The symptoms depend on the type of the mite involved, the variety of grape and the environmental conditions. The most common symptoms appear in late spring, when some areas of the upper surface of young leaves bulge upwards and develop into blister-like swellings (also called erineum). A layer of short fine hairs, varying in color from white to pinkish red, can be found in the cavities beneath these raised areas. The tiny and translucent mites are protected by these dense hair coatings. Later on, the swellings and the hair, that coat them inside, dry up and turn brownish. In some countries, these mites cause a different type of damage, for example distortion of basal leaves as well as deformation of buds and leaf curling.
The blister-like outgrowths on leaves are caused by Colomerus vitis. Despite the evident symptoms, it is not considered a major pest of grape. The tiny and sap-sucking mites affect predominantly grapevine. While feeding on the epidermis of the leaves they inject hormone-like substances into the cells that alter their growth, resulting in the characteristic swellings. The blister mite overwinters on the grape plant, for example hiding beneath bud scales. They become active in spring, when they move to the underside of young leaves and start feeding on them. At the end of the summer, they leave the foliage and seek refuge for the winter. The covering on the underside of leaves should not be wrongly identified with a fungal disease, such as mildew. The symptoms are more severe during rapid leaf growth in warm humid weather but the mite does not seem to have any harmful effect on fruit yields.
The predatory mite Galendromus occidentalis feeds on blister mites and has been shown effective in reducing their numbers. Insecticidal soap or neem oil can also be used, but these could also reduce the populations of beneficial insects. Furthermore, treatments with wettable sulfur could be helpful.
Always consider an integrated approach with preventive measures and biological treatment if available. Spirotetramat has been used successfully against blister mites. Ensure that there is adequate foliage for absorption of the compound and allow 30 days between applications. Wettable sulfur could also be used.