The weeping willow is a popular and beautiful tree with long, slender leaves that bend gracefully to the earth. Weeping willows are large (they can reach a height of 70 feet) and need plenty of room to spread out. They love moist soil and are quite hardy, being resistant to many pests and even verticillium wilt, which is a deadly disease that affects many trees. Still, weeping willows can suffer from some pests and diseases.
Weeping willows are affected by many boring and chewing insects, including caterpillars and aphids. Most of these insects only superficially affect the tree, causing minor damage. The gypsy moth, however, favors the weeping willow and can quickly defoliate the tree. The larvae of the moth emerges in the spring, just as the tree is budding, and make quick work of the young, tender leaves. If the weeping willow is healthy and mature, the tree will produce new foliage mid-summer.
Willow scab is a fast-moving fungal disease that will attach and kill new leaves very quickly. The fungus enters through the twigs at the point where new shoots are emerging. The weeping willow will develop olive green masses of spore growth on the undersides of the leaves, and cankers (cracked, sunken areas of wood) will appear on the twigs and branches. When cankers completely encircle a branch, they can cut off the flow of nutrients to that branch, causing it to die. In young willow trees, cankers can even girdle the trunk.
If another fungus, Physalospora miyabeana, attacks the willow at the same time, the combination of the two fungi will cause willow blight, which causes the tree to begin to die from the outermost tips of the branches inward. Affected branches should be immediately pruned off the tree to prevent the spread of either fungus.
Leaf spot is caused by several different types of fungi. Rust can cause yellow spots to appear on the leaves, while tar infects the leaves with large, sticky black spots. Powdery mildew appears as white spots that can spread and eventually cover all of the leaves with a fine, powder coating. None of these will permanently harm the weeping willow tree, but they are unsightly. Rake up infected leaves in the fall so that the fungi do not overwinter in the soil.