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Bitter Rot of Apple
Bitter rot is an important disease in the southern states but occurs in Pennsylvania only infrequently. It is caused by the fungus Glomerella cingulata. Its hosts are apple and pear trees. On peaches and nectarines the same fungus causes a disease known as anthracnose; on grapes it causes ripe rot; and on chestnuts it causes blossom-end rot of green burrs. The discussion below is limited to the disease as it affects apple and pear trees.
Symptoms
Bitter rot occurs only on fruit. Cankers can form on twigs, but they are rare. The fungus is one of the few fruit rot organisms that can penetrate the unbroken skin of the fruit. The disease is noticed first, during midsummer or later, as a small, light brown, circular spot. One or many spots may appear; if temperatures are high they enlarge quite rapidly and soon change to a dark brown. By the time the spots are 1/8 to 1/4 inch in diameter, they are distinctly sunken or saucer-shaped. When they reach 1/2 inch in diameter, small black dots, the fruiting bodies of the fungus, appear in the sunken lesion. These may be arranged in concentric rings. Later, they ooze a gelatinous, salmon-pink mass of spores, washed by rains to other fruit. Beneath the surface of the spot, the flesh is light brown and watery in a cone-shaped area, with the small end of the cone toward the fruit center. As the fruit ripens, it decays rapidly and finally shrivels into a mummy.
Disease cycle
The fungus overwinters in mummied fruit and in cracks and crevices in the bark. Jagged ends of broken limbs are ideal sites. With the advent of warm weather the fungus produces spores washed by rains to developing fruit. Often the first infections appear as a cone-shaped area on the tree and can be traced to a source of spores at the tip of the cone. The optimal conditions for the disease to develop are rains, relative humidity of 80 to 100 percent, and a temperature of 85°F.
Disease management
Routine fungicide sprays normally control bitter rot in Pennsylvania. Summer fungicide applications should not be extended beyond 14-day intervals.