At Winchester area, the second series of rain started around 5:20 pm on Sunday, then it continued on and off until around 8:30 pm on Monday with about 0.17 inches of precipitation. Thus, there was about 27 hours of wetness with average temperature of 58F or so. It was long and warm enough for Phomopsis, and Black Rot to cause infection. Also, these rains from Saturday probably initiated ascospore release of powdery mildew. (Powdery mildew does not require water for its infection, but it requires water to release ascopsores from its over-wintering structure.) The average temperature was low for downy mildew, but between noon to 5 pm Monday, we observed 60-65F while it's raining. Thus, it might be accounted for downy mildew as well.
At Winchester, we had light rain events during the night of 6/12/09, but it was short events and the relative humidity was low (80% or so), thus it probably did not promote any infections. However, we are experiencing continuing favorable nights for downy mildew sporulation (average T>55F, high RH (80-100%)) for 10 days now. Yesterday, we conducted a formal disease assessment, and observed first incidence of powdery mildew for this season. We had plenty of infection events in last two months, so it was not surprising. At this point, it is a trace level of infection on untreated vines. Downy mildew was the major disease so far. We had up to 40% incidence on untreated vines. Next runner-up was black rot. It varies vine to vine, but some of vine had 10-15% incidence. Phomopsis was omnipresent as I expected from early May rain falls, but severity was low overall. We will examine diseases again in the near future, and I will update as the season goes. Here is downy mildew ga...
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