Almost all the vines growing in moist and cold regions are going to be affected by mildew. Some of my vine growing friends control this mildew very easily by powdering them every three or four weeks with a mixture of ground sulfur dust and Bordeaux powder. Do not inhale this dust. This must be done before the trouble begins. It is usual to give the first powdering, just before the vines start to blossom.
You may need to powder 3 to 4 times if the season is wet. During the dry season, it may not be necessary and two times is enough to control the mildew.
This is the only time I am recommending something which is not organic. Do not powder the moment the plant starts fruiting because that is going to affect the grapes, and their flavor. Besides Bordeaux powder is not natural or organic and sulfur too.