Anthracnose is a name for a group of diseases caused by several closely related fungi that attack many of our finest shade trees. It occurs most commonly and severely on sycamore, white oak, elm, dogwood, and maple. Other host plants that are usually only slightly affected include linden (basswood), tulip tree, hickory, birch, and walnut.
Each species of anthracnose fungus attacks only a limited number of tree species. The fungus that causes sycamore anthracnose, for example, infects only sycamore and not other tree species. Other anthracnose-causing fungi have similar life cycles, but require slightly different moisture and temperature conditions for infection.
Anthracnose fungi may cause defoliation of most maple, oak, elm, walnut, birch, sycamore, and hickory species and, occasionally, of ash and linden trees. Damage of this type usually occurs after unusually cool, wet weather during bud break. Single attacks are seldom harmful to the tree, but yearly infections will cause reduced growth and may predispose the tree to other stresses.
Damage may be in the form of:
- killing of buds, which stimulates the development of many short twigs or "witches' brooms;" these may spoil the shape of the tree
- girdling and killing of small twigs, leaves, and branches up to an inch in diameter
- repeated early loss of leaves, which over several successive years weakens the tree and predisposes it to borer attack and winter injury
- premature leaf drop, which lessens the shade and ornamental value of the tree
Specific symptoms of anthracnose vary somewhat depending on the tree species infected. On sycamore, leaves and growing tips of the twigs may die as they emerge from the bud. This damage is often confused with late frost injury. Sudden browning and killing of single leaves or leaf clusters may occur as the leaves expand. The disease continues to develop later in the season, resulting in irregular brown to nearly black, dead areas between or along the main leaf veins and extending to the margin. Infected leaves fall when the petiole is girdled or when several lesions enlarge and coalesce to form large, dead blotches.
After defoliation from spring infections, the tree may appear bare except for tufts of leaves at branch tips. Regrowth appears by midsummer. Sunken cankers form on larger twigs during cooler weather in fall, winter, and spring. Twigs may die as a result of canker formation. When terminal twigs are killed, lateral twigs take over as leaders. Thus, repeated twig dieback results in the formation of crooked branches.