functional canopy to avoid boron deficiency.
Spring fever is sometimes referred to as false potassium deficiency because the leaf symptoms resemble and are sometimes confused with potassium deficiency. Alternating warm and cold weather patterns before bloom, as has been observed this spring, can cause a temporary nitrogen metabolism disorder associated with high levels of ammonium and the polyamine putrescine in the leaves.
Symptoms occur in basal leaves and leaves in the fruit zone. Lower leaf color fades and becomes chlorotic in spring, beginning at the leaf margins and progressing between the primary and secondary veins. Leaf margins may become slightly necrotic, marginal necrosis is significant and affected leaves can drop.
There is no cure for spring fever, and petiole/blade laboratory analysis can differentiate true K deficiency from spring fever. Spring fever typically will fade as the weather warms up and the onset of symptomatic leaves decreases around bloom; however, blades with existing symptoms will remain.
Spring fever in basal leaves of Thompson Seedless, showing chlorosis, curling and browning of leaf margins.
References Bettiga, Larry. Grape Pest Management, Third Edition. University of California Agriculture and Natural Resources. 2013. Christensen, Pete. Raisin Production Manual. University of California Agriculture and Natural Resources. 2000. Williams, L. (2012), Effects of applied water amounts at various fractions of evapotranspiration (ETc) on leaf gas exchange of Thompson Seedless grapevines. Australian Journal of Grape and Wine Research, 18: 100-108. https://doi.org/10.1111/j.1755-0238.2011.00176.x
Heavy infestation of California red scale. Infestations of leaves, twigs and branches can cause overall negative impacts to plant health (photo courtesy Suterra.)
California red scale, Aonidiella aurantii, is one of the primary insect pests of citrus crops in California and other states where commercial citrus is grown. Originally native to southeast Asia, it is believed to have arrived in California between 1868 and 1875 and is now distributed worldwide. California red scale (CRS) is a cosmetic pest of fruit that can result in costly economic damage due to downgrades at the packinghouse. In addition, infestations of leaves, twigs and branches can cause overall negative impacts to plant health. As a result, reliance on insecticides has been historically necessary to mitigate damage caused by this pest.
During the last half of the 20th century, CRS control relied on the use of broad-spectrum insecticides in the carbamate and organophosphate classes. By the late 1990s, these chemistries were no longer effective in many orchards due to the development of insecticide resistance. In the early 2000s, additional chemistries came to the market and proved valuable to aid in managing CRS. These newer-generation insecticides, based on insect growth regulators, proved effective for several years.
Adult female California red scale. Adult females are sessile and attached to the plant surface (photo courtesy UC Statewide IPM Program.)
Unfortunately, there were still a limited number of insecticide modes of action available for control, and by 2006, resistance to one of the primary insect growth regulators (pyriproxyfen) was confirmed in CRS populations in the Central Valley of California. An integrated pest management approach is necessary to maintain effective control of CRS and extend the lifespan of the remaining available insecticide chemistries.
The foundation of integrated pest management (IPM) programs for CRS includes biological control, cultural practices and judicious use of selective insecticides as needed based on monitoring and treatment thresholds. There are effective biological control agents for CRS, including multiple parasitic wasps and general predators. Unfortunately, necessary insecticide applications for the invasive Asian citrus psyllid have reduced the impacts of these natural enemies. More recently, mating disruption has emerged as a fundamental component of successful IPM programs and does not adversely affect natural enemies. Incorporating each of these IPM approaches will benefit growers by reducing crop damage and mitigating further insecticide resistance development.
California red scale male and female mating. Males are extremely short-lived as adults, living approximately six hours, and their only purpose is to locate females for reproduction (photo courtesy UC Statewide IPM Program.)
California Red Scale Biology
California red scale typically has four to six generations each year in California depending on growing region and environmental factors. Adult females are sessile and attached to the plant surface, and each produces on average 100 to 150 crawlers. The crawlers are mobile, however longer distance dispersal of crawlers is often due to wind, birds, equipment, or orchard workers. Adult males are the only life stage with wings and capable of flight. Males are extremely short-lived as adults, living approximately six hours, and their only purpose is to locate females for reproduction. Mate location is facilitated by a sex pheromone emitted by adult female scales.
