 |
 IDENTIFY AND CONTROL LEAF SPOT DISEASES OF BLACK WALNUT |
| 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.
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:
Control: 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.
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:
Control: 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.
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:
Control: 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. W.M. BLACK Graduate Research Assistant University of Illinois Urbana, Illinois 61801 DAN NEELY Plant Pathologist Illinois Natural History Survey Urbana, Illinois 61801 JAMES A. MATTEONI Graduate Research Assistant University of Illinois Urbana, Illinois 61801 For more information on these diseases, contact:
|
