Heat waves were a major driver of the INSV disaster of 2020. Although researchers have established a link between warmer temperatures and population increases of thrips, science still has a lot to learn about those disease vectors (photo by Richard Smith.)
Although Salinas Valley lettuce growers enjoyed a reprieve from virus pressure in 2021, the plentiful December rains have sparked intense weed growth, providing prime habitat for Western flower thrips, a vector for impatiens necrotic spot virus. The UCCE publication Salinas Valley Agriculture reports that this plant virus caused $100 million in lost gross revenue for Salinas Valley lettuce growers in 2020.
“It’s a blessing, yes, we need the water,” said Tony Alameda, managing partner of Topflavor Farms, which grows a variety of produce in Monterey and San Benito counties. “But, oh gosh, with that water, here come the weeds, here comes the habitat, here comes all the other problems that go along with it.”
The agricultural community called it “the biggest problem we’ve seen in a long, long time,” said Mary Zischke, facilitator of a task force convened by the Grower-Shipper Association to address INSV and a related affliction, Pythium wilt.
Even though 2021 was a “good” year, about one-third of all lettuce plantings in the Salinas Valley had at least a low level of infection, according to Zischke.
INSV on lettuce causes characteristic patterns of chlorosis and necrosis on the inner leaves of the plant as well as significant stunting. However, INSV can cause significant necrosis and lesions on and at the base of the ribs of lettuce plants. Lettuce plants infected with only INSV do not exhibit wilting of the outer leaves of the plant or show root rot or root discoloration.
Salinas Valley Agriculture reports that recent studies have identified several weeds as key reservoirs of thrips, including malva, marestail and hairy fleabane. Mustards, fortunately, appear to be poor hosts for thrips, although their pollen serves as potential food sources.
Aggressive weed management was an important factor in limiting the virus’ spread in 2021. Because weeds recognize no boundaries, managers of non-agricultural lands are being urged to keep their properties as clean as possible, including industrial sites, equipment yards and the edges of roadways, namely U.S. Route 101, which runs through the center of the valley. Some growers have been volunteering to weed their neighbors’ vineyards.
“We’re encouraging everybody, as best they can, to knock down known weed hosts; that’s really critical,” Zischke said.
Heat waves were a major driver of the INSV disaster of 2020. Researchers have established a link between warmer temperatures and population increases of thrips.
Alameda and Zischke point to the breeding of more resistant lettuce varieties as the ultimate solution to INSV.
Southern California avocado growing regions face uncertain water supplies, mandatory reductions of water use and the rising cost of water, making efficient use of irrigation water one of the highest conservation priorities (photo by A. Montazar.)
With funding from CDFA’s Specialty Crop Block Grant Program, UCCE Irrigation and Water Management Advisor Ali Montazar will work on development of irrigation tools and strategies for avocado growers in Southern California.
Montazar, who works with growers in Imperial, Riverside and San Diego counties, said avocado growers in Southern California face uncertain water supplies and mandatory reductions in water use. Improved irrigation and water use efficiency are critical to sustainable avocado production. Efficient use of irrigation water is one of the highest conservation priorities, he added. Proper irrigation management is also crucial to managing tree nutrition as well as preventing Phytophthora cinnamomi or avocado root rot.
Avocado water demand may vary depending on canopy features, row orientation, weather, soil types and conditions, and irrigation practices. Different avocado varieties do not necessarily have different water demands while they might have different responses to water or salinity stress. The negative impact of stress can be different for different varieties.
Montazar said one of the main purposes of this study is to develop information on avocado water demands under different conditions. Deficit irrigation trials will be used to see responses in fruit quality and quantity.
New tools and resources developed in this project will help growers achieve water use efficiency goals, Montazar said.
The research team is collaboratively working with California Avocado Commission and several growers. In this project, a combination of field experiments, case studies and a robust outreach program are used to develop and disseminate information and tools to growers and stakeholders. These tools and information may have a significant impact on water quality and quantity issues, Montazar said, bolstering the economic sustainability of avocado production not only in the well-established production region of Southern California, but also in Kern and Tulare counties where new avocado plantings are growing.
Botryosphaeria is associated with band cankers that infect cracks in the bark and pruning wounds. Two- to five-year-old trees with vigorous cultivars are most affected by this disease (photo by F. Trouillas.)
With 26 fungal species associated with canker disease, it is important for growers and farm managers to determine which pathogen is attacking their almond trees. Field diagnosis plus a laboratory test can reveal the responsible pathogen or point to an abiotic reason for canker formation.
Florent Trouillas, UCCE plant pathology specialist at the Kearney Agriculture Research and Education Center, noted during a presentation at The Almond Conference that in order to use the most effective control options, it is critical to understand which diseases can affect the trunks and scaffolds of almond trees. UCCE Plant Pathologist Themis Michailides and USDA researcher Greg Browne also provided information on control and prevention of canker diseases.