California red scale crawlers. Crawlers are mobile, however longer distance dispersal of crawlers is often due to wind, birds, equipment, or orchard workers (photo courtesy UC Statewide IPM Program.)
Mating Disruption for CRS
Mating disruption for CRS is a technology that utilizes the insect’s sex pheromone as a management tactic to interfere with the ability of males to locate females for reproduction, thereby reducing pest populations and crop damage. This technology has been commercially available in California since 2016, and adoption is increasing each year as positive benefits are observed.
A recent publication by leading University citrus researchers summarized a multi-year study demonstrating the efficacy and value of CRS mating disruption in California’s Central Valley citrus production. These studies used the commercially available CRS mating disruption product CheckMate CRS dispensers at the label rate of 180 dispensers/acre. The research team, led by University of California citrus specialist Elizabeth Grafton-Cardwell, concluded several key findings as a result of their 2016-19 studies in 12 commercial citrus orchards.
Significant reductions in pheromone trap capture, twig and leaf infestations and infested fruit were consistently demonstrated where CRS mating disruption was applied compared to non-mating disruption reference plots. Suppression of male capture in pheromone traps, an indicator of effective mating disruption, averaged 90%. Twig and leaf infestations were reduced by an average of 75%, and the percentage of highly infested fruit at harvest was less than 0.5% in 9 of the 10 mating disruption blocks in 2018 and 2019.
Summary conclusions within the publication state, “mating disruption using CheckMate CRS can be an effective method to reduce California red scale populations throughout the four-plus generations that occur in central California,” adding that “mating disruption has the potential to reduce or eliminate pesticide applications.” It is important to note that the decision to reduce or eliminate any inputs in the overall IPM program should be determined on an individual basis based on grower and crop adviser consultation.
Adult male California red scale. The dark band is a key feature for identification (photo courtesy UC Statewide IPM Program.)
Implementing CRS Mating Disruption
In general, there are three primary types of mating disruption products or delivery methods. These include vapor dispensers (also called passive dispensers), aerosols and sprayable microencapsulated formulations. The type of delivery method or product that is most effective for a given pest will be based on the pest’s biology and flight behavior as well as the architecture of the cropping systems (i.e., orchard, vineyard or field canopy structure). For example, aerosol-style mating disruption products have been used for many Lepidopteran pests, including codling moth and navel orangeworm in orchard crops for several years. Vineyard crop systems are effectively battling vine mealybug with sprayable formulations or vapor dispensers. Vapor dispensers have also been particularly effective in citrus crop systems to reduce California red scale.
The distribution of very small pests, like CRS, is often aggregated in the environment with distinct hotspots. Males are not strong fliers and tend to stay within the tree canopy when searching for mates. This is different from moths, which are stronger fliers and spend more time flying within the orchard rows in search of mates. Because of this, the current high-density vapor dispenser is the most effective pheromone delivery method for disrupting mating of CRS. Aerosols in their current configurations, which are highly effective for moths, are not as effective for pests like California red scale and vine mealybug (VMB) because of these biological and behavioral differences. Sprayable microencapsulated formulations can be thought of as billions of microscopic dispensers and are therefore also effective mating disruption delivery systems for pests like CRS and VMB.
California red scale mating disruption with CheckMate CRS is designed to consistently permeate the orchard environment with the insect’s sex pheromone precisely within the tree canopy where the pests are located. They are applied at a rate of 180 per acre by hand from the ground. Because of pest biology and where populations are most abundant, dispensers should be placed within the tree canopy and not on outer branches. Deployment patterns are based on row and tree spacing and total number of trees per acre.
Common row and tree spacing patterns are available on the manufacturer website (Suterra.com) and the manufacturer field team is available to assist with designing dispenser patterns for orchards with other spacings. When possible, dispensers should be placed after pruning, or if before, care taken not to prune out the current season’s mating disruption dispensers. The controlled release of pheromone lasts a full calendar year. Therefore, precise deployment timing is not critical. Many users choose to re-apply their CRS mating disruption dispensers before the historical first or second flights each year.
