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During the summer the foliage of grape vines can become distorted and the underside of the leaves covered with a dense coating of fine hairs caused by the feeding activities of a gall mite. This does not affect the overall health of the vine.
Grape vine blister mite
Grape vine blister mite is a type of gall mite. Gall mites are tiny (less than 0.3mm) sap-sucking creatures that cause a variety of abnormal growths on various plants. 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 sometimes called a felt gall or erineum.
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, the hairs may be pinkish red. In late summer the hairs start to dry up and they can become brownish.
While the mites are sucking sap from the foliage and hairs, 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 much less serious problem. It does not affect the fruit and does not seem to have any harmful effect, apart from distorting the leaves.
The mite can be tolerated as it will not affect fruiting or health of grape vines.
If 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 must be tolerated, as removing infested leaves will cause more damage than the mite.
None of the pesticides available for use on grapes in gardens and conservatories is effective against gall mites.
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.
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