This disease should not be confused with the fire blight or leaf spot diseases of pears. Leaf blight and fruit spot are caused by the fungus Fabraea maculata, which infects the leaves, fruit, and shoots of pear and quince and the leaves of apple trees. 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.

Susceptible pear varieties often are defoliated by mid summer, resulting in dwarfing of fruit and reduction of fruit buds. Infected fruit is unmarketable since fruit are usually cracked and misshapen. In the nursery, early defoliation and twig infection result in stunted growth of the tree.

Symptoms

Leaf spot can be found on petioles, leaves, shoots, and fruits. Leaf spots first appear as small purple dots on the leaves nearest the ground. They grow to circular spots 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 ooze from the pimple and give the spot a creamy, glistening appearance. Spots coalesce and severely infected leaves fall to the ground prematurely.

Fruit lesions are larger than those on leaves and cause the fruit to crack and drop. 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 have no sale value and often are cracked and misshapen.

Disease Cycle

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. The susceptibility of leaves and fruit to infection does not decrease with maturity.

Disease Management

Routine fungicide sprays normally control this disease in Pennsylvania. In the northeastern states, fungicide applications in June and July will generally control this leaf spot; however, mid-August and September applications are advisable in wet seasons especially on late cultivars such as Bosc.