Bitter rot on apple Bitter rot on 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. 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. 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.