Grape vine blister mite is a type of eriophyid gall mite. These gall mites are tiny (less than 0.2mm) sap-sucking creatures that cause a variety of abnormal growths on various plants, they white are sausage shaped and unlike most arachnids only have two pairs of legs. Although the mites are too small to be seen without a microscope, the galled tissues that they create are often very distinctive and easily recognised. The excessive growth of hairs on the underside of leaves caused by some gall mites, including grapevine blister (or erinose) mite Colomerus vitis, is called a felt gall or erineum.

There are many species of eriophyid mite which are usually host specific this means that each species of mite will only feed on a single, or several closely related, plant species. Eriophyid mites feed by sucking

Sap is a fluid that circulates through a plant's vascular system, in a similar way to blood moving through our veins. Phloem sap carries the sugars produced in the leaves by photosynthesis down to roots and other storage organs, as well as carrying minerals and hormones. Xylem sap is watery and transports nutrients absorbed by the roots upwards to the rest of the plant. Some plants leak sap from wounds or pruning cuts, and this is known as bleeding.

Meristem is a type of tissue that is responsible for growth and development in plants. It consists of unspecialised cells that can divide and diversify into various types of specialised cells. Meristems are only found in certain parts of a plant, such as at root and shoot tips and in bundles between xylem and phloem vessels.

meristem

Symptoms begin to appear in late spring. The upper surface of affected leaves bulges upwards, giving the leaves a puckered appearance. Beneath the raised areas, a dense coating of short fine hairs develops, amongst which the mites live and feed. The hairs are usually creamy white but on some grape cultivars, may be pinkish red. In late summer the hairs start to dry up and can become brownish.

While the mites are sucking sap, they secrete chemicals into the foliage that induces the abnormal growth. The hairy covering on the underside of the leaves could be mistaken for a fungal disease, such as mildew, but the mite is a not a problem. It does not affect the fruit and does not seem to have any harmful effect, apart from distorting the leaves.

The mite should be tolerated as it will not affect fruiting or health of grape vines. 

If the mite cannot be tolerated and only a few leaves or shoots are affected by galling, these can be cut off and disposed of. If many leaves are affected, the presence of the mite should be tolerated, as removing affected leaves will cause more damage than the mite.

Grape vine blister mites overwinter on the grape vine by hiding away beneath bud scales. The mites emerge in spring and begin feeding on the new foliage. Several generations develop in the underside of the leaves during the summer. In late summer, the matted hairs created by the gall mites begin to dry up and this causes the mites to leave the foliage and seek places where they can overwinter.