Hickory anthracnose or leaf spot is caused by the fungus Gnomonia caryae. In wet years the disease causes severe defoliation. The symptoms are large reddish brown spots on the upper leaf surface but brown on the lower surface. The the spots may not have distinct marginal markings. The disease overwinters on fallen diseased leaves so dispose of infected leaves in the fall.
A large number of fungi cause leaf spots on hickory. Severe infections can cause defoliation. Many of the fungi overwinter on diseased leaves, dispose of infected leaves when they fall.
Witches broom can follow a leaf spot disease caused by Microstroma juglandis. Symptoms are yellow blotches on the upper sides of the leaves but white on the undersides. Severe infections may cause defoliation and when the fungus invades the stem a witches broom forms. Leaves on the stems of the witches broom are yellowish, small, and fall prematurely. Prune out witches brooms when they form. No chemical controls are suggested.
Powdery mildew causes a white powdery growth on the leaves. If the disease occurs late in the season, no chemical control will be needed, otherwise use benomyl.
Try to prevent cankers by keeping trees healthy through watering during dry weather and regular fertilization. Avoid wounding the trees unnecessarily and prune out weak or dead branches.
Verticillium wilt kills individual branches or sometimes entire sections of the tree. Foliage on affected branches wilts and later turns brown. Keep trees healthy and prune out small infected branches.