Most infections of fungal canker diseases occur at pruning wounds made for primary and secondary scaffold selection. Fungal pathogens that cause gumming and dieback in almond trees are the leading cause of death in young orchards, Trouillas noted. These pathogens include Botryosphaeria, Ceratocystis, Diaporthe, Eutypa and Phytophthora.
Botryosphaeria is associated with band cankers that infect cracks in the bark and pruning wounds. These cankers often present in a row around the trunk. Two- to five-year-old trees with vigorous cultivars are most affected by this disease.
Phytophthora and Ceratocystis infections also cause gumming on the tree trunk and scaffold branches. Ceratocystis cankers are more elongated and are common in prune and cherry trees. The pathogen is transmitted by insects and affects both young and old trees. Phytophthora infections are notable for their quick development as well as gumming of the trunk and scaffolds.
Eutypa infections begin in pruning wounds or in cracks near the tree crotch.
There are also abiotic reasons for trees to develop cankers. Those include herbicide injury, acid burn and boron toxicity. Foamy canker, marked by copious amounts of reddish gum that flows from the cankers, can be caused by a number of abiotic stressors.
Location on the tree and infection sites can help with a field diagnosis, but Trouillas said a molecular diagnosis is a tool that can now be made in 24 hours using a species-specific primer that targets all canker pathogens.
Michailides noted that band canker in almonds has been on the rise since 2005 and has the potential to kill trees. Bands of cankers develop in a circular pattern around the tree trunk. In severe cases, two to three bands may be present.
Preventative measures in young orchards involve obtaining clean trees from nurseries. Once planted, the fungicide Topsin-M should be applied to the trunks at label rates during the first-, second- and third-leaf years. Avoiding wetting tree trunks during irrigation and protecting pruning wounds with Topsin at label rates is advised.
When band canker is present in young orchards, it is recommended to keep tree trunks dry, apply Topsin-M to trunks and scaffolds and to also protect pruning wounds. Killed trees and stumps should be removed from the orchard and wood piles should not be located near orchard sites to lower inoculum levels.
Best management of perennial Phytophthora Canker (PCC) and Phytophthora crown and root rots (PCRR) involves integrated cultural, genetic and chemical control.
Phytophthora, a ‘water mold,’ is adapted to being spread by surface water and reaching plant roots, persisting in adverse conditions.
Browne said that PCC, which mostly results from scion infections, is most prevalent in mature orchards, while PCRR results mostly from rootstock infections and is most prevalent in young orchards.
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Spotted wing drosophila has a prominent saw-like ovipositor that enables it to lay eggs in undamaged fruit. The larvae are tiny, white cylindrical maggots a little longer than 1/8 inch when full grown. After maturing, the larvae partially or completely exit the fruit to pupate (left photo by Martin Hauser, CDFA;
New information on the invasive fruit fly spotted wing drosophila (SWD) comes from Dr. Artyom Kopp at UC Davis. Published in the January 2022 ANR Blog Strawberries and Caneberries, the information sheds light on origins of SWD, how and why it attacks fruit and possible biological control tactics.
SWD was found in 2008 damaging fruit in many California counties. It infests ripening cherries throughout the state and ripening raspberry, blackberry, blueberry and strawberry crops, especially in coastal areas. It also has been observed occasionally attacking other soft-flesh fruit such as plums, plumcots, nectarines and figs when conditions are right.
SWD, Drosophila suzukii, is native to East Asia and has been established in Hawaii since the 80s, where it is found in large numbers on guava and other fruit. In California, the first SWD were collected in 2008, and by 2009-10, they were a major problem in California, Oregon, Washington and Florida.
Unlike most fruit fly species, SWD has a prominent saw-like ovipositor that enables it to lay eggs in undamaged fruit. The larvae are tiny, white cylindrical maggots a little longer than 1/8 inch when fully grown. One to several larvae can be found feeding within a single fruit. After maturing, the larvae partially or completely exit the fruit to pupate.
All Drosophila species use a mix of sensory cues to identify places suitable for egg-laying. They smell with their antennae, taste with their legs and mouthparts, and measure hardness with their ovipositors. SWD prefer the smell of intact fruit and do not mind hard surfaces.
Spotted wing drosophila prefer the smell of intact fruit and do not mind hard surfaces (photo by Larry L. Strand, courtesy UC Statewide IPM Program.)
Kopp noted that once SWD damages fruit by opening the skin, the fruit is accessible to Drosophila species such as D. melanogaster, D. simulans, and D. hydei. These species are more abundant, more prolific, faster-developing and more heat-tolerant than D. suzukii, so they tend to out-breed it once it provides them with access to the fruit. Kopp said most flies hatching are probably those other species, but it was likely D. suzukii that let them in.