California red scale mating disruption vapor dispensers are designed to consistently permeate the orchard environment with the insect’s sex pheromone precisely within the tree canopy where the pests are located (photo courtesy Suterra.)
Benefits of Mating Disruption in the Overall IPM Program
In addition to the validated efficacy in reducing CRS populations and damage, incorporating mating disruption into the overall IPM program for any pest provides additional benefits. Pheromone mating disruption is non-toxic; it does not kill anything.
Its efficacy is based on interfering with reproduction, thereby preventing significant portions of subsequent generations from ever existing. This allows all other inputs (cultural, biological, chemical) to have greater impact, simply because there are fewer individuals to have to kill or remove from the environment. This technology is safe for non-target species (natural enemies, pollinators, humans) and is MRL exempt.
Mating disruption is not subject to resistance development and may help delay insecticide resistance due to reductions in the number of insecticide applications needed for effective CRS control. Dispenser-based mating disruption for CRS has zero re-entry interval, zero pre-harvest interval and is approved for use in organic agricultural production.
References Grafton-Cardwell, E. E., J. T. Leonard, M. P Daugherty, and D. H. Headrick. 2021. Mating Disruption of the California Red Scale, Aonidiella aurantii (Hemiptera: Diaspididae) in Central California Citrus. Journal of Economic Entomology 114(6): 2421-2429.
Figure 1. Andros pre-pruner in a spur-pruned vineyard. The machine snaps canes and removes part of the wood materials before workers cut canes to spurs (all photos courtesy T. Tian.)
Mechanization is clearly one of the best solutions to reduce labor requirements for vineyard management and lower production cost. Even though pruning machines are extensively used in wine grapes grown in the San Joaquin Valley, the mechanization of pruning in table grapes remains challenging.
Most of the table grape vineyards grown in the San Joaquin Valley use a “Y” trellis system (open Gable system). This highly structured trellis provides adequate support to large canopies of table grapevines but limits the options of mechanization. In addition, table grapes have high aesthetic requirements. Growers rely on vineyard crews to adjust the location and numbers of spurs and canes at pruning to maintain vine shape and achieve desirable yield and fruit quality. In this regard, mechanical tools that are currently available cannot mechanize the entire pruning process in table grape vineyards. However, mechanizing part of the process is possible.
Cane pruning, spur pruning and spur pruning with kicker canes are used in table grape vineyards, and the choice of method is based on basal bud fruitfulness of specific varieties and growers’ preferences. Despite the difference in pruning methods, there are three general steps involved, including 1) selecting canes and cutting canes to desirable length; 2) pulling pruned canes off the catching wires and placing them between rows; and 3) tying canes or new cordons to the wire.
Mechanization has the potential to improve efficiency in the latter two steps. Andros Wye trellis pre-pruner (Andros pre-pruner) snaps canes and removes part of the wood materials before workers cut canes to spurs (Figure 1). KLIMA cane pruning system (KLIMA shredder) strips canes from the wires and mulches them after an initial pass of cutting canes (Figure 2). Battery-powered shears are alternatives to conventional hand shears, expected to reduce the fatigue of workers and lower the risk of repetitive motion injuries (Figure 3). Battery-powered tying machines simplify the tying process and may reduce the time needed to tie individual canes.
Despite the promising outlook of mechanical pruning tools, more evaluation is needed on their suitability in different training systems and whether mechanization can decrease labor requirements for pruning. Thus, as supported by the California Table Grape Commission, we evaluated those four mechanical pruning tools in four different table grape vineyards in 2022 between January and March.
Figure 2. KLIMA cane shredder in a spur-pruned vineyard. The machine strips canes from the wires and mulches them after an initial pass of cutting canes.
KLIMA Cane Pruning System
We evaluated the KLIMA shredder in a spur-pruned ‘Holiday’ vineyard and a cane-pruned ‘Autumn King’ vineyard. Self-releasing clips were installed on the “V” cross arms in both vineyards prior to the evaluation (Figure 4). Those clips allow the shredder to directly pull catching wires from cross arms without human assistance. In each vineyard, 7 to 10 continuous rows were hand-pruned, and the adjacent 7 to 10 rows were pruned using the adjusted procedure.
