Weeping willow trees are graceful, elegant trees that do well in a wide variety of environmental conditions. They grow well in full or part sun and can tolerate either well-drained or particularly damp soils. Weeping willows are susceptible to a number of different tree diseases, particularly fungal infections. These disorders cause a range of symptoms from cosmetic damage to tree death.
Cytospora chrysosperma is an opportunistic fungus. It commonly infects trees that have been weakened by insect damage, water stress, extreme weather conditions, insufficient nutrients or transplanting, according to the University of Illinois Extension. A Cytospora fungal infection initially appears as a canker, which is a shallow sunken area in the branch that forms at the site of a wound or scar. Other symptoms include dead or dying branches. The University of Minnesota recommends removing infected branches during dry weather and tending to the plant's overall health.
Willow scab is caused by the Venturia saliciperda fungus. According to the University of Minnesota Extension, early symptoms of willow scab include brownish or black spots that develop on leaves. The infected leaves wither and fall to the ground. Olive-coloured spore masses are present on the undersides of dead leaves and tissues. The fungus forms small cankers that encircle branches and twigs in a process called girdling, which severs the cells that send nutrients from the roots to the foliage. This results in twig and branch death. Remove fallen foliage before spring and prune infected branches. Fungicides are effective if applied in the fall after the leaves have fallen.
Black Canker and Willow Blight
Weeping willows are also infected by black canker, a fungal disease caused by the Glomerella miyabeana fungus. Dark brown or black lesions form on leaves, which shrivel and die. Cankers form on wooden stems and produce peach-coloured spore masses in early summer, according to Bartlett Tree Research Laboratories.
Black canker often occurs in combination with willow scab in an infection known as willow blight. Willow blight results in the loss of leaves and branches and can kill the tree in severe cases. Use a fungicide to minimise the symptoms. Prune out infected branches and fertilise the tree.
Aesthetic Fungal Infections
Weeping willows are also infected with several other fungal infections that mainly infect the leaves but do not normally cause serious harm to the tree. Tar spot is a fungal infection that forms raised black spots on leaves. These spots are unsightly but harmless; rake up the leaves at the end of the season and dispose of them so that the fungus does not overwinter on the ground and cause new infections the following year. Powdery mildew is characterised by a powdery white dusting on the leaves. According to the University of Florida Extension, this disease is also harmless. Lastly, rust causes yellow spots and occasional defoliation. Rake up and destroy the diseased leaves.