(I'm still at a meeting, so, I will make it quick.) At Winchester, the series of rain started around 2 AM on this Monday (5/17/10), it was not a continuous rain, but we had frequent enough rain events during Monday, Tuesday, and this morning to keep the relative humidity high (>90%) until ~9:30 AM today. During Tuesday, there was a moment when the RH was little less than 90%, but it was still in high 80's. Thus, I would say this rain event accounted for about 55.5 hours of wetness and average temperature during this period was about 58F. Thus, it was long and warm enough for downy mildew, black rot, and Phomopsis. Some of us are expecting bloom very soon. Please remember that the critical time for downy, black rot, and powdery mildew berry infection is from bloom to 4-5 weeks after bloom.
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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