On the first day, workers cut canes to desirable lengths without pulling pruned canes off the wire. On the second day, the KLIMA shredder was hooked to a tractor for passes in the vineyard. In each pass, the shredder caught the catching wires, releasing them from crossarms, stripped canes from the wires and mulched those canes. A worker was assigned to guide the tractor driver. The time required for shredding ranged between 1.5 and 2 hours per acre. On the third day, workers replaced catching wires back to crossarms.
When the KLIMA shredder was used, the non-machine labor hours required to prune vines reduced by 26% to 34% (Table 1), leading to a savings of $150 to $200 per acre on labor costs. A greater reduction in labor requirement was observed in the spur-pruned vineyard than in the cane-pruned vineyard. It seems the KLIMA shredder conveys a greater benefit in vineyards where pulling canes off the wire takes similar or more time than cutting canes. Indeed, we found that workers spent a similar amount of time on cutting and pulling canes in the spur-pruned vineyard. In the cane-pruned vineyard, on the other hand, workers used 65% of the time on cutting canes and 35% of the time on pulling canes.
Figure 3. Battery powered pruning tools (left, tying machine; right, pruning shear). These tools are alternatives to conventional hand shears, expected to reduce the fatigue of workers and lower the risk of repetitive motion injuries
Andros Pre-Pruner
We assessed the single-head model of Andros pre-pruner in the spur-pruned ‘Holiday’ vineyard that was used for KLIMA shredder evaluation. Ten continuous rows were pre-pruned and then hand pruned. The adjacent 10 rows were hand-pruned. Andros pre-pruner covered one acre in an hour, leading to a 10% decrease in non-machine labor required for pruning (Table 1). The reduction of labor requirements resulted in a savings of $40 to $80 on a per-acre basis. Given the trellis configuration of the experimental vineyard, the cutting system was placed 20 to 25 inches (51 to 63 cm) above the cordon to avoid the cutting system hitting the horizontal bars of crossarms. In vineyards with larger “V” crossarms, the cutting system can be placed closer to the cordon and thus a larger portion of the wood materials can be pre-pruned. In this case, the cost savings reached $120 to $130 per acre. In addition to cost savings, workers suggested benefit on their end; it was easier to pull canes off when they were pre-cut.
Figure 4. Self-releasing clip installed on a “V” cross arm. These clips allow the shredder to directly pull catching wires from cross arms without human assistance.
Battery-Powered Pruning Tools
We tested battery-powered pruning shears in a spur-pruned ‘Scarlet Royal’ vineyard. Workers who used battery-powered shears can prune two to four more vines per hour as compared to workers using conventional hand shears, leading to a 10% reduction in manual labor input (Table 2). Workers using battery-powered shears felt less fatigue and pruned at a similar pace throughout the day, while workers using conventional hand shears clearly slowed down after 11am.
We also examined battery-powered pruning shears and tying machines in a cane-pruned ‘Autumn King’ vineyard. The use of tying machine allowed workers to tie five to nine more vines per hour than manual tying, improving tying efficiency by 15%. However, battery-powered shears did not improve pruning efficiency, even though workers felt more comfortable making difficult cuts when using those shears (Table 2).
Growers and the manufacturer observed a larger decrease in labor requirements when battery-powered tools were used under similar conditions. We suspect workers who participated in the trial did not fully adapt to the new tools, given that they only practiced for a day prior to the evaluation. We plan to perform more evaluations on those tools in 2023.
Mechanization of pruning can effectively reduce labor requirement and improve efficiency. Depending on the trellis configuration and production scale, growers could save $40 to $200 per acre by adopting mechanical pruning tools. Besides cost savings, pruning mechanization conveys benefits in other aspects. For example, workers could pull off pruned canes more easily after pre-pruning. KLIMA shredder shreds canes in pieces smaller than regular ground shredders, allowing wood materials to break down faster. The quality of cuts and ties improves when battery-powered tools are used.
