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 serious problem. It does not affect the fruit and does not seem to have any harmful effect, apart from distorting the leaves.