By Teo Spengler Willow scab disease attacks different types of willow species in Europe and the United States. It can attack weeping willows but is not one of the more common weeping willow diseases. Willow scab is caused by the fungus Venturia salciperda. Scab on willow trees usually doesn’t cause serious harm unless the black canker fungus (Glomerella miyabeanais) is also present. Read on to learn about how to recognize and how to treat willow scab. Scab on Willow Trees Willow scab is a fungal disease that causes leaf symptoms, followed by brown spore masses at the base of leaves. The symptoms of scab on willow start with dark spots on the leaves. These can be brown or black, and cause the leaves to wilt, shrivel up and die. In time, as the willow scab disease progresses, the fungus spreads to the stem tissue at the bases of leave petioles.