Wayne F. Wilcox, David M. Gadoury, and Robert C. Seem
Department of Plant Pathology
Cornell University, NY State Agricultural Experiment Station
Geneva, NY 14456
Over the last 20 years, grapevine powdery mildew has been the subject of intensive research at the Geneva Experiment Station Experiment Station Experiment Station Experiment Station Experiment Station. Following is a summary of some recent findings with practical implications for controlling this disease on labrusca, vinifera, and hybrid grapes.
We analyzed powdery mildew epidemics over a number of years, comparing the severity of fruit infection on unsprayed 'Rosette' vines with different weather factors during the season. This revealed a very strong relationship between the severity of fruit infection at harvest and the number of primary infection periods (>0.10 inches rain + temperatures >50°F) that occurred from just before bloom until fruit set (Table 1). Similarly, years in which 4 or more such events occurred during this fruit growth stage were typified by severe outbreaks of powdery mildew region-wide. Thus, we speculated that fruit might be most susceptible to infection during a relatively brief period during the initial stages of fruit development.
Table 1. Relationship between powdery mildew severity on 'Rosette' fruit and
the number of primary infection periods from prebloom through fruit set.
Year % Fruit surface Number infection periods* infected prebloom to fruit set 1988 <1 2 1989 25 7 1991 1 2 1994 61 7 1995 7 3 1996 31 6 *Infection period = > 0.1 inch of rain and temperature > 50° F
Field trials were established to test this hypothesis on Concord berries in 1995 and 1996, and on Chardonnay and Riesling fruit in 1996. Different clusters were inoculated with powdery mildew spores at approximately 2-week intervals beginning near the start of bloom and extending into the summer. Concord fruit were not sprayed with fungicide throughout the experiment. The vinifera vines were kept clean with Nova, but inoculated fruit were shielded within plastic bags during all spray applications from 3 weeks before their inoculation onwards.
Concord berries were highly susceptible to infection at prebloom and fruit set, but they were nearly immune when inoculated 2 weeks or more after fruit set (Table 2). Results were nearly identical in both years, and have been confirmed by five different spray timing trials on Concords in the Lake Erie region over the last two seasons. That is, fungicides applied immediately before bloom and 2 weeks later provide as much protection against fruit infection as did sprays applied from prebloom through August. Rachises (cluster stems) and leaves retained their susceptibility later into the season.
Table 2. Development of powdery mildew on berries and rachises of Concord following inoculation at different stages of growth.
Cluster surface Rachis surface Inoculation dates Growth stage infected (%) infected (%) 1995 9 June Prebloom 69 95 21 June Fruit set 50 79 6 July 6 mm fruit 3 28 17 July 10-13 mm fruit <1 19 Uninoculated 0 8 1996 20 June 10% bloom 5 88 4 July 4 mm fruit 6 88 17 July 10 mm fruit 1 71 2 August (4.2° brix) 2 82
Results similar to Concord were obtained with Chardonnay and Riesling fruit (Table 3). Although Chardonnay and Riesling fruit retained some susceptibility into mid-summer, berries were much more susceptible to infection during the prebloom through fruit set period than they were 4 weeks or more after bloom . In fact, fruit were relatively resistant to infection a full month before reaching 8° Brix.
Table 3. Severity of powdery mildew on Chardonnay and Riesling fruit
inoculated at various stages of growth.
Cluster surface infected (%) Inoculation date 1996 Growth stage Chardonnay Riesling 21 June 10% bloom 50 16 4 July 2-5 mm fruit 74 70 17 July 4-6 mm fruit 6 2 2 August 4.0° Brix <1 <1 15 August 4.5° Brix 0 0 29 August 7.2-9.3° Brix 0 0
Why do these results conflict with "conventional wisdom," which holds that fruit are susceptible to infection until sugar levels reach 8.0° Brix, and that established infections continue to sporulate and expand until 15.0° Brix? There are two likely reasons:
The take-home message: Serious fruit disease that you see in mid-tolate summer is usually the consequence of events that occur during the early stages of fruit development, e.g., rainy weather and/or a problem with the spray program.
Recall that initial (primary) powdery mildew infections are caused by spores (ascospores) that overwinter in fungal fruiting bodies on the bark of the vine. Such primary infections occur during favorable weather between bud break and fruit set. Yet, even in a vineyard with a large supply of overwintering inoculum, the early waves of infection often are present in very low numbers that are hard to detect. Nevertheless, they are important. By studying the progress of powdery mildew epidemics on both sprayed and unsprayed vines over a number of years, we have determined two basic principles that govern how the disease increases from this inconspicuous phase into the all-too-conspicuous phase that everybody knows.