Several generalist wasps native to North America and Europe can parasitize D. suzukii under laboratory and field conditions. Some Leptopilina and Ganaspis wasps, abundant in Asia, seem to be specialist parasitoids of D. suzukii, preferring it to other Drosophila species. They prefer to attack the larvae that are feeding on ripening fruit and cause high mortality in D. suzukii in its native range.
Kopp said use of transgenic tools to introduce artificial constructs can suppress population growth. Transgenic tools for D. suzukii have recently been developed, and several female-killing approaches have been tested in lab trials.
Delaying vineyard pruning until late in the dormant period is effective in reducing fungal infections in grapevine trunks. The delay avoids the highest spore release time of December and January (photo by Jack Kelly Clark, UC Statewide IPM program.)
Extending the profitable lifespan of a table grape vineyard includes management practices to prevent trunk diseases, weed and nematode control.
The UC Integrated Pest Management guidelines for table grape vineyards note that delaying vineyard pruning until late in the dormant period is effective in reducing fungal infections. The delay avoids the highest spore release time of December and January.
Preventative practices are most valuable in young vineyards, but they have also shown some efficacy in older vineyards with prior trunk disease infections. The guidelines report that post-infection practices, including sanitation and vine surgery, are more expensive than preventative practices adopted early in a vineyard’s life. Those include delayed pruning, double pruning and use of pruning wound protectants.
Dormant-season sanitation practices include destroying prunings of older, infested wood to reduce pest sources. Dried grape clusters should be removed from vines and weeds disced to remove overwintering sites for orange tortrix or omnivorous leafroller. Where branch and twig borers have been a problem, old pruning scars and dead parts of vines should be scouted for evidence of brown frass and wood dust. Where vine mealybug has been found in a vineyard, care should be taken to prevent spread of this pest to uninfested areas of a vineyard by sanitizing machinery.
Weed control is essential in young vineyards until canopies are large enough to shade out competition. Surveys can allow for identification of weeds that escaped control in the last growing season and to determine which perennial weeds are present. Finally, surveys help with choosing appropriate herbicides of cultivation.
Cultivation may be preferred to herbicide use in young vineyards to prevent injury to vines. In mature vineyards, herbicide application in the vine row should be used together with mowing or cultivation between the rows. In the absence of herbicide applications, mowing may be needed when weeds exceed six to eight in height. Cultivation in the rows may be necessary after irrigation causes weed seeds to germinate.
In the San Joaquin Valley, sampling soil for nematodes is done from November to February when X. index ordagger nematode populations are most likely to be detected. Root knot nematodes are found at any time of the year. Soil samples should be sent to a diagnostic laboratory for identification.
Xiphinema index can cause yield reduction in some varieties but is more important for its transmission of grapevine fan leaf virus, the cause of grapevine fan leaf degeneration disease.
Cause of fresh peeling on young almond tree branches was found to be from poling damage at harvest. A UCCE research associate reported the damage was probably due to the suppleness of the wood due to the trees’ age, irrigation making the wood swell, or a combination of the two, increasing the chance of damage from a pole (photos by C. Zuber.)
An investigation into mid-canopy wood peeling in young almond trees by Cameron Zuber, a UCCE staff research associate in Merced County, found pole harvesting to be the likely culprit.
Zuber reported that fresh peeling of tree branches was typically seen in the fall and winter periods. Only a small portion of the branch was affected, and they were mid-canopy branches. The peeling exposed a large area of the inner wood and there was concern that it could be a site of infection.
In September 2021, fresh peeling was observed in a third leaf almond orchard in Fresno County with peeling ranging from the bark to sap wood layers. The peeling was found in about a quarter of the trees on the outer and inner canopy branches. Zuber said the outer branches seemed to have more damage.
He said the damage was new as the orchard was under observation to collect information for almond production research. The orchard observations included walking the entire orchard site and working within the tree canopy. The peeling was not present until it was observed in September.
The puzzle was that no orchard activity had occurred in the orchard that could explain the wood peeling. When it was learned that harvest at the orchard was done with a poling crew using metal poles and tarps to collect the nuts, Zuber began to examine the practice. It was done, he said, due to the young age of the trees and concern about shaker damage. The research being conducted at the site also required that nuts be collected from specific sets of trees.
Given the timing of the pole harvest and peeling damage, this was the most likely cause. The height of the peeling also lined up with the main area the poling crew was hitting trees the hardest at.
The reason for the peeling may be due to the energy from a hard-hitting pole causing fractures to form parallel to wood fibers. Tension from the undamaged fibers pulled this fracture and an opening formed, thus exposing the inner layers of wood. If the pole harvesting caused this peeling, it provides insight into the fresh peeling seen at other locations during the fall and winter since poling is also used to perform sanitation.
Zuber said that the wood peeling possibly caused by the pole harvest in the summer does not tell whether pole sanitation in the fall or winter would result in the same type or amount of damage. The high amount of peeling from pole harvesting a third-leaf orchard was probably due to the suppleness of the wood due to the trees’ age, irrigation making the wood swell or a combination of the two, increasing the chance of damage from a pole.
