Powdery mildew on grapes

Asked September 6, 2017, 1:50 PM EDT

Could you please tell me if the problem with these grapes is Powdery Mildew or Downy Mildew. If so, how do you go about treating them. Any and all information would be greatly appreciated.

These were brought into our Extension office by an Amish Fellow. He said he had put manure around the bases as well as some milk-water(?). He has had the problem for 2 years now.

Thank you.

Gladwin County Michigan

1 Response

Hard to tell from the leaf it could be both. I assume this is Concord which is more susceptible to powdery and pretty resistant to downy. Captan fungicide is not good for powdery mildew control and sulfur which works fairly well should not be used on Concord because it is sensitive to sulfur and it will cause the leaves to fall off.
Powdery mildew only grows on living grapes so it over winters on the grape vine.
The overwintering fungus is easy to see and a black sooty covering on the shoots.
When he prunes the vines this winter he should try to cut as much of it off as possible. I usually suggest that 100 buds is a good number to leave on the vine.