Weeping willow trees are gorgeous ornamental species, nonnative to North America, but now firmly entrenched across the continent as landscaping plants and growing in the wild. Various diseases can take their toll on the weeping willow, with some precipitating serious symptoms in the tree.
Willow scab is fungal in nature, brought about on a weeping willow by an agent known as Venturia saliciperda. Willow scab attacks the fresh foliage on the weeping willow. Symptoms include reddish patches and blackened blotches on the leaf surface. In a short time, the leaf falls from the weeping willow. Willow scab then makes its way into the branches and the twigs of the tree. Once established, willow scab opens the door for other fungal ailments, including black canker.
Black canker goes hand in hand with willow scab, allowed into the weeping willow once willow scab infects a specimen. Black canker's fungus, Glomerella miyabeana, results in darkened areas of brown showing up on the weeping willow's narrow leaves. The stems then develop light grey lesions that have black borders surrounding them. Once black canker makes its way all around a branch, the branch will perish. The tree can die if the black canker appears several years in a row.
A less serious disease that can affect weeping willows is powdery mildew. The malady takes its name from its main symptom, which causes a whitish coating to appear on the leaf surface, resembling powder or dust. Powdery mildew is another fungal disease of plants, with lilacs one of the most common victims of this problem. Powdery mildew typically shows up late in the growing season, according to the Cornell University website. In some plants, the disease can kill the leaves or make them distorted and drop off prematurely; in weeping willows powdery mildew usually just adversely affects the tree's appearance.
Other weeping willow tree diseases include tar spot, a relatively benign condition that sees black raised spots emerge on the leaves. Another fungal disease, leaf spot, is a problem cosmetically for the foliage of the weeping willow. Rust is more problematic for a weeping willow. This disease shows up on the undersides of the leaves in the form of yellow spots. Rust can bring about a severe defoliation of the tree, which can hurt the willow, especially if this occurs year after year.