Cause of fresh peeling on young almond tree branches was found to be from poling damage at harvest. A UCCE research associate reported the damage was probably due to the suppleness of the wood due to the trees’ age, irrigation making the wood swell, or a combination of the two, increasing the chance of damage from a pole (photos by C. Zuber.)
The American plum borer larvae is one pest to be on the lookout for in spring (photo by Jack Kelly Clark, UC Statewide IPM Program.)
Prune bloom time will soon be here. UCCE orchard systems farm advisors have noted in Sac Valley Orchard Source that there are several management options that should be considered at this time of the year, though timing can vary depending on growing region.
If the forecast is for cold weather at bloom, tall weeds or a cover crop will cause colder orchard temperatures than a mowed floor. Newly disked soil provides the coldest conditions. On the other hand, if temperatures reach into the 80s during or soon after bloom, sprinklers can help reduce orchard temperatures one or two degrees.
Protecting this year’s crop from fungal disease begins at this time. To prevent brown rot, a single bloom spray is recommended at 25% to 40% bloom. Dew can provide enough moisture for brown rot infection and a second spray may be necessary. If rain is forecast during bloom, a spray at white bud and again at full bloom is advised. The full bloom spray is critical. A scab material can be included in the tank.
Significant rainfall during or right after bloom can trigger russet scab. Once the fruit is through the jacket, risk is minimal.
Monitoring for peach twig borer should be done during and after bloom. If this pest is present, chewing on buds is evident. Place pheromone traps at two per block no later than mid-March to determine biofix. If moths are caught on two consecutive trap checks, begin accumulating degree days.
For aphid control, two 440 oil sprays at four gallons per acre at bloom can be used to control mealy plum and leaf-curl plum aphids. Applications should be at slow ground speeds seven to ten days apart. For bee safety, alternative management timing avoiding bloom should be considered.
Weed control should also be part of an early spring management plan. Rotating or mixing herbicides with different modes of action will help with herbicide-resistant species.
Brad Hanson, UCCE weed specialist, has organized a chart to help, with herbicide name, a common trade name, the site of action group and the crops for which an herbicide has been labeled for use. Labels may have changed.
Delayed and erratic bud break from DSG vines (courtesy G. Zhuang.)
After 2021 grapevine budbreak, we received many calls about dead spurs, delayed bud break, stunted shoot growth and poor fruit set. In Fresno County, some severely impacted vineyards suffered a substantial yield loss. In many cases, the problem was delayed spring growth (DSG), and the classic vine symptoms include:
Delayed and erratic bud break
Stunted shoot growth
Excessive berry shatter and poor fruit set
The situation was apparently spread across different grape growing regions in California, and UC Davis Department of Viticulture and Enology held a virtual grower meeting to discuss it (the recorded presentation can be found on the UC Davis AggieVideo website.)
Delayed Spring Growth
Grapevine DSG is associated with insufficient rehydration of the vines and may be due to vascular tissue injury, insufficient carbohydrate reserves, excessively dry soil over winter or some combination of these factors. Symptoms can result in significant yield loss and permanent vine damage, resulting in economic hardship for growers. Some vine DSG symptoms are similar to other pest/disease symptoms (e.g., vine trunk disease or soil pests like nematodes/phylloxera.) However, most vineyards we visited had little or no sign of trunk disease or soil pests. Several factors this past fall and winter contributed to widely observed and severe vineyard DSG symptoms:
Ongoing drought and increasingly dry soils, especially over winter in vineyards which were not sufficiently irrigated postharvest, or during winter
Warmer than normal fall temperatures, including a particularly warm October
A sudden freeze in early November
According to CIMIS station data at Five Points, October 2020 was warmer than the last five years’ average and followed a sudden freeze event in November (Figure 1), and warmer-than-normal autumn is a risk factor for DSG. In addition to the November freeze event, October and November 2020 were mostly dry, and a drier autumn could make the freeze worse. Even though the minimal temperature of the 2020 winter might be lower than the last five years’ average according to the CIMIS station data, the DSG’s occurrence and severity varied significantly across vineyards in Fresno County and other parts of California.
Figure 1. Daily minimal temperature from September 2020 to April 2021 at CIMIS Station near Five Points, Calif.
Also, vineyard management, particularly postharvest and winter irrigation, could make a big difference on the results of DSG even if the ambient weather condition was similar. The geographic location as well as vineyard microclimate can sometimes mean quite different consequences in the face of freeze damage. Figure 2 illustrates the variation of daily minimum temperatures from five locations in Fresno County. Typically, vineyards located on the west side had a lower minimal temperature and suffered more freeze damage than vineyards located on the east side. Sand Ranch in particular had the lowest daily minimum temperature among five locations from October to March.
Figure 2. Daily minimal temperature from September 2020 to April 2021 at five UC IPM weather stations in Fresno County.
