We have planted a small vineyard in our yard (36 plants), mostly Pinot Noir, and also have a couple of unknown variety table grape vines. The vineyard was doing well until a couple of weeks ago, when most of the vines started showing dark spots, and the grapes look mottled and sick. One of the vines has what appears to be powdery mildew on the leaves, but the others do not show this. I am uploading images of them, taken today.
You have a severe infection of grape powdery mildew. This is a fungal disease that requires the application of preventative sprays (fungicides) that begins in spring. However, now that you have infection, there is not much you can do. A small infection (~3%) can cause off flavors in wine and any infection can cause table grapes to be inedible.
It is not possible to eradicate the disease to save the crop this year. What you can do is treat the vines by dousing them with water and possibly following with a horticultural oil spray to knock down the population and possibly save your canopy. For more information about grape powdery mildew management and prevention, please see the following resources:
Grape Powdery Mildew article on eXtension:
Pest Management Guide for Wine Grapes in Oregon (2013):
PNW Disease Management Handbook:
If you need more assistance, please contact the Master Gardeners at your local county Extension office. For contact details, please click here: