First sign of the pest is the stippling of leaves, something that can be fully appreciated when leaves are placed against the sun. The number of tiny translucent necrotic dots per leaf gives an idea of the degree of infestation of the plant. The presence of profuse white hairs is also a sign of infestation. Leaves later take a dark greenish-purple color and grow distorted as a consequence of the injury. Heavy infestations early in the season can cause serious damage to shoots and leaves. Defoliation and stunted growth can ensue. Fruit production is reduced either because the flowers are injured or because the development is delayed. Normally, rust mites are a minor problem because grapes can outgrow the stunting late in the season. However, if all the conditions are met for a rapid development of the population, this can cause yield and quality losses.
The symptoms are cause by the grape rust mite (Calepitrimerus vitis), an obligate pest of Vitis vinifera. They overwinter as adult females under the bark or in crevices of the vines, and they migrate from there to the developing shoots in early spring. Their microscopic size and translucent color makes them difficult to identify. On leaves, they are often surrounded by white plant hairs. They feed gregariously on young leaves and shoots in early spring, inserting their stylet (mouhtparts) into the epidermal cells and sucking the content. Some of the substances their inject into the cells while feeding have hormonal properties and this causes the deformation of tissues. By mid- to late summer, the mites seek the protection of their overwintering sites. They are usually not a problem because of the numerous predatory mites and insects feeding on them.
Rust mite are easy prey to a number of natural enemies, particularly predatory mites. Timely wettable sulfur applications during dormant stage and at bud break can also help to ward off the mites. However, populations can increase once these sprays are stopped. Application of extracts of neem oil or some insecticidal soaps can also be used as foliar spray.
Always consider an integrated approach with preventive measures together with biological treatments if available. In most cases, miticides are avoided to prevent significant reductions in populations of predatory mites, which can help control this pest.