To make matters worse, Fresno County saw much less precipitation during the months of November and December 2020 than the last 20 years’ average (Figure 3). These drier months might offer the perfect conditions for DSG. Although precipitation amount was normal in January 2021, February was yet another dry month in comparison to historical averages. Lack of soil moisture before bud break is another major risk factor for DSG.
Figure 3. Monthly precipitation from October 2020 to March 2021 in Fresno County.
Grapevine winter freeze damage and DSG have similar symptoms and can be difficult to differentiate. Winter cold damage or freeze injury damages vascular tissues and can thus interfere with water, carbohydrate and mineral translocation, causing symptoms similar to DSG. A lack of soil moisture can impair vine rehydration, making vines suffer water stress and causing DSG symptoms directly. Additionally, vines might be more vulnerable to cold injury even though the minimal temperature in the past winter, such as 25 degrees F at Sand Ranch, might not cause significant freeze damage on most Vinifera grapes.
Maintain Vine Health
Vineyard conditions should be considered to avoid DSG and possible cold damage:
Abiotic or biotic stressed vines (e.g., severe water stress and overcrop, nutrient deficiency, pest/disease)
Young vines
Late ripening and cold tender varieties
Certain rootstocks, including Freedom and Harmony
Insufficient soil moisture during the dormant period (e.g., October to March in the San Joaquin Valley)
Generally, maintain vine health over the growing season and assess soil moisture as the vines enter dormancy, watering if needed. Too many clusters with not enough leaf area can weaken the vines and deplete the trunk and root’s carbon reserves, which are needed to maintain respiration over winter, help prevent freezing and nourish the vines as they regrow in the spring. To maintain a functional vine canopy, irrigate as necessary to support photosynthesis without stimulating excessive growth. If pests or diseases are present in the vineyard, such as powdery mildew, nematodes, grapevine trunk disease, virus, mites and leaf hopper, a good assessment of canopy health is important. Grapevines with severe defoliation or small canopies will be of great concern, and management should focus on better addressing pest and disease problems to avoid early defoliation.
A young vine has its inherent nature of vulnerability due to a lack of sufficient carbon reserves. Therefore, severe water stress and overcropping should be avoided, and irrigating the soil before a freeze event (e.g., late October and early November) can be greatly beneficial to provide heat protection for young vines.
This past spring, we noticed some susceptible varieties might suffer greater damage from DSG, and that has been consistent with the reports from other growers. Chardonnay and Pinot gris have been reported frequently on DSG, although both varieties are also susceptible to winter freeze.
Rootstock can also play an important role in DSG. Certain rootstocks (e.g., 5BB and Freedom) are more susceptible to DSG than others (e.g., 1103 P), according to results of UCCE rootstock field trials in different growing regions of California. Thus, growers who have the susceptible rootstock might want to take extra care of the vines, such as irrigating the soil during the dry winter, so that the risk of potential DSG might be minimized.
Manage Soil Moisture
Last but not least, lack of soil moisture might be the most important yet manageable factor contributing to most DSG farm calls. As discussed previously, drier October, November, December and January months posed a great risk of DSG as well as inhibited rehydration of the vines, which can also lead to a greater risk of freeze damage. However, water availability during the drought years might be significantly reduced or expensive.
Therefore, irrigation during the drought years can become the dilemma. Growers need to balance the cost and reward of irrigation vs. no irrigation during drought years. Greater than 20% yield loss has been reported for some vineyards in Fresno County in 2021, and DSG might play a large role in it, although the record summer heat and seasonal variation could also result in loss. Finally, the consequence of DSG on Fresno vineyards varied greatly. Some vineyards appeared to be significantly stunted after budbreak and later fully recovered due to irrigation. Some vineyards might suffer multi-year yield loss due to the weakened canopy and few desired canes to prune.
In the face of upcoming potential drought, growers can use multiple tools to reduce or eliminate the effect of soil moisture deficit. Many tools (e.g., shovels, soil augers, moisture sensors) can provide great benefits for assessing soil moisture and help growers determine whether or not to irrigate. Weather stations can also provide great amounts of information regarding the minimal ambient temperature as well as the amount of local precipitation, since temperature and precipitation can vary greatly from one vineyard to another. UC IPM has seven weather stations in Fresno County and one station in Madera County in cooperators’ vineyards, and those stations can offer both temperature and precipitation amount, serving the growers whose properties are nearby the station.
Though vineyard managers continually face many challenges to optimal productivity, Pierce’s Disease (PD) represents a particularly formidable threat due to limited options for effective prevention and control. PD is a degenerative, deadly and costly disease of grapevines caused by Xylella fastidiosa subsp. fastidiosa (Xff) bacteria, Gram-negative rod-shaped microbes with characteristic large pili. Though hosted by many plant species, these bacteria are easily spread to grapevines by insect vectors such as blue-green and glassy-winged sharpshooters (Figure 1, see page 41). Xff colonize the gut of sharpshooters and are transmitted to grapevines as the insects feed on vines.
