What is in this article?:
- A model approach for tracking powdery mildew threat
- Timing your sprays
- University of California plant pathologists have developed the powdery mildew index (PMI) model to battle the challenging grape disease.
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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