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 can 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 and become 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 might have dozens of spots, resulting in extensive defoliation. Fruit lesions are much like those on leaves, but they are black and slightly sunken. They can be so numerous that they 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 can run together and form a superficial canker. Early defoliation leads to small fruit, weak bud formation, and fall blossoming. Infected fruit have no sale value and often are cracked and misshapen.
The sexual spore stage develops on fallen, overwintered leaves. Conidia--asexual spores--might also develop in the spots on overwintered leaves, or they can 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. In the northeastern states, fungicide applications in June and July generally will control this leaf spot; however, mid-August and September applications are advisable in wet seasons, especially on late varieties such as Bosc.