The damage is not a disease, but from erineum mites, Colomerus vitis, an occasional pest of grapes. The mites feeding produces what appear to be blisters on the top surface of grape leaves. The corresponding depressions on the reverse are filled with a white cottony-appearing material which is deformed leaf tissue that will age to tan, then brown. For more information see the following link to the Univ. of California Integrated Pest Management Handbook. The link will take you right to the relevant page. http://www.ipm.ucdavis.edu/PMG/GARDEN/FRUIT/PESTS/grerineummte.html.
Pesticides aren’t needed against these particular mites because their feeding doesn’t affect grape yield, quality, or flavor. 

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OK. I've noticed they multiply close and eventually cover the bottom the leaf with the white fibers. After a while the leaf dies. So I have just picked off all leaves with these mites. They seem to prefer a cooler temp. as most are under shade, a few are on the top/outer layer of leaves that get the most sun/heat.
I'll watch and see if they develop more after my "purging" of infected leaves.
Thanks for your reply.

I have another leaf problem ~ a separate question?
Attached are pix. It's a rust color that dries and kills the leaves.
 - Brandon

I am assuming from your note that the infected leaves are not those affected by the mites and this is an entirely different problem.

There is a grapevine viral disease called Red Blotch that infects certain wine grape varieties. The web page in the Pacific Northwest Disease Handbook https://pnwhandbooks.org/plantdisease/host-disease/grape-vitis-spp-grapevine-red-blotch-disease has pictures and descriptions you can compare to your grape leaves. The article suggests keeping vines well-watered can mitigate some of the effects of the disease. 

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