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Improving management of grape powdery mildew with new tools and knowledge.

Many vineyard managers observed in 2013 that grape powdery mildew (caused by Er ysiphe necator) could explode from seemingly being nonexistent to looking like a 3-year-old child was playing with a bag of flour. This explosion is in part due to the pathogen's reproductive potential, and that the disease is extremely hard to detect below 1% leaf incidence without very expensive (e.g. 1,000 leaves per acre) weekly scouting.

Since E. necator can have a generation of five days and produce more than 100,000 spores per day from a colony, a single colony can become 10 million colonies within a month, even if only one in 10,000 spores successfully infects grape tissue. Couple this with developing shoots and laterals rapidly producing susceptible tissue between fungicide applications, and it is easy to see why grape powdery mildew (GPM) epidemics are so difficult to control.

In a previous PWV article (spring 2011) we presented research on the use of GPM inoculum detection to initiate fungicide applications and the rationale behind using this approach. Basically, 17 years of research (1).(2) has shown that significant ascospore release can occur when the temperature is above 45[degrees] F and the bark is wet. In numerous regions, these conditions occur when there is no susceptible host tissue available and the spores do not survive once released in absence of susceptible host tissue. This effectively reduces the amount of overwintering de-istothecia that can cause infection in the spring.

We have been studying the delay since 1997." We first observed that there was a delay in the release of ascospores from the remaining overwintering cleistothecia such that release did not occur even though conditions were suitable until a period of warm weather occurred. This indicated that some portion of the overwintering cleistothecia was not mature enough for ascospore release to occur and suggested that we did not understand the biology of E. necator overwintering. This lack of understanding led to a closer examination of the development of cleistothecia as it relates to timing of ascospore release.

In order to address this question, a group under the leadership of Gary Grove (Washington State University) and Walter Mahafee (USDA, Oregon) began developing molecular techniques to monitor pathogen dispersion of grape and hop powdery mildew.

Over several years, we demonstrated that these techniques were specific and sensitive (detect 1-2 spores of E. necator) and were useful in monitoring for the presence of airborne inoculum early in the season. We also showed that delaying the initial fungicide application until airborne E. necator is detected resulted in saving an average of 2.3 applications during a seven-year period without increasing disease 'development.

Spore trapping in California

Beginning in 2011, both Doug Gubler's group (University California, Davis) and Seth Schwebs (Coastal Viticultural Consultants) began examining the utility of spore trapping in various California grapegrowing regions. Schwebs began exploring how to commercially implement this technology. Weekly trap sample collection occurred during normal vineyard visits for monitoring soil moisture and vine stress. This approach significantly reduced the labor and travel needed for sample recovery and made the sampling economically feasible.

In 2013, as in 2011-12, there was a high degree of variability among vineyards in the amount of powdery mildew inoculum detected (Figures 1 and 2), which suggested two different approaches to managing the epidemic. Some vineyards were able to reduce the number fungicide applications.

For example, the vineyard in Figure 1 was treated with one early spray (about 3-4 inches of shoot growth) containing 1% mineral oil but not again until late May due to spore trap data, which resulted in one less fungicide application than the five they typically applied.

The second approach was to alter the fungicide program. Based on the spore trapping data, Coastal Viticultural Consultants made suggestions to either change the timing or type of fungicide used at specific times in the season.

For example, the Chardonnay vineyard in Napa, Calif. (Figure 2), had a significant spore load all season long, which resulted in Coastal Viticultural Consultants making recommendations to adjust sprayer speed and application intervals and alter cultural practices. Both of these approaches resulted in better disease control than in previous years.

Francesca Hand (Doug Gubler's group), in conjunction with local crop production advisors, tested the inoculum detection technology in Fresno, Napa, Monterey, Sacramento, Solano and Sonoma counties of California in 2010-12.

Unlike in Oregon and Washington, the traps were only collected on a weekly basis and shipped to their lab in Davis, Calif., for processing. They found that spore detection occurred prior to visual symptoms. Similar to observations in Oregon, they did not find a significant relationship of disease incidence or severity to spore density.

The lack of correlation in California and Oregon is likely a function of how weather impacts conidia development and not just the amount of infected tissue. This suggests that knowledge of spore concentration throughout the growing season could be useful in more accurately estimating risk of disease spread and whether fungicide application intervals need to shorted or extended.

