Crown gall causes galls to form near the soil line or farther up the plant. Take out infected plants and do not replant in the same area for at least 2 years.
Willow scab attacks and kills young leaves within a very short time. The fungus enters twigs, kills back the young shoots and causes cankers. Olive green spore masses can be seen along the veins on the undersides of leaves. Another fungus, Physalospora miyabeana attacks willow and the two fungi in combination cause willow blight. Prune out infected branches and use resistant species.
Black canker is caused by Physalospora miyabeana and is one of two fungi in the willow blight complex. Black canker causes dark brown spots on the leaves. Whitish gray lesions with black borders appear on the twigs and stems. Prune out infected branches and use resistant species. weeping willow appears to be resistant.
Many fungi cause cankers on willow and infected branches are pruned out. If the trunk is infected and girdled, the tree will die. Keep trees healthy by regular fertilization.
Many fungi cause leaf spots but are not serious enough to warrant preventive sprays. Rake up the fallen diseased leaves in the fall.
Powdery mildew causes a white coating on the leaves. The disease is usually not serious. Sprays of benomyl control powdery mildew.
Rust causes yellow spots on the lower surface of leaves and, if severe, defoliation. Chemical controls are usually not needed. Rake up and destroy leaves from diseased trees.
Tar spot causes black, raised spots on leaves. Rake up and dispose of fallen leaves from diseased trees at the end of the growing season.