Insects and Diseases of Trees in the South

by Anonymous

WALNUT ANTHRACNOSE

Walnut anthracnose is a fungus disease caused by Gnomonia leptostyla. This worldwide disease attacks most species of walnut in the United States. Black walnut is most severely affected, but with favorable weather for the fungus, even less susceptible individuals may be defoliated. Butternut, Persian walnut, and two species from California (Hinds walnut and California walnut) are all susceptible. Anthracnose has also been reported on species of walnut from most of the European countries, Argentina, Canada, and South Africa.

Wet weather greatly favors this leaf disease which may defoliate black walnuts by late July or early August. Defoliation slows growth, weakens trees, and sometimes causes mortality. Infected leaves reveal circular spots of dark brown or black. These spots often grow together, leaving large dead areas. These spots or blotches are bordered with yellow to golden tissue. While severely affected leaves fall, some “anthracnose” leaves remain on the tree. This disease also affects the growth and quality of the nuts. Nutmeats from infected trees are dark, unattractive, and shrivelled. Sunken, killed areas appear on the husks as dark circular spots smaller than those on the leaves. Infected nuts, like the leaves, may also fall from the tree. Lesions may appear on the current year’s shoots and later form dead sunken areas that are oval to irregularly circular with reddish brown margins.

As with the other anthracnose diseases, no practical control is available for forest trees. Control of walnut anthracnose on ornamentals and nut trees is partially achieved by raking and burning of old leaves.