Winchester area received a shower yesterday, resulted in 5+ hours of wetness in lower 70F. It was an infection risk event for downy mildew, and very low infection risk event for Botrytis. In addition, we observed dews in the last two nights, indicating high humidity, which can trigger downy mildew to produce spores. As usual, late season downy tends to appear on younger leaves on the top of the canopy. Please keep eyes on them! There is a chance of rain on Sunday and Monday.
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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