Pierce’s Disease: A Major Threat
Once inside a grapevine, Xff bacteria impede the normal function of xylem tissue (transport of water and nutrients ‘up’ from roots to stems and leaves.) This damage induces the characteristic chlorosis and scorching of leaves, causing early symptoms of PD which mimic water stress. However, the insidious and cumulative damage caused by PD eventually kills entire vines in one to five years.
Figure 3. Viral bacteriophage particles of XylPhi-PD precisely targeting their bacterial host (photo courtesy Inphatec.)
PD represents a major threat to U.S. wine regions, accounting for widespread economic damage (e.g., roguing and replanting of vines, low fruit production, etc.) and costly deployment of resources aimed at disease moderation. PD has been reported in 28 California counties, covering most of all wine-producing regions. State Extension teams in Texas, Arizona and North Carolina have reported significant outbreaks in 2021. Lost production and vine replacement has been estimated to cost grape producers about $56.1 million annually as of 2014. Further, a 2016 survey of nearly 200 growers and managers in Napa and Sonoma counties revealed that 73% of respondents identified PD was one of their top three management problems.
Figure 1. Blue-green sharpshooter (top) and glassy-winged sharpshooter (bottom), two of the main insect vectors for spread of Xff (photos courtesy Inphatec.)
Few methods for controlling and treating PD have been made available, with efforts historically focused on controlling the sharpshooter vector (e.g., insecticides, trapping, monitoring, inspections) or roguing seriously ill vines (rogue, replant), all of which has achieved only limited success. However, a new option that reduces PD in grapevines is now available.
New Bacteriophage Injection for PD
XylPhi-PD is a novel, OMRI-listed, biological treatment for PD, a cost-effective break-through technology developed exclusively for viticulture by A&P Inphatec (Figure 2, see page 32). XylPhi-PD contains a cocktail of viral bacteriophages (Figure 3, see page 32) that are injected into a grapevine (‘bacteriophage’ are bacteria-killing viruses that selectively infect bacteria but do not infect the eukaryotic cells of plants or animals.) These virus particles enter and destroy Xff bacteria, thus limiting bacterial growth and the xylem-clogging damage to the plant. Hundreds of phage particles can be manufactured inside an Xff bacterial cell after infection by a single phage particle. The Xff bacterial cell eventually dies and releases all newly created phage particles to seek and destroy more Xff bacterial cells (Figure 4).
XylPhi-PD applications are made by injection of the product into the vascular system of grapevines. Applications are made directly into the active xylem tissue of the plant using a pressurized injection device, the Xyleject Injection System (Figure 5).
XylPhi-PD can be flexibly applied as a treatment when disease symptoms appear, as a preventative to protect growing vines, or whenever conditions may lead to disease. As with most any disease situation, disease prevention or treatment early in the disease process provides much better outcomes than treatment of later-stage severe infections. XylPhi-PD is available in 100-mL vials (treats up to 300 mature vines or 600 young vines.) The product has no restricted-entry interval (REI), requires minimal personal protective equipment (PPE) when used in accordance with label directions and is approved for use in organic production (OMRI-listed, Organic Materials Review Institute).
Efficacy Overview
Multiple studies have been conducted to support the development and commercialization of XylPhi-PD, and results have provided much insight regarding the product’s efficacy profile and use strategies. Brief overviews of some of these studies follow:
Greenhouse pilot study (Texas A&M, 2014):
Design: 30 greenhouse grapevines inoculated with Xff. 15 vines treated with XylPhi-PD once at three weeks post-inoculation, 15 vines received only buffer. Visual symptoms of PD assessed 12 weeks post-inoculation.
Results: XylPhi-PD reduced PD incidence by 87%.
Natural infection field trial (Texas A&M, 2015):
Design: 30 Chardonnay and 30 Cabernet Sauvignon vines in an area with high natural PD pressure randomly assigned to each of the four groups. Vines treated zero, one, two or three times with XylPhi-PD during the summer.
Results: Disease incidence in the XylPhi-PD-3X group was significantly reduced by 44% (P = 0.047) compared to controls (10% vs 18%). Activity of vectors positive for Xff was high during the trial period.
Figure 4. Death and rupture of a bacterial cell, releasing newly created phage particles to seek and destroy more bacterial cells (photo courtesy Inphatec.)
Challenge studies, prevention and therapeutic treatment (California University, 2017):
Design: Prevention study – 15 Chardonnay and 15 Cabernet Sauvignon vines treated with XylPhi-PD and then challenged twice with Xff.
Therapeutic study – 30 Chardonnay and 30 Cabernet Sauvignon vines/group challenged twice with Xff and then treated zero, one, two or three times with XylPhi-PD.
