Just a quick note that recent rain events were risk events for downy mildew based on the NEWA DMCast model. It seems that northern Virginia received more and longer rain events. In addition, we may see more rain toward the end of next week.
Since we have been experiencing a very dry season, I was not really thinking about downy mildew. However, during recent vineyard visits, I have noticed downy mildew on leaves, and sure enough, I saw some downy mildew lesions on leaves in one of my research blocks too.
If you are due to spray soon, grow downy mildew susceptible cultivars (e.g., Chardonnay, Vidal, etc.), and/or have seen a considerable amount (or duration) of rain recently, it may be a good idea to include a Phos acid material (e.g., Prophyt, Phostrol, Agri-Fos, etc.) in the tank mix as insurance. Please note that if you use a copper material, a combination of copper and phos acid can cause phytotoxicity. Since many of our vines are in the critical period to protect clusters from downy mildew, powdery mildew, and black rot, the use of a Ridmil product may also be a good idea. — By Mizuho Nita, Grape Pathologist, Virginia Tech