What to do about the spots on your apple leaves

PRESTON — Blister mites are a common sight in backyard orchards. In the summer and fall, their damage can be seen as brown raised “blisters” on the leaf surface. In our area, the most common blister mite in the orchard is pear and apple-leaf blister mites. Blister mites are too small to be seen without magnification.

These mites belong to a group called eriophyid (er-ee-oh-fy-id) mites. Eriophyid mites are translucent, cigar-shaped microscopic mites that cause deformities on many plant species.

These mites are noticed when their feeding causes abnormalities of plant tissues such as galls, brooms, leaf curling, blisters, rusts, silvering, fruit russeting and deformed buds, etc. Fortunately, these mites rarely cause serious harm to plants, and control is seldom needed.

Some mites are host specific, while others attack many plant species. The mites generally overwinter as fertilized adult females under bud scales or protected sites on or near the host plant, emerging at bud break in spring. Both males and females are present throughout the growing season. Reproduction is continuous, with generations completed every two to three weeks.

If populations are high, blister mites on apple and pear leaves can reduce photosynthesis and tree vigor. Treat heavy infestations in early fall (October) before leaf drop, when mites are migrating from leaves to buds. Options include carbaryl, horticultural oil or lime-sulfur. Horticultural oil applications in spring at bud break are also effective. Blister mites cannot be treated in summer.

Because eriophyid mites are generally a cosmetic issue, you should favor reduced-risk treatment options (oil or sulfur). Beneficial predatory mites feed on eriophyid mites and other pests. Reduced-risk treatment options minimize impacts to the predator mites as well as beneficial insects. As you preserve beneficial species you maintain local ecological integrity and will experience fewer pest outbreaks in the future.