Weeping willows are flowing graceful trees that do best near a water source. If you are planting new willows, make sure that you don't plant them near underground pipes or septic systems. Their roots can damage property in their efforts to get to water. Be on the lookout for certain diseases that can plague these trees. Most won't cause any long-term damage, but there are a few that will require some action on your part.
Black canker creates infected areas on the bark of branches, leaving a gray mark with a black outline. Leaves will show brown spots. Trim out all diseased areas by cutting out the branches. Willow trees are fairly hardy when it comes to this disease and should bounce back fine.
Crown gall is a serious disease that causes knots, called galls, to develop on the trunk of the tree. The knots form near the base of the trunk as well as further up into the tree. It is the formation of the galls around the base of the tree, or crown, located near the soil that give it the name crown gall. The trees will have to be removed and disposed of. You won't be able to garden or plant in the vicinity of where the trees were for two or more years.
Fungus causes many different issues with weeping willows. Willow scab is a disease caused by the fungus Venturia saliciperda, which goes after young leaves. It enters through the new growth stems and kills the new leaves as they develop. Olive green spore congregates underneath leaves, causing spots and killing leaves. Physalospora miyabeana, or black canker, in combination with a second fungus disease called willow scab, can bring on willow blight, which sometimes results in the death of the tree.
The best way to rid a tree of these diseases is to cut out the infected branches. There are also less evasive fungi that do little more than create spots on leaves. Removing leaves in the fall will help prevent the spread of fungus.
Powdery mildew causes white residue on foliage. Tar spot creates black bumps on foliage. These are not typically something to worry about it. Just remove the leaves that fall in order to prevent spread or re-contamination the next year.