Adult blister leaf mites are tiny — smaller than a mote of dust. But if you could view them with the naked eye, you would see cream-colored worms with two pairs of legs. Grape erineum mite damage appears on young leaves as dark green to pink tinged swellings on upper areas. The underside of leaves have a concave appearance, littered with blister-like edemas covered with a felted carpeting of dense long leaf hairs.
Erineum mites overwinter on the grapevines and move on to the new growth in the spring. They feed in groups beneath the swellings and, as their numbers increase, move to new areas of the vine. From late summer into autumn, the mites move back to the bud scales to overwinter.
While unsightly, treating grape leaf blister mites is generally unnecessary. Leaves afflicted with the erineum galls or swellings function normally and there is no effect on grape production unless the vine is suffering from additional grapevine diseases, pests or environmental stresses. These mites can affect growth and production of newly planted, very immature vines, however, so blister mite control in these cases may become needed.
Different grape varieties are more susceptible to erineum mites. In young plants, removing and disposing of infested leaves can control light infestations.
A natural predator, Glaendromus occidentalis, feeds on erineum mites. Introduction of this predator has some effect on reducing their numbers; however, the tiny mites are often protected by the dense hairs of the galls.
In vineyards, blister leaf mites are rarely an issue when the property has been routinely treated for powdery mildew with application of sulphur early in the growth season. A number of other chemical sprays used for the control of leafhoppers and spider mites also stem the population of blister leaf mites.
For the home grower, however, again, there is very little need for treating grape leaf blister mites with a chemical measure. The affects done by these tiny mites is primarily aesthetic and should simply be tolerated. You should still get a bumper crop of grapes provided all other conditions are favorable.