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Oak Wilt DiseaseOak wilt disease 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.RegulationsTo prevent the introduction of oak wilt disease to Canada, the Canadian Food Inspection Agency (CFIA) prohibits importation of all wood with bark of oak (Quercus spp.), chestnut (Castanea spp.), chinquapin (Castanopsis spp.) and tanbark oak (Lithocarpus spp.) from 22 U.S. states under Directive 99-03. Propagative nursery material from these states can be imported only with a Permit to Import and a U.S. Phytosanitary Certificate stating that the producing nursery is free of the disease on the basis of inspection during the previous growing season. De-barked forest products and non-propagative material, including logs with bark for processing can be imported only if certain conditions are met.[Note: Wood and nursery stock imported from the continental U.S. may be subject to other regulations depending on the state of origin. Check the CFIA website for other directives that may apply. ]
SymptomsThe first symptoms are usually yellowing along leaf veins (vein clearing) or brown scorching of leaf margins. Leaves usually drop from the tree. Branches wilt and die back and when cut open, a brown discolouration can sometimes be seen in the sapwood, under the bark, especially in red oak. 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 and white oak species wilt rapidly from the top down and are killed completely within four to six weeks. Moderately susceptible species wilt more slowly. Trees of species with low susceptibility may "wall off" the disease and survive with only partial death of branches. However, these 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 BC. HostsAll 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.
Disease CycleHealthy trees are infected by fungal spores that enter wounds on branches and trunks. Insects such as sap beetles (Nitidulidae) and bark beetles (Scolytidae) are attracted to oozing sap from fresh tree wounds. The beetles become contaminated with fungal spores and are believed to be the primary agents of long-distance spread from diseased to healthy trees. Thus, the importation of all wood with bark is prohibited since it may contain contaminated beetles. The disease also spreads slowly from tree to tree via natural root graphs.Further Information:
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