Pear Leaf Spot
The pear leaf spot fungus, Mycosphaerella pyri, infects the leaves of pear, quince, and occasionally apple trees. Numerous leaf spots can produce defoliation. Fortunately, this does not occur often before fall, except in nurseries.
Mature leaf spots are recognized easily by their grayish-white centers with sharply defined margins. Appearing first on upper leaf surfaces as small, brown lesions, they enlarge from 1/8 to 1/4 inch in diameter. The borders become dark brown, and small, black pimples appear in the centers.
Sexual spores are produced on overwintered, fallen leaves and are carried by air currents to newly formed leaves. About a month after infection, new spores are generated in the centers of the grayish-white leaf spots, from which they are washed by rain to other leaves. These secondary infections usually peak in late summer or early fall.
Routine fungicide sprays normally control this disease in Pennsylvania. The control program for Fabraea leaf spot will usually control this leaf spot as well.