The phrase “willow cherry tree” actually refers to the weeping variety of cherry tree, not to a willow tree. Both have a weeping or drooping appearance, and that’s where the phrase originates. However, cherry trees belong to the Prunus family while weeping willows belong to the Salix family. Both are Pendulas, or weeping, but that is where the similarity ends. The weeping cherry is prone to a number of diseases, most caused by infection.
The fungus disease sooty mold affects weeping cherry trees. It causes a charcoal-black coating that spreads over fruits, leaves and branches. It caused by a combination of insects, like aphids, leaving a clear, sticky substance on surfaces called "honeydew" and wind-borne spores landing on this honeydew. The honeydew becomes a growing medium for the spores and the fungus spreads.
The fungus disease powdery mildew coats leaves in a gray-to-white, dust-like film. It is prevalent later in the growing season during periods of high humidity or when humidity is created by dense foliage of plants that are too close together.
A serious bacterium-caused disease is X-disease, sometimes called cherry buckskin disease. It can destroy a cherry tree. X-disease causes a dieback of leaves that can be gradual or rapid, depending on the tree's variety. Leafhoppers enhance the disease's spread by carrying it from diseased tissue to healthy tissue either on the same tree or on nearby trees.
Black knot develops 1/2 inch to 1 foot warty growths, called "galls," on branches. By the time these knots appear, the fungus has been at work for 2 years. Once the galls are large and dried out, they can split open and allow boring insects to invade the tree.
This fungus affects all aspects of the tree: twigs, blossoms and fruit. Blossoms turn brown, wither and die. Brown spots followed by powdery blemishes cover fruit or seeds and quickly spreads. Twigs develop cankers, and leaves on those twigs turn brown and wither. The loss of too many leaves can cause sunscald.
Cankers are sections of dead bark or wood on a tree caused by a variety of bacterial or fungal infections. Once present, the openings created by the cankers invite insects inside, which can further harm the health of the weeping cherry tree. Large cankers can kill a tree by girdling branches or even the trunk.