How to...

North Central Forest Experiment Station
Forest Service
U.S. Department of Agriculture
St. Paul, Minnesota

Growing black walnut in pure stands is becoming more and more common. Although this practice has definite economic advantages, it can aggravate disease problems. This is especially true of leaf spot diseases, which multiply rapidly where large numbers of susceptible leaves are concentrated in a small area.

Three such diseases are especially common on black walnut: walnut anthracnose, bull's-eye leaf spot, and white mold. All are becoming more prevalent as more walnut plantations are established. The symptoms produced by each of the three diseases are distinct and the grower can easily learn to tell them apart.

Gnomonia leptostyla

Found wherever walnut is grown, walnut anthracnose causes premature loss of leaves, usually resulting in reduced growth and increased susceptibility to other diseases. Anthracnose also occurs on other nuts, causing the nutmeats to shrivel and darken.

Look for:

Dark angular spots on the leaves ranging from pin-prick size to 1/2 inch (12 mm) in diameter.

Very small black bumps (acervuli) within the spots, especially on the underside of the leaf near veins. You can easily see them through a hand-held magnifying lens. Acervuli are the fruiting bodies of the fungus;

Prematurely defoliated trees. Examine some fallen leaflets. The presence of numerous spots with acervuli on the leaflets is a clue that anthracnose was the primary cause of the defoliation.

crecent-shaped, bicellular spores (conidia) which are responsible for the spread of the disease.

they contain many crescent-shaped, bi-cellular spores (conidia: 20 x 3 u), which are responsible for the spread of the disease.


Trees not severely defoliated every year are not sufficiently harmed by the disease to warrant control measures. However, should control be required, spray benomyl or dodine fungicide on the foliage. (Benomyl is not registered for use on walnut trees grown for nut production.) Proper maintenance of soil fertility, especially with applications of nitrogen fertilizers, will also help minimize the damage.

Cristulariella pyramidalis

Bull's-eye leaf spot, like anthracnose, causes premature defoliation of black walnut trees. The range of bull's-eye leaf spot is not known. Recently, however, sever outbreaks of the disease have occurred in southern Illinois and Ohio, defoliating entire stands of walnut trees. Cristulariella also attacks maple, hickory, and many common weeds.

Look for:

Dark, round, dead, or dying areas with concentric white rings on leaves. These spots are more rounded than anthracnose spots and the rings give them a target-like appearance.

The white Christmas treelike fruiting body of the fungus (approximately 350 x 100 u) on the underside of the leaf. No other pathogen of walnut produces a fruiting body resembling Cristulariella.

Target-shaped spots on newly fallen leaflets, and for the Christmas treelike sporophore. An abundance of these prove bull's-eye leaf spot to be the cause of defoliation.

No means of controlling Cristulariella has yet been developed. Fertilization with nitrogen should help minimize the harmful effects of the disease. Also weed control in and around the planting may help reduce early-season infection.

Microstroma juglandis

White mold, or downy leaf spot, is common wherever walnut is grown. The disease is more unsightly than damaging. Microstroma does not kill the leaf, nor is it known to cause defoliation. Any harm to the walnut is probably secondary due to the shading of a portion of the leaf surface and hence a decrease in photosynthesis.

Look for:

A whitish growth on the underside of the leaf, often concentrated along the veins, and for a yellowish discoloration on the top surface of the leaf.

No control is recommended.

Leaf diseases do not cause all defoliation of black walnut trees. Insects (e.g., the walnut caterpillar) and drought can also be the culprits. The fallen leaflets may show black specks of flecks but seldom spots typical of the three diseases described.


Graduate Research Assistant
University of Illinois
Urbana, Illinois 61801

Plant Pathologist
Illinois Natural History Survey
Urbana, Illinois 61801

Graduate Research Assistant
University of Illinois
Urbana, Illinois 61801

For more information on these diseases, contact:

NA State & Private Forestry
100 Matsonford Road
5 Radnor Corp. Ctr., Ste. 200
Radnor, PA 19087-4585
North Central Forest
Experiment Station
1992 Folwell Avenue
St. Paul, MN 55108