The black rot fungus, Botryosphaeria obtusa, covers a wide geographical range, attacking the fruit, leaves, and bark of apple trees and other pomaceous plants. The fungus is a vigorous saprophyte and may colonize the dead tissue of many other hosts. However, its parasitic activities are confined mainly to pome fruits.

The disease may occur in three forms: a fruit rot, leaf spot, and limb canker on apple trees. It also causes a fruit rot on pear and quince. In northern regions of the United States, losses from black rot result from the cankering of large limbs and dieback of twigs and branches and from fruit rot.

Symptoms

The first signs of black rot are small, purple spots appearing on the upper surfaces of leaves 1 to 3 weeks after petal fall. Leaf margins remain purple, while the centers turn brown, tan, or yellowish brown. After a few weeks, secondary enlargement of leaf spots occur. Because this is not a uniform expansion, the spots become irregular or lobed in shape, at which time they assume a characteristic "frogeye" appearance: a purple margin with a zone of dark brown surrounding the tan-to-gray center. Small, black pycnidia may appear in the centers. Leaves that are heavily infected will drop from the tree.

Infected areas of branches and limbs are reddish brown and are sunken slightly below the level of surrounding healthy bark. These cankers may expand each year, a few eventually reaching several feet in length. Pycnidia are produced abundantly on limb cankers.

Fruit rot usually appears at the calyx end of the fruit. It can originate at any wound that penetrates the epidermis, including insect injuries. There is usually one spot per fruit, a characteristic that distinguishes black rot from bitter rot. Initially, the infected area becomes brown and may not change in color as it increases in size, or it may turn black. As the rotted area increases, often a series of concentric rings form . The flesh of the decayed area remains firm and leathery. Eventually, the apple completely decays, dries, and shrivels into a mummy. Pycnidia, containing spores of the black rot fungus, appear on the surface of rotted tissue.

Disease Cycle

The fungus overwinters in dead bark, dead twigs, cankers, and mummified fruit . It can invade almost any dead, woody tissue and is frequently found in tissue killed by fire blight. Early leaf infections often are visible as a cone-shaped area on the tree, with a dead twig or mummified fruit at the apex. Spores (conidia and ascospores) are released during rainfall throughout the season. Conidia may continue to be produced during wet periods throughout the summer and may remain viable for long periods. Disseminated by splashing rains, wind, and insects these spores can infect leaves, the calyxes of blossoms, tiny fruit, and wounds in twigs and limbs. Leaf infection develops during petal fall, at which time conidia attach, germinate, and penetrate through stomata or wounds. Infections of fruit and wood may not become visible for several weeks. Initial fruit infections occur during the bloom period but are not usually apparent until midsummer as the apple approaches maturity. Throughout the growing season, infections occur through wounds.

Disease Management

Cultural control strategies can affect the level of black rot in an orchard. Management programs based on sanitation to reduce inoculum levels in the orchard are very important aspects of control.

• Carefully prune and dispose of dead wood. This should be an important component of both current-season and long-range management.
• Prune and remove cankers; properly dispose of prunings by burial or burning.
• Remove all mummified fruit.
• Control fire blight by pruning out infected wood or controlling insect vectors.
• Regular protective fungicide applications are important in controlling the disease, especially during wet years.