Black Rot of Apple
The black rot fungus, Botryosphaeria obtusa, covers a wide geographical range and attacks the fruit, leaves, and bark of apple trees and other pomaceous plants. The fungus is a vigorous saprophyte and can colonize the dead tissue of many other hosts; however, its parasitic activities are confined mainly to pome fruits. The disease can occur in three forms: as a fruit rot, leaf spot, or limb canker on apple trees, and as a fruit rot on pear or quince trees. In northern regions, losses from black rot principally result from the cankering of large limbs and the dieback of twigs and branches. Losses from fruit rot and defoliation resulting from leaf spot can be considerable, especially in warm, humid areas of southern and central fruit-growing regions of the eastern United States.
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 occurs. Because this is not a uniform expansion, the spots become irregular or lobed in shape, at which time they assume a characteristic "frog-eye" appearance: a purple margin with a zone of dark brown surrounding the tan-to-gray center. Small, black pycnidia (pimple-like fruiting bodies of the fungus) might appear in the centers. Leaves that are heavily infected will drop from the tree.
Infected areas of branches and limbs are reddish brown and sunken slightly below the level of the surrounding healthy bark. These cankers might expand each year, with 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. Usually only one spot occurs per fruit--a characteristic that distinguishes black rot from bitter rot. Initially, the infected area becomes brown and stays brown or turns black as it increases in size. As the rotted area increases, a series of concentric rings often forms. The flesh of the decayed area remains firm and leathery. Eventually, the apple completely decays, dries, and shrivels into a mummy. Pycnidia that contain spores of the black rot fungus appear on the surface of the rotted tissue.
The fungus overwinters in dead bark, dead twigs, cankers, and mummified fruit. It can invade almost any dead, woody tissue and frequently is 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.
In the spring, spores (conidia and ascospores) are released during rainfall throughout the season. Conidia might continue to be produced during wet periods throughout the summer and can 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 might 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.
Cultural control strategies can affect the level of control achieved for black rot. Management programs based on sanitation to reduce inoculum levels in the orchard are the primary means 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.