Weeping willow trees grow very fast, provided they have the right conditions to thrive. They also die very quickly, however, as they are susceptible to many diseases that kill them slowly as they grow older. Weeping willows can withstand a few diseases, but eventually a combination of these plus other infections will kill them.
A number of fungi cause leaf spot on the weeping willow tree. The diseases in this family that affect the tree include tar spot, gray scab, leaf and twig spots and marssonia spot. These diseases do not warrant the use of preventative sprays, as they do not destroy the willow. Simply rake up infected leaves and twigs at the end of the season and throw them away to prevent the spread of disease.
Powdery mildew occurs on weeping willows in shady areas exposed to long periods of moderate temperatures. In these conditions, fungi begins to grow on the leaves, creating a white coating. The spores can travel to other plants and infect them. The disease is not serious for the willow, but it could spread to other plants in the area. To prevent this, prune away infected branches if possible, or apply fungicides to the tree.
Willow scab is a fast-moving infection that invades young shoots on willow trees and kills them. Dark green spots and cankers appear on the underside of the leaves. Weeping willows have some resistance to the disease, and are thus not as susceptible as other willow cultivars.
The fungus Physalospora miyabeana causes the black canker disease. It causes dark brown or black spots on leaves and white lesions on branches. Black canker combined with willow scab causes the deadly willow blight. Weeping willows, however, are resistant to black canker as well as willow scab.