Grape mites are probably the least serious disease in vines. While the development of small bumps appearing on the leaf surface, resembling the "smallpox" disease, is very alarming at first glance, the mites are actually relatively harmless. On the underside of the leaves there are small hollows with white furry patches. The minuscule mites live in the patches. By sucking on the leaf, they produce growth deformations but don't actually limit the function of the leaf and are harmless to the plant; the little mites themselves are harmless to humans. No other plant species is infested by them. The only problem is that they may multiply considerably and aesthetically, the vine may suffer. Infested leaves can be removed to stop the spread of the mites. Further measures are usually not necessary -- after bad infestation, a light sulfur treatment in the next spring might be in order.
(Grape mites - eriophyes vitis)