It also indicates that we need to significantly improve our understanding of the factors that impact spore formation and dispersion in order to efficiently manage plant diseases. Our current research on spore dispersion shows that wider tractor rows cause more spores to be lost to deposition onto the vineyard floor. This indicates that wider spaced vineyards have less risk to disease development that tightly spaced vineyards under the same conditions.

The California results further indicate that monitoring of inoculum availability may be useful in improving disease management of grape powdery mildew.

What does a trap represent?

The short answer is we do not know. It could be a single vine or hundreds of acres, and it literally depends on which way the wind is blowing and how fast.

The size and origin of the air mass transporting the spores will change with wind speed and direction. This air mass interacts with terrain features and canopy architecture (vine-row spacing, height and training system) to cause air turbulence. This turbulence is responsible for the number of spores in the air mass moving over the trap since it drives spore release, deposition and how high the spores are lifted into the air mass above the canopy (Figure 3). The turbulence is also likely to impact the spread of some insects and the disease they spread (such as mealybugs).

Pathogen infection, growth and sporulation are also governed by the microcli mate created by all the above interacting factors and local weather patterns. While this complexity would seem to indicate that one cannot rely on the trap results, we have found that strategically targeting trap placement can still yield actionable information.

In the early spring, when ascospore release is the most likely inoculum. source, the vineyard area represented by a trap is not really a concern for most vineyards.

Traps should he placed in regions of the vineyard that were highly diseased in the previous year (look for cane scaring closest to the spur or head) or placed in a vineyard with a history of disease showing up at the beginning of each new growing season. We found that these locations had the greatest probability of high levels of overwintering inoculum and serve as "canaries" for when ascospore release does occur.

As the canopy develops and the potential for longer distance spread of the pathogen from other vineyards increases, the sampling head should be placed into the turbulent mixing layer at the top of the canopy. One might even want to move the trap, if there has been no detection at the current location, to a block in your vineyard that is perennially the first to have observable disease or regions that have the most fruit damage the previous season. These locations likely have terrain features that cause more spores to be preferentially deposited from external inoculum sources.

An example of this phenomenon is a site in the northern Willamette Valley of Oregon. A trap placed in one block was repeatedly testing positive, but no disease could be found in the block despite looking at thousands of leaves. Abandoned vines were found up wind and were likely the source of E. neca for being detected in the trapping results. Since the offending inoculum source was not from the vineyards he managed, his only knowledge of the problem was from the spore detection results. The grower has significantly reduced the fruit infection in this block using the spore data to shorten spray intervals, thus managing the external inoculum source. This problem block is now a reserve block.

Grower innovation

During the testing of the spore trapping in Oregon, we gave growers estimates of spore concentration whenever there was a positive detection. One organic grower started using these numbers to help him decide the fungicide application interval all season. He would decrease the application interval when the spore concentration was increasing or extend it when the concentration was decreasing or remaining low or not detectable. Over the course of two years, he was able to save one to two additional fungicide applications.

I LI In 2013, we began exploring whether this approach was commercially feasible at six different Oregon vineyards. Paired blocks were established with each grower, where one block was maintained according to the grower's standard program and the first fungicide application delayed for the paired block until inoculum detection and then the spray interval was adjusted based on the spore concentration.

Results from 2013 in Oregon were inconclusive, since only one vineyard reached an action-threshold that altered the application interval. However, the experience of Coastal Viticultural Consultants indicates that this approach has value in improving disease control in California.

In 2013, Oregon growers still made two fewer applications to the blocks managed based on the inoculum detection data compared to their standard program with similar disease control. Since a typical fungicide application can cost $50 -$80 per acre depending on terrain and material sprayed, a reduction of two sprays realized by all growers in Oregon (approximately 35,000 acres) equates to saving the industry $6 million each year! models for how air turbulence impacts pathogen dispersion.

Adapting a modeling approach developed to predict particle movement in urban environments, we have begun investigating how canopy architecture (vine row spacing and leaf area) influence air turbulence and particle movement in vineyards. Using these models we have predicted several phenomenon that have been observed in vineyards.

For example, GPM disease foci tend to be elongated along vine rows (particularly in vineyards with wide tractor rows). The models indicate that this elongation is caused by eddies that form and move up and down the wider tractor rows. The eddies deposit spores onto vines further down the same vine row or on the ground and rarely eject them into the turbulent mixing layer for long distance dispersion.

Data on the spatial distribution of E. necator clones from Michael Milgroom's group in Cornell (New York state), also supports the modeling results. They showed that clones of E. necator were located mostly along vine rows or several vine rows away. These models have the spatial resolution to predict E. necator deposition at a scale smaller than a vine and we are developing them to run on a laptop or over a server that is accessed with a smart phone.

