Year in and year out, powdery mildew remains a challenging disease to control. Just ask growers last year who had a real battle on their hands. That was despite being aware of the potential for damage posed by conditions at the time that favored spread of the disease.

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.

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Except for oil, all powdery mildew fungicides are best used as protectants, note University of California Cooperative Extension farm advisors and specialists. They advise beginning to treat vines promptly and to repeat treatment at appropriate intervals.

Timing of the first treatment depends on your choice of fungicide and growth stage. After that, frequency of treatment is based on the particular fungicide you use and weather conditions.

To help you in managing this crop threat, University of California plant pathologists have developed the powdery mildew index (PMI) model. It 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 this index, UCCE guidelines call for monitoring the vineyards for signs of the diseases and to stop treatment, if there is no recent evidence of the disease.

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 can monitor temperatures in your own vineyard and calculate the PMI using the following rules

Starting 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 85F 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.

Timing your sprays

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.

• If fewer than 6 continuous hours of temperatures between 70 and 85 F occurred, subtract 10 points.

• If 6 or more continuous hours of temperatures between 70 and 85 F occurred, add 20 points.

• If temperatures reached 95 F for more than 15 minutes, subtract 10 points.

• 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.)

Treatment for wine and raisin grapes may be discontinued when fruit reaches 12 Brix, according to UCCE recommendations. However, table grapes should be treated up to harvest.

Details on powdery mildew and guidelines for managing the disease are available in the Grape section of the UC IPM Online website – http://www.ipm.ucdavis.edu. It includes a table indicating how you can shorten or lengthen spray intervals depending on disease pressure.

Also, it has a link to weather equipment that has the UC Davis PMI built into its software; and details on general properties and efficacy of useful IPM fungicides and fungicide resistance management.

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