How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Symptoms of powdery mildew include red blotchy areas on dormant canes. On leaves, initial symptoms appear as chlorotic spots on the upper leaf surface. Signs of the pathogen appear a short time later as white, webby mycelium. As spores are produced, the infected areas take on a white, powdery or dusty appearance. On fruit and rachises the pathogen appears as white, powdery masses that may colonize the entire berry surface.

COMMENTS ON THE DISEASE

The fungus survives the winter as dormant mycelium in buds or as chasmothecia (spore structures). Chasmothecia are the most important sources of overwintering inoculum. They mature in late summer and fall on infected green tissue and are washed onto the cordons and spurs with fall and winter rainfall. On warm winter and spring days when moisture is abundant, cleistothecia burst and release ascospores. Conidial spore production occurs 7 to 10 days after primary infection by ascospores and will continue throughout the season as long as moderate temperatures (70° to 85°F) exist.

MANAGEMENT

Season-long control is dependent upon reducing early-season inoculum and subsequent infection. Thus treatment must begin promptly and be repeated at appropriate intervals. Timing of the first treatment depends on fungicide used and growth stage. Frequency of treatment thereafter depends on fungicide choice and weather conditions. Monitor and use the powdery mildew index (PMI) model to determine necessary spray intervals. Treatment may be discontinued for wine and raisin grapes when fruit reaches 12 Brix but should be continued up to harvest for table grapes.

All powdery mildew fungicides, with the exception of oil, are best used as protectants. Discontinue the use of soft chemistry products (sulfurs, biologicals, systemic acquired resistance products, and contact materials) when disease pressure is high because by themselves they will not provide adequate control. If eradication is necessary, a light summer oil may be used anytime in the season if there is no sulfur residue present (i.e. at least 2 weeks after a sulfur treatment). Basal leaf removal can improve coverage and efficacy of powdery mildew fungicides on clusters.

Organically Acceptable Methods
Sulfur, Serenade, Sonata, and Organic JMS Stylet Oil are acceptable on most organically certified grapes; check with your certifier for details.

Monitoring and Treatment Decisions
In spring, the overwintering chasmothecia produce ascospores, which are the primary source of infection. Ascospores are released when 0.1 inch of rain or irrigation is followed by 13 hours of leaf wetness when temperatures are between 50° and 80°F. Seven to 10 days after this initial infection, monitor vineyards for the presence of powdery mildew by collecting 10 to 15 basal leaves from 20 or so vines at random and examining the undersurface for powdery mildew spores. If spores are found, then monitor disease development by using the powdery mildew risk assessment index.

Powdery Mildew Index (PMI). Once initial infection occurs, ideal temperatures for growth of the fungus are between 70° and 85°F. Temperatures above 95°F for 12 continuous hours or longer cause the fungus to stop growing. The powdery mildew index assesses the risk of disease development by relating it to air temperature and tells you how often you need to spray to protect the vines. When using the powdery mildew index, always monitor the vineyard for signs of the disease. If evidence of the disease is not recent, don't treat. You may monitor temperatures in your own vineyard and calculate the PMI using the rules below, or you may use weather equipment that has the UC Davis PMI built into its software.

Initiating the index. After you find powdery mildew, an epidemic will begin when there are 3 consecutive days with 6 or more continuous hours of temperatures between 70° and 85°F as measured in the vine canopy.

1. Starting with the index at 0 on the first day, add 20 points for each day with 6 or more continuous hours of temperatures between 70° and 85°F.
2. Until the index reaches 60, if a day has fewer than 6 continuous hours of temperatures between 70° and 85°F, reset the index to 0 and continue.
3. If the index reaches 60, an epidemic is under way. Begin using the spray-timing phase of the index.

Spray timing. Each day, starting on the day after the index reached 60 points during the start phase, evaluate the temperatures and adjust the previous day's index according to the rules below. Keep a running tabulation throughout the season. In assigning points, note the following:

• If the index is already at 100, you can't add points.
• If the index is already at 0, you can't subtract points.
• You can't add more than 20 points a day.
• You can't subtract more than 10 points a day.

1. If fewer than 6 continuous hours of temperatures between 70° and 85°F occurred, subtract 10 points.
2. If 6 or more continuous hours of temperatures between 70° and 85°F occurred, add 20 points.
3. If temperatures reached 95°F for more than 15 minutes, subtract 10 points.
4. If there are 6 or more continuous hours with temperatures between 70° and 85°F AND the temperature rises to or above 95°F for at least 15 minutes, add 10 points. (This is the equivalent of combining points 2 and 3 above.)

Use the index to determine disease pressure and how often you need to spray to protect the vines. Spray intervals can be shortened or lengthened depending on disease pressure, as indicated in the table below.

SPRAY INTERVALS BASED ON DISEASE PRESSURE USING THE POWDERY MILDEW INDEX

RESISTANCE MANAGEMENT

Alternating fungicides with different modes of action is essential to prevent pathogen populations from developing resistance to fungicides. This resistance management strategy should not include alternating or tank mixing with products to which resistance has already developed. Do not apply more than two sequential sprays of a fungicide before alternating with a fungicide that has a different mode of action.

PUBLICATION

UC IPM Pest Management Guidelines: Grape
UC ANR Publication 3448
Diseases
W. D. Gubler, Plant Pathology, UC Davis
R. J. Smith, UC Cooperative Extension, Sonoma County
L. G. Varela, UC IPM Program, Sonoma County
S. Vasquez, UC Cooperative Extension, Fresno County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Research Center, Parlier
A. H. Purcell, Environmental Science, Policy and Management, UC Berkeley
Acknowledgment for contributions to Diseases:
G. M. Leavitt, UC Cooperative Extension, Madera County

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