We truly appreciate the support from table grape growers in Tulare and Kern counties, Pellenc America Inc. and Andros Engineering. This project is funded by the California Table Grape Commission.
Please contact Tian at titian@ucanr.edu for more information.
Themis Michailides and Plant Pathologist Mark Doster examine AF36 seeds spread in the field. The atoxigenic AF36 strain was registered for use in pistachio orchards in 2012.
Aflatoxins are a category of mycotoxins, which are toxic compounds produced by fungi (Myco= µύκης, which means fungus in Greek.) So, aflatoxins are toxins produced by specific types of fungi. These compounds are by-products (secondary metabolites) produced mainly by two fungal species, Aspergillus flavus and Aspergillus parasiticus, which are present in California nut crops and fig orchards when they grow in susceptible substrates. These compounds are very toxic, carcinogenic and cause disease when consumed in large amounts. Because of the high toxicity, aflatoxins are regulated worldwide by governments, dealing with marketing products susceptible to contamination with these mycotoxins. There are several kinds of aflatoxins, such as B1 and B2, produced by the above Aspergillus species. In addition, the latter species produces two more kinds of aflatoxins, G1 and G2. The letters “B” stands for the blue color that these compounds show and the “G” for the green color when contaminated products are placed under a UV light (365 nm wavelength).
Figure 1. (A,B) Early split pistachios serve as the “Achilles Heel” for navel orangeworm infestation and aflatoxin contamination. (C) Suture staining of early split that helps their removal in the sorting belt at the processor.
In the U.S., the tolerance for aflatoxins is 20 ppb (1 part per billion =0.000000001 gr). In the European Union (EU), for all (total) aflatoxin, the tolerance is 10 ppb while for B1 is 8 ppb, and these values are for pistachios and almonds for direct consumption. The tolerances are even stricter for walnuts and dried fruit (4 and 2 ppb for toral and B1 aflatoxins, respectively), and for dried figs, total aflatoxins are 10 ppb while for B1 is 6 ppb.
Because of these strict tolerance thresholds, the U.S. (and to a lesser degree other countries that produce and market products susceptible to aflatoxins) takes extraordinary measures to reduce this contamination as much as possible.
Figure 2. Effect of level of navel orangeworm damage to aflatoxin contamination
Early Aflatoxin Research
For a decade (1991 to 2001), the research in our plant pathology laboratory focused on cultural practices and reduction of damage by the navel orangeworm (NOW) to reduce aflatoxin contamination in almond and pistachio. For instance, in early research on pistachio, we discovered that the early split (ES) nuts (Figure 1) serve as the “Achilles Heel” for infestation by NOW and infection by aflatoxigenic fungi, leading to aflatoxin contamination.
In fact, when samples were collected from several orchards and separated into ES without NOW damage, ES with NOW-damaged nuts and normal nuts (mature nuts but having hulls intact) and analyzed for aflatoxins the results showed that ES contribute a lot in aflatoxin contamination of pistachio as follows: ES (shriveled, i.e., those that had developed early and infested by NOW) had an average of 84% of the total aflatoxins in these samples; if one includes the ES with no NOW infestation, the total levels of the aflatoxins could explain 99.9% of aflatoxins in these samples. Although 20% of the samples represented ES that developed close to harvest had aflatoxins, the levels were only 2 ppb, representing only 0.1% of the total aflatoxins.
Figure 3. Effect of feeding damage by navel orangeworm to aflatoxin accumulation (laboratory experiment with almonds)
These results made us investigate ways to reduce the incidence of ES and also methods to reduce NOW damage. Normal mature nuts had no aflatoxins in these samples. The results were very consistent in two consecutive years. Moreover, when samples were collected and were separated based on their levels of NOW damage, we showed that as the level of damage increased, so did the aflatoxin contamination frequency and amounts (Figure 2). Similarly in almonds, as the feeding sites caused by the NOW larvae increased on almond kernels, so did the aflatoxin levels (Figure 3). Therefore, it is obvious reducing the damage by NOW will result in lower risk for aflatoxin contamination, and this is accomplished by doing sanitation (“mummy shake”) in both almond and pistachio. Avoiding water stress is another approach to reduce susceptibility of trees to aflatoxin contamination. Specifically in pistachio, water stress in early season (May) increases the incidence of ES, and thus increases the risk for aflatoxin contamination.