The future Where is this all going? This research is currently moving along two paths that will eventually merge. We are working with robotic engineers to automate spore trap placement and collection and add other vineyard monitoring sensors to the traps. This research is also helping us develop a modeling system that could become a vineyard simulation environment that growers could use to test management decisions before applying them in the field.

Can you imagine plugging in a few management options into your smart phone and seeing the differences in how a simulated vineyard develops? How about spending a few weeks running simulations training to manage a new vineyard or bring a new manager up to speed?

We realize this all sounds a bit farfetched; however, how many of you would have guessed that that the smart-phone in your pocket has the processing power of an $8 million Cary-1 supercomputer in 1976? The speed of technology development continues to increase and will likely make the goals above seem short sighted in 20 years.

Caption: Spore trap with solar panel (SP) and adjacent temperature/humidity datalogger (THD) in a California vineyard. The aluminum sampling arm (SA) is placed at the top of the canopy in the turbulent mixing layer where spore movement is the greatest. A sealed lead acid battery (B) provides power to sampling arm.

Caption: The relationship of the annual cycle of the grapevine (green and orange boxes) and pathogen (blue boxes). Both the pathogen and host are dormant in the winter and emerge in the spring when conditions are favorable. The powdery mildew epidemic begins when either ascospores or conidia from flagshoots are dispersed from the bark or infected buds, respectively, and infect grape tissue. The infections develop and begin producing conidia within as few as five days. The conidia are released and begin another cycle. There can be up to 35 generations during one growing season. with each colony producing spores for up to 35 days. In the fall, the pathogen forms cleistothecia, which overwinter on grapevine bark in most regions. Sometimes the pathogen can infect grape buds before veraison and overwintering in the buds. These infected buds become flagshoots in the spring.

Caption: Powdery mildew infection of the inflorescence can cause caps to remain on flowers, preventing fertilization and increasing the chance of Botrytis bunch rot.

Caption: Figure I: A spore-trapping report from Coastal Viticultural Consultants for a Napa Valley Cabernet Sauvignon vineyard. The blue line is daily high temperature, and the red line is daily Gubler Thomas index on the left y-axis. Green bars indicate no powdery mildew detection, and yellow bars indicate powdery mildew detection and concentration on the right y-axis.

Caption: Figure 2: A spore-trapping report from Coastal Viticultural consultants for a Chardonnay vineyard in Carncros, Calif. Blue line is daily high temperature, and red line is daily Gubler-Thomas index on the left y-axis. Green bars indicate no powdery mildew detection, andconcentration of the right v-axis.

Caption: Figure 3: Modeled spread of grape powdery mildew in a vineyard with 9-foot-wide tractor rows and leaf area index of 3. Blue dots are mildew spores released from the top of the canopy, and red dots are spores released from the fruit zone. Upper and lower images show the effects of 5 mph wind coming from the southwest and west, respectively. More spores are transported into the air when air movement is not perpendicular or parallel to the vine row.

Caption: Diffuse infections of berries are not visible until the gray scarring appears well after infection.

Caption: Severe powdery mildew infection of the cluster without the gray scarring seen later in the season.

Caption: Powdery mildew infection of clusters often radiates from infections that occur on the inner portion of the clusters.

Caption: The SMART-Dart device and related equipment are required to perform the LAMP assay.


(1.) Falacy, J.S., G.G. Grove, W.F. Mahaffee, H. Galloway, D.A. Glawe, R.C. Larsen and GJ. Vandermark. 2007 "Detection of Erysiphe necator in air samples using the polymerase chain reaction and species-specific primers." Phytopathology 97: 12901297.

(2.) Mahaffee, W.F. 2014. "Use of airborne inoculum detection for disease management decisions." In: Detection and Diagnostics of Plant Pathogens, M. L. Guilin[degrees] and P. Bonants, Eds. Springer Verlag, N.Y. (in press).

(3.) Hall, T. 2000. Epidemiology of grape powdery mildew, Uncinula necator, in the Willamette Valley (master's thesis). Oregon State University, Corvallis, Ore.
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Comment:Improving management of grape powdery mildew with new tools and knowledge.
Author:Mahaffee, Walter; Schwebs, Seth; Hand, Francesca; Gubler, Doug; Baily, Brian; Stoll, Rob
Publication:Wines & Vines
Geographic Code:1U9OR
Date:Apr 1, 2014
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