Detection: In red oak, leaves wilt and turn brown starting from the top of the tree and the tips of the branches before spreading down through the crown. Individual leaves brown from the tips and margins inward. White oak species may or may not have leaf browning, when present, it is more subtle than on red oak species, and there is less defoliation.
Sporulating mats (pressure pads) develop under the bark of red oak - in the fall on trees infected in spring or early summer, or the following spring for trees that are infected later in the summer. Look for bark cracks on the trunk or large branches of newly dead trees. Drumming on the trunk over a pressure pad produces a hollow sound. Pull off the bark around the crack or hollow-sounding region and look for an oval shaped area of fungal growth with a characteristic fruity smell likened to fermenting apple cider, or Juicy Fruit chewing gum. Diagnosis of oak wilt is by the presence of pressure pads, or the culturing of lab testing of wood tissue samples.
Leaf symptoms on red oak. Image: D. W. French, University of Minnesota, Bugwood.org
What you can do: There is no cure for oak wilt, however, it is preventable, at least at early stages. Minimize tree wounds during the flight season of the insect vectors. Refrain from pruning, harvesting, or otherwise harming oaks during the growing season. If injuries occur, mitigate it by immediately covering the wounds with tree paint; this prevents nitidulids from detecting the wound.
If you think you have found oak wilt, contact the CFIA's Plant Health Surveillance Unit.
Trees at risk: All oak species are at risk. Species in the red oak group (e.g. red, black and pin oak) are at greatest risk and tend to die quickly, usually within a year, and sometimes within a few weeks of infection. Species in the white oak group (e.g. white, swamp white, bur oak) tend to be more resistant and may show damage on only a few limbs for a time. White oaks can eventually succumb to oak wilt, but this may take a few years; some seem to be able to survive the infection by walling it off. Bur oak is intermediate in its ability to resist infection - some are able to resist infection, while others die quickly.
Oak wilt is a vascular wilt disease, similar disease to Dutch elm disease, however, is much deadlier. It is caused by the fungal pathogen Bretziella fagacearum, which is introduced to a healthy oak by root grafting or by nitidulid beetles. Oak wilt was first attributed to the fungus in Wisconsin in 1944; oak wilt is now found within much of the central and eastern U.S.. It was recently detected in Niagara Falls Ontario.
Pressure pad on oak wilt infested tree. Image: Silv-Econ Ltd.