The most common cause of raised blisters on pear tree leaves is an insect called the pear leaf blister mite (Eriophyes pyri). Left unchecked, this mites moves from a pear tree's foliage to its fruit, where it causes the most serious damage. Trees producing smooth-skinned pears--Bartlett, Comice and Anjou--are the most susceptible to the mites, according to the Washington State University Tree Fruit Research and Extension Center. Pears with russeted skin, including Bosc and Hardy, are immune.
Belonging to the Eriophydae mite family, the pear leaf blister mite arrived in the United States from Europe in the 1870s, according to the Michigan State University Extension. It achieved pest status in the United States in 1902. The mites now infect all commercial pear-growing regions in Canada and the United States. The most significant U.S. infestations occur in the Pacific Coast states.
The mites hatch from oval white eggs. Although they are smaller, mites in the three nymphal stages resemble adults. The adults are white or yellow, with females 25 percent larger than males. They have long bodies with sharply tapering hind ends and two pairs of legs directly behind their heads.Their bodies are similar to those of small worms, notes WSU.
Adult mites burrow into the pear tree's bud scales--the leafy sheaths that protect developing buds--in late summer or early fall. They overwinter there and resurface when the trees' leaves emerge in the spring. They burrow into the undersides of the new leaves and emerging flower buds to feed, irritating the tissues. The irritated tissue forms telltale raised blisters. Female mites lay their eggs inside the blisters. The nymphs mature there, emerging to feed on other leaves and buds and form more blisters. The cycle repeats throughout the tree's active growing season until the last generation of adults migrates back into the bud scales for the winter.
Pear leaf blister mites overwintering on trees feed beneath the bud scales, possibly preventing them from developing properly in the spring. They also eat emerging leaves and developing fruit. Serious leaf infestations can hinder the trees' photosynthesis and cause leaf drop. Infested fruit develops 1/4- to 1/2-inch, indented russet spots. The spots often merge, severely scarring the pears. Mature pears may be misshapen, according to the University of California's Integrated Test Management Program.
Because pear leaf blister mites are not pesticide-resistant, spraying them with a pesticide on the state's extension service recommended list is the best means of control. The pesticides will work most efficiently after the pear harvest, and when the mites are exposed. The insects are most vulnerable during their migration from the leaf blisters to leaf and fruit buds.