We are starting to see diseases pop up on both European vinifera and hybrid grapes. For some of these, such as anthracnose and Phomopsis, much of the initial infection occurred much earlier. The goal now would be to prevent secondary spread — especially to developing clusters. Relative downy mildew, Sarah Campbell (UGA grad student) reported limited downy mildew leaf infections in all untreated and strobilurin (Abound and Pristine) test plots this week — all three locations. This clearly proves that we can’t trust this fungicide class for management of downy mildew, and strobilurins would need to be mixed with either mancozeb or Captan products at a minimum for downy mildew control. The other downy mildew fungicides appear to be holding well for the moment. Cheng-Fang Hong (UGA grad student) indicated that the Italian disease prediction model is now predicting strong secondary spread of downy mildew, and both powdery mildew and black rot environmental conditions are now ideal for the development of these diseases. Dew is sufficient for infection of downy mildew and black rot, so even though we have had a little less rain over the last couple of weeks, we are still having more than enough moisture for epidemic development. Powdery mildew actually will infect better under drier conditions, so you really need to concentrate your fire on this disease at this time.
