Fungicide now won't get rid of the disease this year. Here is a link to "Grapevine Powdery Mildew" from Washington State University Extension. http://wine.wsu.edu/research-extension/files/2011/02/2012-EastPMWhitePaper.pdf It will give you more information than you ever wanted to know about powdery mildew! Your grapes are now at post-fruit set so your efforts need to be concentrated on minimizing the disease level. Improve the air circulation by pruning. Don't water overhead. Next year you can be proactive and hopefully, avoid this problem.
http://pnwhandbooks.org/plantdisease/node/6097/print This publication, "Grape Powdery Mildew" is from the PNW Plant Disease Management Handbook.The spray options marked with an "O" are allowed for organic growing. The spray options marked with an "H" are available for homeowners.