Grape leaf rust mite (Calepitrimerus vitis), which belongs to the same family as grape erineum mite, was identified for the first time on grapes in the Okanagan Valley in 2009.
A microscope is required to properly identify these 0.2 mm long, pale white, wormlike mites that closely resemble erineum or blister mites but do not form leaf galls.
They are much smaller than spider mites and elongate rather than round, tapering at the rear, and having two pair of legs near the head. Their presence is best determined by the bronzing of leaves during July and August, a result of their feeding.
Unlike spider mites that cause similar bronzing to leaves, grape leaf rust mites are microscopic and do not produce webbing.
Adult rust mites that spend the winter under bark or bud scales move, in the spring, to the developing buds to feed and lay eggs.
Large numbers of adults concentrated on the available green tissue in early spring can damage buds, resulting in deformed leaves, scarring of shoots, reduced shoot growth, reduction or loss of fruit clusters and even death of buds. Early season rust mite damage can be confused with cold injury or damage from other causes. Damage becomes less visible as the season progresses and shoots lengthen.
Immature mites that hatch over a period of several weeks feed under the bud scales, eventually moving to feed on developing leaves. Unlike spider mites, rust mites equally infest the upper and lower leaf surfaces. Many generations are produced throughout the summer.
Before leaves drop in fall, adult rust mites migrate to the cordon and trunk for the winter. Bronzing of leaves is most obvious during July and August, but damage to leaves at this time is not thought to seriously damage vine health. Extensive bronzing of leaves is an indication, though, that rust mites need to be controlled the following spring.
There are no established thresholds for this pest. Control the following spring is indicated when a significant proportion of leaves become bronzed in late summer.
For more information about chemical control, see the Pesticides page.
As with erineum mite, western predatory mite (Galendromus occidentalis), other mite predators and predacious thrips are capable of keeping rust mites in check.
To maintain healthy populations of beneficial insects and predatory mites, it is important whenever possible to limit the use of insecticides and to apply selective materials that are less toxic to predatory mites.
