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.
Powdery mildew causes a white powdery growth on the leaves.
If the disease occurs late in the season, no chemical control will be needed. 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.