The willow family of trees are commonly found growing along the banks of rivers and streams. These moisture-loving trees are well known for their rapid growth. Unfortunately, this quick growth produces a weaker wood than other deciduous trees, causing the willow tree to become highly susceptible to fungal diseases.
Fungi causing powdery mildew disease are commonly found on the willow tree. This fungi covers the leaves, twigs and flowers in a white or grey mould, according to the Yardener website. Most often, the mould is velvety in texture and can cause budding growth to die.
To treat powdery mildew, remove all fallen leaves and infected branches from the trees. In most cases, the powdery mildew infection does not seriously damage the tree. In severe cases, treat willow trees with a sulphur-based fungicide, according to the North Carolina State University Plant Pathology Extension.
Heart rot is one of the most severe fungal diseases of the willow tree, according to the North Dakota State University Agriculture website. Since the wood of the willow tree is commonly weak, wood-decaying fungi may invade the tree. Heart rot fungi cause the trees to develop hollow trunks, dead branches and eventually kills the entire tree.
Mushrooms and conks developing on the trunks of the willow are the most common symptoms of heart rot. To help prevent heart rot, avoid pruning older trees. Quickly remove any broken branches or stumps following a storm. Have an tree specialist check infected trees and remove the trees if necessary.
Rust disease causes the development of yellow spots on the underside of willow leaves, according to the Yardener website. As the rust disease progress, the leaf spots develop into fungi spores, continuing to spread the disease. Most often, rust disease is not severe and will not cause significant damage. Young willow trees infected with rust fungi may drop leaves. To prevent the fungi spores from spreading to other trees, treat with a rust disease fungicide for seven to 10 days, according to the North Dakota State University Agriculture website.
Fungi also contribute to shoot blight in the willow tree. As the willow tree produces new shoot growth, these shoots turn black and eventually die. The fungi responsible for shoot blight most commonly develops during wet weather. While unsightly, shoot blight rarely damages the entire tree, according to North Dakota State University. To control the shoot blight disease, prune out the infected shoots. Treat young trees with a copper-based fungicide to avoid lasting damage.