I. Introduction: The pear rust mite (PRM)
is found throughout the mid-Atlantic area where it
occasionally causes injury to foliage and russetting of the fruit. The
importance of this pest has
increased in some orchards where pyrethroid insecticides used in pear
psylla control have reduced
early-season populations of mite predators.
II. Hosts: PRM is found only on pear. Attempts to determine whether it can become established and reproduce on apple and related wild plants, such as hawthorn, have failed.
III. Description: PRM is almost invisible with the naked eye and appears identical in body form to the ARM when viewed with a 20X hand lens. PRM is wider at the anterior end, however, giving this species a more "broad-shouldered" appearance (Plate 85). Adults are dull white to pale brown and have wedge-shaped bodies that are larger at the head than the tail end. The two nymphal stages look similar in shape but smaller. The round eggs are flattened, colorless when first laid, and extremely small.
IV. Biology: The PRM overwinters solely as adult females beneath bud scales of leaf spurs and under loose bark of 1- to 2-year-old twigs. As the weather begins to warm in early April, usually before buds break, mites move to developing clusters and begin feeding on the succulent parts of buds. Eggs are produced shortly after mites become active. As buds open, adults and immatures move to the expanding leaf tissue and eventually to fruit as the leaves mature and harden. Immature mites develop quickly through two instars, each followed by a resting stage.
There are 4 or 5 generations each growing season in the mid-Atlantic area, and each cycle takes only 10 to 14 days under warm summer conditions. During June and July, adult males and females, eggs, and immatures are commonly found together on the lower leaf surface and around the calyx end of fruit. By late summer, only females are produced and they soon move to protective overwintering sites.
V. Injury: PRM feed on the surface of the leaves and fruit, causing bronzing and russetting of the epidermal tissue. Bronzing occurs first on the undersides of the younger leaves near the midrib and then gradually extend outwards. Discoloration is less evident on the upper surface, which often remains green. Leaf injury may stunt the growth of young trees but seldom causes any photosynthetic reduction in older, non-stressed trees.
After petal fall, more PRM move from the foliage to the fruit where they first feed on the calyx end and cause localized russetting (Plate 86). If populations are allowed to increase, especially later in the season, feeding and russetting may spread over the whole surface of the fruit. Pears are downgraded or rejected for the fresh market if the extent of heavy russet exceeds 10% of the surface of the fruit. However, russet fruit is usually marketable for processing since the damage is restricted to the surface and thus removed with the peel.
VI. Monitoring: Because of their small size, sampling of PRM requires careful examination of buds, leaves, or fruit. Terminal and fruit buds can be opened during the dormant season and again just before bloom to determine the relative population level and distribution of mites in an orchard. Since these mites do not disperse readily, initial infestations are often confined to a few trees.
After Bloom, leaf samples can be collected for brushing to detect the presence of adult and immature mites. If mites are active, a sample of 100 developing fruit should be examined around the calyx end for the presence of mites. For fresh market pears, treatment is suggested if PRM infestations exceed 10 adults or nymphs per fruit. For processing fruit or naturally russeted cultivars, the action threshold is much higher.