The take-home message: Early fungicide sprays hold down the number of leaves that become infected. Later sprays reduce the severity of those infections that do slip through. Both are important for maintaining healthy leaves.
Resistance to the sterol-inhibitor (SI) class of fungicides (Bayleton, Nova, Rubigan, Procure) is complicating powdery mildew control programs throughout the region. (For an in-depth treatment of this topic, see the Spring 1996 issue of Grape Research News). Bayleton resistance is severe and widespread enough that we no longer recommend its use against powdery mildew. Surveys of resistance levels that we conducted in Finger Lakes and Lake Erie vineyards during 1996, plus recent grower experience, both suggest that the remaining SI's are still effective if thorough spray coverage is provided. However, previous margins for error are gone.
The explanation is simple. Individual powdery mildew isolates have a wide range of sensitivities to individual SI fungicides. To obtain a given level of control, a small percentage of individuals in a wild population need a very low dose, most need an "average" dose, and another small percentage need a significantly higher dose than average. After years of spraying, most of the survivors tend to be those individuals needing doses in the higher range. That's why 2 oz per acre of Bayleton routinely provided good control in the early 1980's, but 5 oz per acre now gives poor control in many vineyards. For this reason, we have suspected that the control problems sometimes encountered with Si's are typically rate-related, i.e., low dosages on certain tissues (incomplete spray coverage) are not sufficiently inhibiting the increasing population of less-sensitive isolates.
In order to maintain the usefulness of the SI's and avoid disease losses, we've promoted three basic anti-resistance strategies:
These three concepts were tested during 1996 in a Seyval vineyard near Dresden, New York, where Bayleton no longer provides adequate control but Nova and Rubigan have remained effective. Various fungicide programs were imposed, using six spray applications at 14-day intervals from early June (10-inch shoots) through mid-August. Sprays were applied with a hooded boom unit delivering 100 gal per acre before bloom and 200 gal per acre after, so coverage was thorough (thus, we figured that our 2 oz per acre rate might approximate the deposits provided on some tissues in a commercial vineyard when a "4 oz" rate is applied with poor coverage). Disease severity was rated in September, and a number of individual powdery mildew "survivors" were tested from each plot to determine their sensitivities to Nova.
Results (Table 4) indicate several things: (1) Approximately 36% of the powdery mildew isolates in the vineyard were controlled by a 4 oz/A rate of Nova but not by a 2 oz/A rate (76% control vs. 40% control). (2) When three sprays of Nova were followed by three of sulfur, control was as good or better than when six seasonal sprays of Nova were applied, but fewer resistant isolates were selected (the survivors made it simply because sulfur had its problems on a 14-day schedule in a wet year, rather than because they were resistant to the Nova). (3) Allowing mildew to build up with sulfur applications early and then spraying Nova provided poor control with the 4 oz rate and virtually no control with the 2 oz rate.
Resistant isolates % Cluster % of % Treatment, rate per acre, (timing) area infected Control* Selected** Nova, 4 oz (Sprays #1-6) 6.2 76 21 Nova, 2 oz (Sprays #1-6) 15.9 40 38 Nova, 4 oz (Sprays #1-3) + Microthiol, 4 lb (Sprays #4-6) 5.2 81 9 Nova, 2 oz (Sprays #1-3) + 23.1 12 57 Microthiol, 4 lb (Sprays #4-6) Microthiol, 4 lb (Sprays #1-3) + 14.8 44 data not available Nova, 4 oz (Sprays #4-6) Microthiol, 4 lb (Sprays #1-3) + 25.3 3 data not available Nova, 2 oz (Sprays #4-6) Unsprayed 26.3 -- * % control relative to the unsprayed check ** Product of (% isolates surviving treatment) x (% isolates resistant)
Take-home message: The anti-resistance strategies listed above really work. Poor spray coverage = low rates on susceptible tissues = poor control and increased resistance development.
Start spray schedules early to keep mildew colony numbers down. Do everything right in the immediate prebloom and first postbloom sprays: good material, good rate, good spray conditions, good coverage (every row and enough water for canopy penetration). Recognize that this will be doubly (triply?) important if weather is wet during the bloom and early postbloom periods. Use SI's early and finish with alternative materials rather than the other way around.