I have been receiving more than usual number of calls and emails about powdery mildew this year. It seems that consensus is that it started to show up on clusters last week and just keep going. We have passed critical time for cluster infection at this point, thus, you may wonder how it happened? Since it takes 7-10 days to develop symptoms (powdery appearance on the surface, i.e., spores), and it probably takes a few generations prior to the explosive outbreak, thus, the initial infection events probably took about 20-25 day ago when berries are still susceptible. Management options on on-going powdery mildew infection is limited. As with other diseases, we do not have real curative or eradication material against powdery mildew. Also, it is not recommendable to apply protectant fungicides because there will be higher risk of fungicide resistance development; however, as I mentioned in the previous post, there are a few options that can help you out....
Grape Disease Management Tips from Mizuho Nita (Grape Pathologist at Virginia Tech)