Apple Rust Mite

Apple rust mite is a tiny mite living an apple foliage. This eriophyid lives in apple orchards world-wide. Although more abundant in unsprayed trees, it is common in managed blocks. High population densities can cause injury, but more usual populations are beneficial because Apple rust mite is an alternative food source for the predatory mite, Amblyseius fallacis, and the black hunter thrips, Leptothrips mali.

Apple rust mite, Aculus schlechtendali, is commonly found, but is rarely an important pest of apple in connecticut. 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 in some situations.

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.

Damage:Apple rust mite inserts its mouthparts into plant cells and sucks up their liquid contents. This feeding produces a silvery cast to the leaf in the early stages which tends to get browner as the season progresses. Bronzing caused by apple rust mite is more finely textured than spider mite bronzing and lacks the stippling produced by spider mites. Rust mite damage sometimes causes leaves to roll lengthwise.Like other pests that affect the foliage, damage disrupts photosynthesis and the trees’ water balance. Excessive amounts of damage, with peak populations greater than 300 mites per leaf or 4,800 mite days, can reduce fruit growth. Populations in excess of 2,000 per leaf have been noted.

As well as affecting fruit size, rust mite feeding can cause premature terminal bud set. Depending on the overall vigor of the orchard and other cultural factors, this may not be a problem.Rust mites can also feed directly on the skin of fruit, causing a tan russeting. Usually this feeding is concentrated around the calyx end. This is only a problem on light colored cultivars such as Golden Delicious.

Control:The predatory mites Typhlodromus occidentalis and Zetzellia mali attack apple rust mites but generally do not control them. Predators can increase their numbers early in the season by feeding on apple rust mites, thus providing better control of spider mite populations that develop later.Control measures for rust mites as foliage feeders are rarely called for. Populations of up to 50 mites per leaf in May or 250 in late June do not warrant control. Even if populations exceed 300 per leaf, the benefit of having them as an alternate food source for predatory mites must be weighed against the possibility of yield loss. An added benefit of rust mites is that their feeding preconditions foliage so that it is less suitable for build-up of the more damaging spider mites.

The time of year and weather should also be factored into rust mite control decisions. Because of the tendency of rust mite populations to drop sharply during hot weather in July and August, control measures applied just prior to this period could be wasted. Consider control only where rust mite populations remain high into July and there are no predatory mites. Make sure that trees are adequately irrigated during this period so they are not subject to water stress, which would exacerbate the effects caused by the mites’ feeding. An exception to the above strategy is in the case of high levels of rust mites prebloom on Golden Delicious. If populations are high, then control to prevent fruit damage is warranted. Chemical controls will be most effective at about the pink stage of blossom development.

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