Tree Diseases | City of Hamilton
Oak wilt is a disease caused by a fungus. It starts with fall coloring out of season—the leaves are wilting and browning, and within a few weeks, the tree is dead. Once a tree has been infected, there is nothing you can do other than remove it promptly to prevent the disease from spreading to nearby healthy oak trees. That’s why it’s so important to know the symptoms of the disease.
The first sign of oak wilt is a rapid wilting and browning of the leaves in late spring to early summer, followed by the leaves dropping. This starts at the top of the tree, which is not easy to inspect in tall trees.
The browning of the leaves starts at the margins of the leaf at the apex, which is the tip of the leaf. From there it progresses downwards along the margins towards the midrib and the leaf stem. The browned leaves drop to the ground. Another symptom are vertical cracks in the bark with mat-like fungal spores underneath. The bark swells and eventually ruptures from the pressure created by the growing fungus.
In the following you might also notice sap beetles. While feeding on the sweet-smelling fungal mat, these beetles, about one-quarter to one-eighth of an inch, pick up the spores and thereby spread the disease further.
What homeowners can do to control Oak Wilt
Oaks that belong to the group of red oaks (pin oak, black oak, northern red oak) are more susceptible than white oaks (white oak, bur oak, swamp white oak). There is no way to save an infected oak tree; the only way to deal with oak wilt is prevention. Since the fungus enters the tree through wounds, avoid injuring oaks between April and August. That means no pruning during those months, and careful maneuvering around oak trees with your lawnmower and other power tools.