Ministry of Agriculture

Oak Wilt

Oak wilt is caused by a fungus called Ceratocystis fagacearum. The disease has been found from the Northeastern U.S. (Great Lakes region) to the southwest as far as Texas and in the upper mid-west as far as Minnesota and Wisconsin. It does not occur in Canada. It is one of the most serious tree diseases in the Eastern United States, killing thousands of oaks each year in forests and home landscapes.

Hosts

All species of red oak, such as northern red oak (Quercus rubra), northern pin oak (Quercus ellipsoidalis) and Spanish oak (Quercus falcata) are highly susceptible, as is live oak, also called Texas live oak (Quercus virginiana). Species of white oak such as Quercus alba or bur oak (Quercus macrocarpa) are low to moderately susceptible. The susceptibility of Garry oak is unknown.


Oak wilt leaf symptoms. Photo courtesy: Minnesota Department of Natural Resources Archive, Bugwood.org.	Vascular streaking caused by oak wilt. Photo courtesy: D. W. French, University of Minnesota, Bugwood.org.


Wilted branch on northern pin oak (Quercus ellipsoidalis) in Minnesota. Photo courtesy: John N. Gibbs, Forestry Commission, Bugwood.org	Oak killed by oak wilt in Texas. Photo courtesy: William M. Ciesla, Forest Health Management International, Bugwood.org

Symptoms

Early symptoms include yellowing along leaf veins (vein clearing) or brown scorching of leaf margins. Leaf scorching occurs from the tip and edges toward the midrib and base with a distinct line between bronze and green tissues. On red oak, foliage wilts and turns brown giving trees a false, fall coloration. Dead leaves may stay on trees but eventually drop. Branches wilt and die back, and when cut open, a brown discolouration may be seen in the outermost xylem ring. On red oak, the fungus forms spore mats or "cushions" under the bark, which crack the bark open and have a fruity smell.

Trees of highly susceptible red oak species wilt rapidly from the top down and are killed completely within four to six weeks. White oaks are less susceptible and frequently recover from infection or take many years to die. Wilting and death of foliage may only occur on a few branches. Trees that survive infection remain possible sources of spread of the disease.

Similar leaf scorch symptoms can be caused by drought or anthracnose disease caused by species of Discula fungi, which commonly attack oaks in coastal regions of B.C.

Disease Cycle

Oak wilt is a vascular wilt disease, with the fungus confined to xylem vessels. When a red oak dies, the fungus often grows out into the inner bark forming spore-bearing fungal mats, which are attractive to sap beetles (Nitidulidae). Beetles pick up spores on the surface of their bodies while feeding, and can carry the spores to infect healthy trees. Oak bark beetles (Scolytidae) can also carry spores of the oak wilt pathogen. They form galleries under the bark of oak trees, and may emerge contaminated with the oak wilt fungus. The disease also spreads slowly from tree to tree through natural root graphs.

Prevention

Early detection is the most important step to prevent the spread of this disease. If you spot symptoms of oak wilt, contact the provincial Plant Health Laboratory for instructions on submitting a sample.

If planting oak trees, obtain nursery stock from a reliable, local source. Do not transport fire wood from other areas into British Columbia.

Regulations

To prevent the introduction of oak wilt disease to Canada, the Canadian Food Inspection Agency (CFIA) regulates the importation of propagative and non-propagative materials of oak (Quercus spp.), chestnut (Castanea spp.), chinquapin (Castanopsis spp.), and tanbark oak (Lithocarpus spp.), originating from the continental United States under Directive 99-03.

[Note: Wood and nursery stock imported from the continental U.S. may also be subject to other regulations depending on the state of origin. Check the CFIA website for other directives that may apply. ]