Anthracnose on Trees
The term anthracnose refers to a variety of fungal infections, with each fungus dependent on a certain host tree for development. In our area, the anthracnose fungi commonly attack ash, elm, oak, maple, black walnut and sycamore.
The symptoms of anthracnose vary according to the host plant and the severity of attack. For instance, the disease can run along the leaf veins in maple and oak, as well as cause malformations of the leaves. Also, on oak, leaf buds and new shoots can be killed. The elm anthracnose fungus appears first as small whitish flecks on the upper surface of the leaf. On walnut, the fungus first appears as small, dark circular areas on the leaves; it will also attack the nuts. But it is the sycamore, more than any other tree in our area, that suffers most from anthracnose. When conditions are right, sycamore leaves, bud shoots and one-year old twigs may be infected, resulting in shoot dieback and a witch’s broom effect. The disease first shows as a scorching and wilting of new shoots and leaves.
Anthracnose fungi overwinter on leaf and twig debris of the host tree and in dead areas of the bark. The severity of the infection depends on the weather; a cool, wet spring favors development of the fungus. When conditions are right, the disease can continue into early summer.
Although the effects of anthracnose are not esthetically pleasing, because it rarely causes significant long-range damage to trees, controls are usually not required. But the incidence of anthracnose can increase a tree’s susceptibility to other diseases or insect problems if the tree suffers infections year after year.
How can you control the disease on landscape plantings?
For accurate diagnosis of anthracnose, take leaf samples showing the disease in progress to your County Extension Service, which can also give you a current list of fungicides that are used for anthracnose control, and a list of trees that are resistant.