Figure 4. The two aflatoxigenic mold species that occur in California tree nut orchard.
Biological Control
Because aflatoxin contamination is very sporadic, and because there are no fungicides that would affect aflatoxin contamination, several countries now emphasize a biological control approach by displacing the toxigenic strains of A. flavus and A. parasiticus with the use of atoxigenic strains that are applied directly on to the orchard floor. In California, in cooperation with Dr. Peter Cotty (USDA-ARS), the A. flavus AF36 strain was registered first in 2012 for use in pistachio and then the same product as AF36 Prevail® in 2017 for use in pistachio, almond and figs. As of August 2021, a second product using a different atoxigenic strain, Afla-Guard® GR manufactured by Syngenta Crop Protection, LLC, was registered for use in almond and pistachio. Pistachio and almond growers can use either product starting in the 2022 growing season.
But before moving into the application of either of these products, we need to provide a brief background into the history how this technology had developed over many years of research and the various challenges orchardists have experienced over the years in using the AF36 and/or the AF36 Prevail products.
Figure 5. Very high displacement of toxigenic strains by the applied atoxigenic AF36 strain of Aspergillus flavus in commercial pistachio orchards during 2008 to 2011 when the Experimental Use Permit was
As mentioned earlier, there are two major fungi that can produce aflatoxin and contaminate the susceptible crops, A. flavus and A. parasiticus. The A. flavus produces two strains in the soil based on the size of the sclerotia (resistant, survival structures), the S strain and the L strain (Figure 4). All the isolates of the S strain are toxigenic, while among the isolates of the L strain, there are strains that produce various levels of aflatoxins and others not producing any strains. These latter strains are called atoxigenic.
All the strains of A. parasiticus are aflatoxin producers. An atoxigenic strain called AF36 was found to be among the most frequently encountered atoxigenic strains ranging in incidence from 4% to 12% while other groups of atoxigenic strains ranged from <1% to 2% among the strains in the soil. This strain was determined to be the same that was isolated earlier in Arizona from cotton fields and used there as a biological control agent to reduce aflatoxin contamination in cotton and corn. Therefore, since all the toxicological studies were done by USDA and because the AF36 was the one most frequently encountered in California tree nut and fig orchards, we selected and used it as a biological control agent in the Kearney experimental orchards first to measure displacement of the toxigenic strains.
After 10 years of research in microplots initially and three years as an Experimental Use Permit compound used in 3,000 acres of pistachio, eventually, the AF36 strain was registered for use in pistachio orchards in 2012. The initial carrier of the strain was wheat seed inoculated in big fermenters with the A. flavus AF36 strain. Five years later, the manufacturer was able to register the AF36 Prevail® product, which contains the same AF36 strain and uses sorghum as the carrier, the surface of which is coated with propagules of the atoxigenic strain via a polymer. Recently, a second atoxigenic strain, Afla-Guard GR, was provisionally registered for use in pistachio and almond. There is also an interest now by the walnut and fig industries to have both these products registered for these crops.
Figure 6. Reduction of aflatoxin- contaminated samples after application of AF36 atoxigenic strain during 2008 to 2011.
Challenges
When experiments using the atoxigenic strain AF36 were initiated in 2002, we determined a very high displacement ability of the toxigenic strains in the soil of microplots, which reached almost to 95% displacement at the end of the fourth year, applied once per year. When the EUP was granted in 2008 and 3,000 acres of pistachio were treated with AF36 once per year, the displacement also reached to 95% by the fourth year of application (Figure 5). It was during this time when analyses of first harvest pistachio library samples showed a 40% reduction in aflatoxin-positive samples and an almost 55% reduction in aflatoxin-positive library samples of the second harvest (or reshakes) (Figure 6).
