On-going disase risk

As usual, we start our season with rains.  It has been more than 13hr of wetness so far at Winchester with an average temperature of low 50s.  It has accounted for a low risk of Phomopsis infection, and a line of rain just passed the area too.  We will see how it will turn out over the weekend.

As shoots extend rapidly in the early part of the season, the new tissues are not protected by the previous application of fungicide(s).  Keep in your mind to protect these new tissues.

I often am asked about when to reapply fungicide(s) after rain.  An often discussed rule of thumb is:

  1. < 1 inches of rain: Continue with intended spray interval
  2. 1-2 inches of rain: Consider shortening your spray interval in half.  (E.g., if there is 7 days remaining for the normal spray interval, shorten it to 3-4 days)
  3. >2 inches of rain: Consider re-application soon.

However, please note that it also depends on the history of your vineyard(s), cultural practice, and also cultivar selection too.  For example, if you have not seen downy mildew in the past 4-5 years, have a nicely kept canopies for air movement, and you have relatively downy mildew resistant hybrids, say, Chambourcin or Norton, you may have less issue with downy mildew even if the fungicide coverage is not perfect.

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