Results: Prevention – prechallenge XylPhi-PD significantly reduced incidence of PD symptoms by 75% in Cabernet Sauvignon vines (P < 0.10) and 100% in Chardonnay vines (P < 0.05) vs controls.
Figure 5. Injection of XylPhi-PD into trunk of mature grapevine. Xyleject Injection System (Pulse Biotech, LLC; Lenexa, KS)
Therapeutic – three post-challenge XylPhi-PD treatments significantly reduced incidence of PD symptoms by 90% in Cabernet Sauvignon vines (P < 0.05) and 77% in Chardonnay vines (P < 0.10) vs controls.
Multi-year natural infection field trial (2017-20; Sonoma, Calif.):
Design: Two groups of healthy Zinfandel vines were tracked in a high PD-pressure, organic vineyard for three seasons (Ridge, Lytton Springs). One group (n=71) received XylPhi-PD three times/summer, while controls (n=94) only received buffer.
Results: After three years of treatment, vines treated with XylPhi-PD show much less PD incidence than control vines as assessed by both qPCR (-60%) and visual PD symptoms (-72%). Vines treated with XylPhi-PD also generated higher fruit yields, averaging +1.34 lb/vine (+21%) more than control vines.
4-site, 3-year, Natural Infection Field Trial (2019-21; Sonoma, Calif.):
Design: A three-year, multi-location commercial (Wilbur-Ellis) field study evaluated the efficacy of XylPhi-PD against endemic PD across four sites and three production seasons. The extensive research effort began in 2019 when a study was conducted that involved 400 vines (300 Chardonnay, 100 Pinot Noir) at three Sonoma County commercial wineries with a history of PD (one winery had two test fields) (Figure 6). All four commercial vineyards were historically high PD sites, and despite continual roguing and insecticide use in the past, a persistent reservoir of Xff remained in the vineyards from previous infection cycles. Thus, each site included vines with both early-stage and chronic/severe PD.
Figure 6. Locations of four sites used for three-year field study.
Vines were randomly selected in treatment blocks at each site and assigned to either of two treatment groups as follows:
-Control (untreated): n=200 (50/site);
-XylPhi-PD: three treatments (Jun/Jul/Aug); 80 µL of XylPhi-PD injected twice in the trunk and once in each cordon (4 to 6 injections = one treatment); n=200 (50/site).
Six petioles from each vine were collected in September for analysis by quantitative polymerase chain reaction (qPCR) and confirmation of Xff infection. All study vines were also visually assessed by trained observers for PD development. Insect traps were placed at each study site in an attempt to monitor vector pressure.
A continuation of the same study protocol was followed in 2020 and 2021, allowing two additional seasons of treatments and observations for the same vines/blocks at the same sites/wineries. In addition, treatments and observations of additional vines were initiated in 2020 at each site (n=50/site, 200 total), repeated again in 2021 (Figure 7). As a result, three groups emerged from the study for tracking and evaluation:
Figure 7. Study design and timeline for three-year multi-site field study.
Vines with three years of treatment (three-year treated, n=200)
Vines with two years of treatment (two-year treated, n=200)
Non-treated controls (n=200)
Results
Comparative outcomes for vines qPCR-positive for Xff are summarized in Figure 8, see page 36. Under the conditions of only mild to moderate PD pressure and low vector populations, sequential year-to-year use of XylPhi-PD generated impressive results. Incidence of Xff positivity fell 24% and 45%, respectively, for vines receiving two or three years of treatment compared to untreated controls. Improvement in the three-year group was significant (P < 0.05) vs controls. The few vines remaining Xff-positive in the two- and three-year groups were chronic infections that would be rogued.
Figure 8. Vines qPCR-positive for Xff, or vines showing visual signs of PD. Summary of four sites in Sonoma County.
The visual assessment of vines for signs of PD was another important study parameter, and outcomes (Figure 8) were similar to those using qPCR confirmation of infection. Vines treated with XylPhi-PD for two years generated a 27% reduction in visual PD incidence compared to controls, while vines treated for three years showed double the benefit, a significant 54% reduction (P < 0.05) of PD incidence. The similarity of these data with qPCR results suggests that visual assessments can help vineyard managers tangibly gauge the efficacy of XylPhi-PD.
Figure 9. Average fruit yield for Chardonnay vines at one site (D).
Notably, XylPhi-PD continued to protect against PD infections at all four trial sites for the three years of observations, with no new infections detected in the three-year treated group. In contrast, the control group had ~2% to 4% new infections.
Fruit yield was measured at one study site (D) at the study’s conclusion in 2021 (Figure 9, Chardonnay, eight- to ten-year-old vines). Compared to untreated controls, vines in the group treated with XylPhi-PD for three years averaged 5.1 lb (+17%) more fruit per vine than controls. Vines treated for two years were intermediate (3.1 lb, +10% more fruit vs controls).
