Just to be sure, you ARE talking downy mildew and not powdery mildew? Just so those answering your post know you know the difference.

Now I let the experts weigh in....

definitely downy

I know you know but could you post a picture so others can see? It would be helpful if you can.

Diagnosing Downy Mildew can be difficult. Photos would be helpful. See URL below for some photos of the disease as it sometimes appears. It can take on dramatically different looks depending on the variety it infects.

Here is a link that might be useful: Downy images

PS: control is affected by sanitation, increasing air circulation and avoiding wetting the foliage. (You don't say whether this is in a greenhouse, or out in the open) Chemical control can be useful as well. I use Aliette when needed.

For identification, the undersides of the leaves are checked for a particular and distinctive mycelia. These mycelia are fairly rare (thus the fungal species name sparsa) and you or your plant pathologist/county extension agent would need freshy dropped leaves and a microscope and patience to find them.