Each lesion may have dozens of spots, resulting in extensive defoliation. Photo by K. Peter.
This disease should not be confused with the fire blight or leaf spot diseases of pears. The disease can build up rapidly, even in orchards where it has not been a problem. If conditions favor the disease and it is not controlled, pear trees may become defoliated in a few weeks.
Leaf spots first appear as small purple dots on the leaves nearest the ground. They grow to circular spots about ¼ inch in diameter, becoming purplish black or brown. A small black pimple appears in the center of the spot. When the leaf is wet, a gelatinous mass of spores oozes from the pimple and gives the spot a creamy, glistening appearance. Each lesion may have dozens of spots, resulting in extensive defoliation. Fruit lesions are much like those on leaves, but they are black and slightly sunken. They may be so numerous as to run together and make the fruit crack.
Lesions on twigs occur on current-season growth. They are purple to black, with indefinite margins. The lesions may run together and form a superficial canker.
Early defoliation leads to small fruit, weak bud formation, and fall blossoming. Infected fruit has no sale value and often is cracked and misshapen.
The sexual spore stage develops on fallen, overwintered leaves. Conidia, asexual spores, may also develop in the spots on overwintered leaves, or they may be produced in the previous season's shoot infections. Often the first infections do not occur until mid-June to the first of July. Secondary infections begin about 1 month later and reoccur throughout the season during periods of rain.
Routine fungicide sprays normally control this disease in Pennsylvania.