Pearleaf Blister Mite

Pearleaf Blister Mite Eriophyes pyri (Pagenstecher) also known as pear bud mite, and also by the synonym Phytoptus pyri Pagenstecher, is most often seen in unsprayed trees. This species is part of a complex of related species, which has confused accounts of life history.

The bodies of these eriophyids are elongate and wormlike. The body length is 1/100 inch 200-240 microns; 0.2 mm. Other details have been published separating it from other eriophyids, but these will not be seen without a compound microscope. While in the bud, mites are white; later in the season they assume a pinkish color.

Pearleaf blister mite adults are white to light red and extremely small. The body is sausage-shaped. Nymphs resemble adults, but are smaller. This species causes brownish blisters that appear on the undersides of leaves and fruit. On pear trees, blisters first appear as small greenish pimples that become reddish, then brown. They may cover the lower leaf surface. On developing fruit, early feeding causes depressed russeted spots surrounded by clear halos that look like blisters. Since these mites do not move very quickly or very far, their infestations are often confined to single trees or even single branches. Pear rust mite is similar in appearance to blister mite, but the injury is characterized by a smooth russeting of the fruit.

Life Cycle : Egg: The egg is oval, pearly white and about 40 microns long. Immatures: The first instar nymph is wedge shaped, tapering toward the rear, and is about 70 microns long. It has two pairs of short legs near the front of the body. There are two nymphal stages before maturation with an inactive period before each molt. The inactive period at the end of the second nymphal stage is relatively long. Adult: The female is 200 to 230 microns long and is light to amber yellow. It is cylindrical, tapered sharply at the posterior end, and resembles a short worm. It has two pairs of short legs near the front of the body. The male is about 150 microns long.

Blister mites overwinter as mature females at the base of buds or under outer bud scales. In spring, when buds begin to swell, overwintered females penetrate deeper into buds and lay eggs on live tissue. Development from egg to adult requires 20 to 30 days during the spring. Feeding of females and their offspring causes blisters on developing leaves. As the blisters form, leaf cells near the center of the blisters die and pull apart as surrounding cells enlarge, creating a hole. Mites of the first spring generation enter blisters through these holes and feed on soft leaf tissue inside.

Several generations develop within blisters during a growing season. Summer generations require only 10 to 12 days to develop. When blisters become crowded or leaves become heavily damaged, mites may migrate to growing terminals where their feeding produces new blisters. Fruit damage is caused by injury to buds before bloom. It is not known exactly how blister mite infestations spread from tree to tree or orchard to orchard. However, there is indication they can be carried by wind or by birds and insects.

Damage :Mites feed under the bud scales during winter and may cause buds to dry and fail to develop in spring. Feeding on pears results in oval russet spots, usually depressed with a surrounding halo of clear tissue. Mature fruit is often deformed and misshapen. Leaf feeding causes small blisters that are first red and later blacken.

Most of the injury produced by the blister mite on pear and apple trees in Utah is found on the foliage. This damage is caused by the feeding of the small mites within the leaf, between the upper and under epidermis. In the last stage of attack many small brown corky areas from one-twelfth to one-eighth of an inch in diameter develop on the leaves. These stand out sharply against the green background of the leaf. The leaf tissue beneath these spots is dead. When the spots are numerous there may be so much dead tissue that there is not sufficient healthy teaf surface to carty’on the manufacture of starch for the food of the tree. A tree which lacks healthy foliage is weakened and will have small fruit. In’ severe attacks the leaves may turn yellow, split, and drop from the tree. In some cases all of the foliage will fall by August.

This complex causes blisters on the undersides of pear and apple leaves, especially younger foliage, usually in a row along the midvein . The blisters are tiny green swellings at first, later expanding and turning red. These blisters eventually turn necrotic and brown (blisters that are not invaded by mites remain green). The leaf epidermis is loose and wrinkled on the underside of the leaf, resembling a blister. The blisters may coalesce, forming larger blistered areas along the midvein. The leaf blade may turn yellow, leaving a dark band along the midvein, dark green on the top and brown on the bottom of the leaf. Small blisters may also occur on stems and around the fruit calyx. This may cause fruit drop, but this is usually less common than the foliar injury. Leaf injury can result in small, sparse leaves.

Control:Blister mites are not normally controlled by natural enemies. The predatory mite, Typhlodromus occidentalis, which can control spider mites on apples and pears, will also feed on blister mites when they are exposed. However, it cannot get into blisters.

Pearleaf blister mites occur throughout pear-growing areas in California. Abandoned and unsprayed young pear trees are subject to severe, periodic blister mite infestations. Blister mite has increased in orchards under long-term mating disruption programs, especially in mating disruption orchards adjacent to abandoned or unsprayed orchards. Monitor and treat in fall or dormant season. the use of pyrethroids may increase blister mite damage.

Examine terminal and fruit buds for mites during dormant and again just before bloom. During the summer, examine shoot foliage and the calyx end of developing fruit. Applications of Vydate or Diazinon at delayed dormant or prepink should provide a good control of blister mite.

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