Causal Agent
Microstroma juglandis
Hosts
Various species of walnut and hickory.
Symptoms
In the spring, the upper leaf surface will show chlorotic (yellow) spots which may become necrotic (brown). The lesions may coalesce so that larger areas are affected. On the underside of the leaves, a white cottony coating can be seen opposite the chlorotic spots. This fungal growth will be most visible during humid periods. Severely affected leaves may fall prematurely. In some cases, the fungus will cause a witches' broom (abnormal clustering of shoots) near the end of branches. The leaves in the broom are often chlorotic and stunted. The fungus most likely survives in twigs and branches.
Control
Downy leaf spot is rarely a significant problem. No fungicides are recommended for this disease. Fallen leaves should be raked up and discarded in the trash. Infected twigs and branches may be removed by pruning and discarded.