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.