Usage Recommendations
XylPhi-PD can be applied as a preventive treatment to protect growing vines, as a therapeutic treatment when disease symptoms become visible, or anytime production conditions may lead to disease pressure.
Table 1. XylPhi-PD dosage options and number of applications per vial (100 mL).
Locations selected for application of XylPhi-PD can be based on the age of the plant, the pruning style and/or the training system utilized for the plant. The product is to be injected into the active xylem vascular tissue above ground level. Two examples of injection strategies appear in Figure 10, see page 37. On established vines, for example, one or two injections can be applied in the trunk with an additional injection in each cordon or spur. For young, recently planted or radically pruned vines, apply two or three injections into shoot, two to six inches above the ground. For all scenarios, the total number of injections administered to a vine define one ‘application’ of XylPhi-PD.
For most production situations (medium to high PD pressure), two or three applications of XylPhi-PD are recommended during each growing season at near-monthly intervals (Figure 11, see page 37). This frequency of application has been demonstrated to provide optimal PD control under various levels of PD pressure. The volume of XylPhi-PD administered can also vary depending on the age of plants being treated and PD pressure.
Figure 11. Examples of XylPhi-PD application programs involving medium to high PD pressure.
The quantity of XylPhi-PD used in XylPhi-PD treatment programs can vary based on the number of injections/vine, the concentration of product/injection and the number of applications/year. Growers and PCAs have options and flexibility to match doses and the number of applications to their specific conditions and risks. Table 1 summarizes some of these options and their impact on the amount of XylPhi-PD used and number of vines treated per 100-mL vial.
Treatment Strategies for PD Management
Several recommendations for managing PD across an entire vineyard have emerged from field use experience with XylPhi-PD. These recommendations largely depend on the scope and distribution of PD in a particular vineyard or block.
Figure 12. Strategies and options for treating areas within vineyards or blocks.
As usual, vines demonstrating severe and/or chronic PD infection should be rogued per IPM protocols.
Vines appropriate for XylPhi-PD treatment should be identified.
Figure 13. Examples of visual damages caused by PD (photos courtesy Inphatec.)
These appropriate vines should be treated with recommended doses of XylPhi-PD at two or three seasonal applications as needed for mature vines or replants (per label directions).
It is the second step, identifying which vines to treat, that can sometimes pose a quandary for production managers. To help in this process, three strategic options can offer direction for developing a customized plan for vineyard-wide PD management. Based on evaluations regarding the spread of infection and site specifics for a particular vineyard or block, a treatment strategy can be selected from the following (Figure 12):
Targeted buffer zone: Treat all vines in any defined area with high PD activity (>2% symptomatic vines) and/or vector activity, such as near a riparian area.
Precision spot: In large areas with few symptomatic vines, mark/identify affected vines and treat only them and their immediate surrounding neighbors.
Entire block: In large areas with multiple scattered symptomatic vines, in the presence of vectors, full-block treatment is needed to reduce overall PD pressure.
In addition to these three options, the duration of treatment (multiple years) is also a critical consideration. As discussed earlier, field studies have demonstrated the cumulative value of consistent treatment with XylPhi-PD over multiple years for reducing the level of PD in a vineyard or block, even when under low PD pressure. As always, prevention of severe, chronic disease is the best approach, and consistent long-duration use of XylPhi-PD appears to substantially diminish PD progression and pressure in vineyards.
Identifying PD
Some of the visual signs of PD damage are presented in Figure 13, including characteristic chlorosis (leaf scorching), irregular lignification and berry shriveling, and ‘matchstick’ petioles. In general, PD is likely present in a vineyard if the following four symptoms are observed late in the season:
Leaves scald in concentric rings or in sections
Leaf blades abscise, leaving petioles attached to the cane
Bark matures irregularly
Fruit clusters shrivel or raisin
However, damage caused by various other diseases, water stress, pests or nutrient deficiencies/imbalances may produce symptoms similar to those caused by PD. Therefore, PD must be laboratory confirmed by detection of Xff by qPCR testing on late-season/fall petioles (e.g., UC Davis Foundation Plant Services, Texas Plant Disease Diagnostic Lab, Arizona Plant Diagnostic Network).
Conclusions
PD is an extremely challenging problem for wine producers and their consultants. A biological control approach with XylPhi-PD offers a fresh opportunity to help manage PD and limit losses associated with the disease. In multiple studies, XylPhi-PD treatment of diverse wine varietals prompted reductions in PD incidence and/or severity under conditions of both natural and challenge infection with Xff. These favorable outcomes distinguish XylPhi-PD as a targeted and cost-effective strategy for effectively protecting valuable vineyards against PD.
Always read and follow product label directions. Not registered in all states. EPA Reg. No. 93909-1. Operators of injector must undergo training and be certified by Pulse and must follow instructions in device manual. XylPhi-PD is a trademark of A&P Inphatec. Xyleject is a trademark of Pulse Biotech.
