These mites cause brownish blisters on the undersides of pear and apple leaves. On pear, the blisters appear as small, greenish pimples that soon take on a reddish color and later turn brownish. On apple, the first stages are pale yellowish and the blisters never become as red as on pear. The blisters, about 3 mm wide, may be massed together to nearly cover the undersides of the leaf surface. If you like at these with a hand lens, you will see many small mites.
Heavy infestations cause blistering of leaves that can seriously impair leaf function.
Early feeding of the mites on the developing fruit causes depressed russet spots, which can be the most serious aspect of blister mite attack. The fruit is often deformed and misshapen. The presence of these oval russet spots, usually depressed with a halo of clear tissue surrounding each spot, is a characterisitic symptom of blister mite attack. The Bartlett pear is very susceptible to fruit damage. Factors Affecting Abundance Failure to apply effective prebloom sprays with thorough coverage can result in a buildup of pear blister mite populations.