You are having problems with downy mildew. The downy mildews are a group of fungal diseases that cause destruction of the leaves, stems, flowers, and fruits of many plant species. Downy mildew is caused by several closely related fungi (e.g., Peronospora spp., Plasmopara spp. and Pseudoperonospora spp.) that survive in plant debris or on infected plants. Downy mildew fungi are fairly host specific. The downy mildew fungus that infects one type of plant is not the same downy mildew fungus that infects another. However, if you see downy mildew on one plant, then environmental conditions (i.e., cool, wet weather) are favorable for development of downy mildews on a wide range of plants. Downy mildews can be controlled using fungicides. Those containing chlorothalonil, copper sulfate and lime (e.g., Bordeaux mix), fosetyl-Al, mancozeb, metalaxyl, mefenoxam, or trifloxystrobin are labeled for downy mildew control. Be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the fungicide in the safest and most effective manner possible.

The following article from PennState Extension contains more information on downy mildew. The article can be found here: Downy Mildew (psu.edu)