Leaf spot diseases commonly occur on black walnut and other kinds of trees. They are caused by species of fungi that produce yellow, brown, or black spots on the leaves. Defoliation usually follows as the disease progresses. Foliage diseases are considered to be less serious than other types of diseases in black walnut because defoliation normally occurs late in the growing season. However, defoliation reduces growth and resistance to other stress factors.

DOWNY LEAFSPOT,
MICROSTROMA JUGLANDIS (BERANG.) SAC.

Description
    Downy leafspot, also known as white mold, is characterized by small, white fuzzy areas on the lower surface of walnut leaves and pale yellow spots on the upper surface. The white fuzzy areas often fuse together so that almost the entire lower leaf surface may appear to be covered with a white powder.

Injury
    The disease is of little significance because it does not kill the leaves or cause defoliation.

Control
    No control is recommended.

POWDERY MILDEW,
MICROSPHAERA ALNI DC. EX WINT.

Description
    Powdery mildew is a common disease on many plants and is caused by many different, closely related species of fungi.

    The fungus attacks leaves, buds, and young shoots covering diseased parts with a white powdery-looking mass of spores. The disease develops best in warm weather and humidity.

Injury
    Heavily diseased leaves may shrivel up and turn brown. However, its occurrence on black walnut is rare.

Control
    No control is recommended.

WALNUT ANTHRACNOSE,
GNOMONIA LEPTOSTYLA (FR.) CES. & DE NOT.

Description
    Walnut anthracnose is the most common leaf spot disease of black walnut and occurs wherever black walnut is grown.

    The fungus causes small dark brown to black circular spots up to 1/2 inch in diameter on the leaves. These spots are usually bordered with yellow. Wet spring weather is ideal for the development of the disease, although symptoms may not become visible until June or July.

Injury
    By mid-August leaves on trees infected with walnut anthracnose begin to turn yellow and drop. The growth rate of mildly to heavily infected trees is reduced. Heavily infected trees may be completely defoliated. Trees are not killed by the disease unless they are under stress from other causes. Anthracnose may also affect the quality of the nuts.
Control
    Contact your service forester or county extension agent for recommended control. Fertilization with nitrogen may help reduce the harmful effects of this disease.

TARGET LEAFSPOT,
CRISTULARIELLA PYRAMIDALIS WAT. & MARSH.

Description
    Target leafspot, also known as bull's eye or zonate leafspot, is a recently discovered disease on black walnut. So far it has been reported on black walnut only in Illinois and Ohio.

    The disease causes characteristic circular spots on the foliage with dark brown concentric rings inside each spot. These spots strongly resemble small targets, hence the name.

Injury
    The disease progresses rapidly in black walnut. The leaf spots fuse together, causing an entire leaf to curl up, turn brown, and fall from the tree. Symptoms first appear early in August. Within only a few weeks the entire tree may be defoliated.

    The disease causes premature defoliation, thus reducing the tree's growth, vigor, and resistance to other stress factors.

Control
    No control has yet been developed. Fertilization with nitrogen should help minimize some of the harmful effects.