Eriophyes vitis Landois

In central Europe, Italy and in California the leaves of the vine are often deformed by attacks of a minute mite. In in- fested leaves the portions between the larger veins puff up, leaving a cavity on the under side which is clothed with a dense felt-like covering. As a rule, thin-leaved varieties of grape are more subject to attack, or at least show greater evidence of the presence of the mite. In California erinose rarely causes serious injury to the vine or to the crop; it has often been confused with a fungous disease, the powdery mildew.

The mite, as usually found on the leaf, is a minute, nearly colorless, elongate, four-legged creature, about .13 mm. in length. The mites hibernate under loose strips of bark on the larger branches of the vine and in the spring migrate to the under side of the opening leaves where they puncture the epidermal cells with their sharp mandibles, thus producing abnormal thread-like outgrowths from the underlying layers of cells, known as erinea. These erinea when abundant have the appearance of dense felt, and it is in the shelter thus afforded that the mites live, lay their eggs and the young find suitable food. From time to time individuals leave the older leaves and start new colonies on the young leaves at the ends of the branches. In the fall some of the mites desert the leaves and go into hibernation on the bark of the older canes.

- Mark Slingerland, Cyrus Crosby: (1915) Manual of fruit insects^©

Reference: https://www.biodiversitylibrary.org/page/30873865#page/443/mode/1up