Powdery and downy mildew can develop on grapes in the late season, post harvest. It is important to keep vines as healthy as possible going into winter. With the long fall we are experiencing, downy and powdery mildew are both becoming prevalent in many vineyards. Growers should consider a late season application of fungicides to keep these diseases under control to protect the foliage and assure adequate cold acclimation. Downy can be controlled with phosphorous acid products, mancozeb, or captan. However, none of those fungicides will control powdery mildew. So a tank mix including one of the above with a sterol inhibitor such as Rally or Tebuzol would be a good approach.
Powdery mildew on grape leaf
Downy mildew (and powdery mildew) on grape leaves