Fungal diseases are common in many varieties of trees, including ash. Sometimes there is no control for such infections, and often they do minimal harm to your trees -- but there are things you can do to deal with them. Whenever possible, plant tree species that are resistant to the fungi found in your area. Gather and destroy fallen leaves in autumn to get rid of the fungi overwintering in them and ensure that your tree gets the fertiliser and water it needs to stay healthy. Healthy trees are more resistant to fungal infections, as well as pests and other diseases.
Caused by the fungus Gnomoniella fraxini, anthracnose attacks leaves and buds in the spring. It causes dead or distorted leaves with brown spots and blotches, sometimes even stunting whole shoots. If the fungus in contracted later than the spring, it appears as brown spots on fully developed leaves. These spots sometimes grow together, expanding to encompass the entire leaf. Branches that are infected develop cankers and must be pruned away. This also improves air circulation and allows leaves and branches to dry more quickly, helping to control fungus reproduction.
The fungus Puccinia sparganioides causes a disease called ash rust that deforms leaves and produces orange to yellow lesions or pustules on these leaves, twigs or petioles. Though it can be unsightly, ash rust does little damage to the tree itself, although it does have an uncommon and interesting life cycle. The spores produced by the orange-yellow fruiting bodies do not spread to other ash trees, but instead spread to cordgrass (Spartina sp.) where it overwinters. In the spring, the spores it produces on the cordgrass reinfect ash trees.
Heart rot in ash trees is caused by several species of decay fungi, including Fomes fraxinophilius and Polyporus sulfureus. Because these types of fungus inhabit the heartwood of a tree, beneath the bark, they can be dangerous and difficult to detect. Heart rot weakens an ash tree's structure, making it harder for the tree to support its own weight. Effected ash trees can lose limbs and branches and in severe cases these fungi can fell whole trees, especially when other factors such as wind or storms are involved. These fungi often enter through wounds in the tree, so inspecting a tree for injury is a good place to start. Peeling away the bark may reveal white fungal matter inside the tree, or if the fungus is sprouting fruiting bodies such as mushroom or conks, these also will be visible.