At Winchester, yesterday’s (5/6/09) rain started from 5pm (60.8F) and ended at 6:40pm (59.0F); however, the RH was >90% until this morning (8:40am). A total amount of precipitation was 0.05 in. Thus, it probably accounted for >14h of wetness with upper 50’s in temperature (F). It would be enough for Phomopsis, powdery mildew ascospore discharge, black rot, and downy mildew infection. (I feel like I’m repeating myself…)
Please do not get overwhelmed by my report of infection events. I’m just giving you the information about what could happen based on weather conditions observed. If you applied your protective fungicide application, it should stop infection. In addition, there are chemicals with curative (or kick-back) activity against powdery mildew, downy mildew, and black rot. Moreover, the risk of infection by these fungi depends on the history of your vineyard too. For example, if you had a history of outbreak of downy mildew year after year, you tend to have higher risk of downy mildew simply because there are more inoculum (winter-surviving oospore of downy mildew fungus) in your field. In that case, you need to be on top of the situation, and you may want to protect vines from early season to minize the risk of an outbreak later in the season. But if your vineyards have been relatively clean, there may not be enough inoculum to be concerend at this moment.
