On the fruit of nectarine, the same fungus causes a disease known as anthracnose; on grape, it causes ripe rot; and on chestnut, it causes blossom-end rot of green burrs. The discussion below is limited to the disease as it affects apple and pear trees.
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 first appears during midsummer or later as a small, light-brown, circular spot. One or many spots might 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 might be arranged in concentric rings. Later, they ooze a gelatinous, salmon-pink mass of spores, which is washed by rains onto 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.
The fungus overwinters in mummified 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 onto developing fruit. Often, the first infections appear as a cone-shaped area within the tree and can be traced to a source of spores at the tip of the cone. The disease develops optimally in rainy conditions, with a relative humidity of 80 to 100 percent and a temperature of 85°F.
Routine fungicide sprays normally control bitter rot in Pennsylvania. Summer fungicide applications should not be extended beyond 14-day intervals. Sanitation practices, such as removing mummified fruit, are important in controlling this disease.