Apple rust mites. Photo: David Biddinger
It is usually controlled by natural predators or pesticide sprays applied against other pests such as Envidor or Portal. However, rust mite populations can build to injurious levels only in rare situations.
Description and life cycle
The motile stages of this mite are cone- or sausage-shaped, and are very difficult to see with the unaided eye. Examination of the leaf midrib with a strong hand lens is usually needed to verify mite presence.
Apple rust mites overwinter as adult females under the bud scales of apple trees. At budbreak they become active and feed and reproduce until July or early August when overwintering females are produced. In low numbers, apple rust mites are considered beneficial in that they provide an alternate food source for the predators of European red mites and twospotted spider mites.
These mites feed on the underside of the leaf, usually causing little if any damage. Under some conditions mite populations may expand, and their feeding can cause the leaf to take on a silvery appearance as leaf cells are destroyed and chlorophyll is lost. Silvering symptoms do not appear until mite populations build beyond 200 mites per leaf. Under increasing damage from larger populations or more prolonged feeding, apple leaves may longitudinally roll, giving the appearance that the tree is under drought stress. Sometimes these mites will move to the fruit and may be found at either the calyx or the stem end. In extremely rare cases they will russet the fruit.
Monitoring and management
Monitoring should begin before petal fall and continue through the first week of August. A good hand lens or microscope is required. Inspect 10 leaves from 10 different trees. Concentrate the inspections on the basal third of the leaf midrib on the underside of the leaf. It is too tedious to count the exact number of mites, so a quick estimate should be made on each leaf. When more than an average of about 200 mites per leaf is found, then a pesticide treatment is recommended. A treatment is also recommended upon the appearance of plant symptoms such as silvering or leaf rolling. Specific chemical recommendations for home gardeners are in Fruit Production for the Home Gardener, and recommendations for commercial growers are in the Penn State Tree Fruit Production Guide.