Question: I have a pear tree that has some bumps on the leaves. Over time the bumps turn into brown spots. Is this zinc deficiency?
Answer: No, this is not zinc deficiency. The damage you see is caused by the pear leaf blister mites (Phytoptus pyri Pagenstecher). This mite was introduced from Europe, probably before 1900. It is a pest of most pear growing areas of the world. Feeding by these mites causes damage on leaves and fruit. Blister mites over winter as mature females at the base of buds or under outer bud scales.
Adults are light to amber yellow in color and cylindrical, tapered sharply at the posterior end and resemble a short worm. In spring, when buds begin to swell, overwintering females penetrate deeper into bud and lay eggs on live tissues. Development from egg to adult requires 20 to 30 days during spring. Feeding of females and their offspring causes blisters on developing leaves.
Blisters are green or red at first but turn light brown to black as affected tissue dies. As the blisters form, leaf cells near the center of the blisters die and pull apart as surrounding cells enlarge, creating a hole. Blisters vary in size, with the largest about 1/8 inch in diameter. Mites do not live in the blisters on the fruit, but the fruit will be scarred. Mites of the first spring generation enter blisters through these holes and feed on soft leaf tissue inside.
Several generations develop within the 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. Severe damage to foliage can cause leaf drop and reduce shoot growth.
Look at young leaves before bloom early in the spring just as leaves are unrolling. Noticeable light green to light red rough areas where mites have been feeding will be seen. This damage becomes more noticeable as the growing season progresses.
Control: Blister mites are not normally controlled by natural enemies. Predatory mites will feed on blister mites when they are exposed. Blister mites often attack weak, neglected or abandoned trees. If a pesticide is used to control blister mites the best timing is after harvest when mites migrate from leaf blisters to terminal and fruit buds. They are exposed on those sites until buds swell in the spring. Pre-bloom treatments can prevent fruit damage that occurs just before and during bloom.
Source: Orchard Pest Management. 1993. Edited by Elizabeth H. Boers, et al. Good Fruit Grower Publications, Yakima, WA. Pages 151-153.