The weeping willow (Salix spp.) is widely planted throughout the United States in residential neighborhoods as a screen tree around retention ponds. It tolerates moist soils and possesses aggressive, shallow roots. Most recognized for drooping, graceful branches, the weeping willow is fast-growing and deciduous. While generally not affected long-term by pests, a few diseases do afflict the weeping willow in residential landscapes.
A soil-borne bacterium known as A. tumefaciens often infects weeping willow trees through damaged roots or trunks. It causes abnormal cell growth that forms galls around the base of the tree. The galls appear round and rough, sometimes up to 1 foot in diameter. Infected trees must be dug up and destroyed; new trees shouldn't be planted in the same area for several years.
The fungi that cause willow scab enter the tree through young twigs. Young leaves and shoots die off very quickly. You can easily identify this problem by inspecting the undersides of the tree's leaves--dark green masses of spores appear along the leaf veins. Use sterile shears to prune out affected branches as soon as the disease is noticed.
Black canker disease often attacks weeping willow trees in conjunction with willow scab. Much like the symptoms of willow scab, black canker produces irregular black spots on the young leaves of willow trees. If allowed to progress, the disease will continue to spread, shriveling the leaves and producing black wounds on the smaller branches. In a short amount of time, younger branches of the tree will look as though they have been burned. Again, the best method of control is to prune out infected branches as soon as symptoms appear, making sure your pruning tools are sterile, lest you will transfer the disease to other parts of the tree.