During 2012 to 2019 and after application of the product yearly, we noticed a lower percentage of displacement that ranged from 50% to 70% (Figure 7). The blocks that were treated last in 2011 showed a continuous decrease of displacement from 65% down to about 30%, which remained stable from 2015 to 2017, but there was a jump to almost 50% in 2018 (Figure 7). The untreated fields ranged from about 32% to 20% during this period (2012 to 2018). During the same seven-year period, when library pistachio samples were analyzed for aflatoxins, only in two years (2013 and 2017) were there major reduction level in aflatoxin-positive samples, while in the remaining years, the reductions in positive samples were very small, ranging from 5% to 30%, and in some years (2014 and 2016), there was no reduction at all (Figure 8).
To overcome these challenges and to minimize the influence of treated orchards with AF36 Prevail® by toxigenic propagules (spores) escaping non-treated orchards, we initiated research projects funded by both the pistachio and almond industries on long-term areawide projects. The rationale was that when large area is treated year after year, the influence by non-treated orchards should be minimal. By doing this, we were able to increase the levels of displacement from about 45% in almond and about 65% in pistachio to 80% in almond and 95% in pistachio (Figure 9). This is a significant increase in the levels of displacement of toxigenic strains and we expect now to significantly reduce the levels of aflatoxin-positive samples. A good example for displacement of toxigenic strains is from our research with almonds, where toxigenic strains ranged from 65% to 83% before treatment with AF36 Prevail to about 25% toxigenic Aspergillus in fields treated, while the untreated orchard maintained an incidence of 68% of the toxigenic Aspergillus strains. In the treated orchards, the atoxigenic strains reached a level of about 80% as measured at harvest time. (Figure 10).
Moving Forward
The new biological product, Afla-Guard GR is now registered and will be available to pistachio and almond growers to use commercially in 2022. The active ingredient of Afla-Guard GR is the NRRL 21882 Aspergillus flavus strain and the carrier is barley seed. In contrast to AF36 Prevail that is sorghum seeds coated with the spores of AF36 strain of A. flavus, the NRRL 21882 strain is inoculated in the barley seed, i.e., the entire barley seed kernels are “infected” (colonized) by the latter strain.
The Afla-Guard GR product sporulated at higher rates under lower temperatures and lower soil moisture than the sporulation produced by AF36 Prevail when the two strains were compared side by side and under the same conditions. However, since we do not have any results from aflatoxin analyses of pistachio samples, it is still unknown how this product will perform under commercial application. The efficacy of each AF36 Prevail and Afla-Guard GR will be studied in side-by-side commercial fields this year for the first time.
In addition, we are doing studies to overcome other challenges with the commercial application of these products, such as managing predation by ants, rodents and birds, the negative effect of the direct sunlight on the seed inoculum under very high temperatures, the coordination of irrigation either just before or immediately after applying the biocontrol product, etc. All of these are challenges that can differ from orchard to orchard and therefore may affect variably the efficacy of the biocontrol products.
(Co-Authors of this article include: Mark Doster, Plant Pathologist, Pummi Singh, Post-Doctoral Research Associate, Giuseppe Fiore, Graduate Student, University of Bari, Italy, Victor M. Gabri, Staff Research Associate and Graduate Student, Yong Luo, Project Scientist, Ryan Puckett, Greenhouse/Cold Room/Physical Plant Officer and Laboratory Assistant, and Apostolos Papagelis, Agronomist, University of Athens, Greece, John Lake, Laboratory Assistant. All personnel are affiliated presently with UC Davis/Kearney Ag Research and Extension Center, Parlier, Calif. 93648.)
We thank the California Pistachio Research Board and the Almond Board of California (Food Quality and Safety Committee) for funding this research. Also, funding was provided by the USDA/Aflatoxin Elimination Technical Committee and the CDFA/Grant 16-SCBGP-CA-0035. Especially, we acknowledge the extraordinary support of Wonderful Orchards Company, which provided multiple and large-acreage sites for experimentation and hundreds and hundreds of library samples for aflatoxin analyses. It would not have been possible to complete this work and get registration of the atoxigenic strains of Aspergillus flavus without such immense support by this company and its dedicated personnel.
