Photographer: Damon Waitt Affiliation: Lady Bird Johnson Wildflower Center Source:www.wildflower.org Copyright: Lady Bird Johnson Wildflower Center
Oak wilt is an infectious disease caused by the fungus Ceratocystis fagacearum. This fungus invades and colonizes the water conducting tissues of Oak trees. The reaction of the tree to this invasion results in the blockage of the water conducting tissues; ultimately resulting in severe die-back or tree death.
Symptoms: Foliar symptoms, patterns of tree mortality, and the presence of fungal mats can be used as indicators of oak wilt. However, laboratory isolation of the fungus is recommended to confirm the diagnosis. A trained expert should be consulted when in doubt.
Host(s): All oaks (Quercus sp.) are susceptible to oak wilt to some degree, but some species are affected more than others. Red oaks, particularly Spanish oak (Q. buckleyi), Texas red oak (Q. texana), Shumard oak (Q. shumardii), and blackjack oak (Q. marilandica), are extremely susceptible and may play a unique role in the establishment of new oak wilt infections.
The distribution and development of oak wilt has been closely linked to changes in forest stand composition, forest management practices, and pathogen dissemination facilitated by human and vector activity.
Red oaks appear to play a key role in the establishment of new infection centers. The oak wilt fungus may be spread overland by insect vectors and by man through movement of wood from infected red oaks to other locations. Fungal mats form beneath the bark of diseased red oaks in late fall and especially in spring, but do not form on live oaks. Individual fungal mats produce spores for only a few weeks. The fruity odor of fungal mats attracts many kinds of insects, the most important of which are believed to be sap-feeding nitidulid beetles. The fungus may be transmitted by these small beetles as they emerge from mats and visit fresh wounds on healthy red oaks and live oaks. Fungal mats are most commonly formed on standing trees, but they also can develop on logs, stumps, and fresh firewood cut from diseased red oaks.
Live oaks tend to grow in large, dense groups (called motts) with interconnected roots. The fungus may be transmitted from one tree to another through these root connections. Root transmission is a proven means of spread from one live oak to another. As a result, patches of dead and dying trees (infection centers) are formed. Infection centers among live oaks in Texas expand at an average rate of 75 ft per year, varying from no spread to 150 ft in any one direction. Occasionally, the oak wilt fungus is transmitted through connected roots between red oaks, but movement through roots is slower in red oaks and occurs over shorter distances than in live oaks.
Reports of widespread oak death that resemble oak wilt date back to the 1930's. The disease was first described by a scientist in 1942 where oak wilt was killing red oaks in the upper Mississippi river valley of Wisconsin. However, it is believed that oak wilt was likely killing trees as early as the 1890's.
Native Origin: The causal agent, Ceratocystis fagacearum, is found only within the borders of the United States, but the origin of the pathogen is not known.
U.S. Habitat: Throughout the range of oak wilt in the United States, red oaks are the most important hosts, but susceptibility varies somewhat by species. Texas live oak (Quercus virginiana) is moderately susceptible to the disease, but because of its tendency to form large, root-connected clones through which the disease can spread, it is also considered to be an important host.
Map of previous survey results in the United States click here
U.S. Present: Oak wilt is reported only in the U.S. It was first detected in Wisconsin in 1942 and is now known in 22 states. In the early years it was known primarily in the Midwest; it is now common east into the central Appalachians and south into Texas. Except for Texas and a few spots in the Carolinas, it is largely absent from the Gulf and Atlantic coastal states and their coastal live oak populations.
Diseases that can resemble oak wilt include Actinopelte sp., Oak leaf blister, Diplodia sp., and Oak rust among others.
There are currently three primary approaches used for oak wilt management in Texas. Successful control usually depends on an integrated program incorporating measures from all three approaches. The first approach attempts to prevent the formation of new oak wilt infection centers by eliminating diseased red oaks, handling firewood properly, and painting wounds on healthy oaks. The second approach involves trenching or other measures to disrupt root connections responsible for root transmission of the pathogen. Finally, injections of the fungicide propiconazole (AlamoTM) into individual, high-value trees help reduce crown loss and may extend the life of the tree. These measures will not cure oak wilt, but will significantly reduce tree losses.
Juzwik, Jennifer, Thomas C. Harrington, William L. MacDonald, and David M. Appel. 2008. The Origin of Ceratocystis fagacearum, the Oak Wilt Fungus. Annual Review of Phytopathology 46: 13-26.
Rexrode, Charles 0., and Thomas W. Jones. 1970. Oak bark beetles-important vectors of oak wilt. J. For. 68: 294-297.
True, R. P., H. L. Barnett, C. K. Dorsett, and J. G. Leach. 1960. Oak wilt in West Virginia. Bull. 448T. Morgantown, WV: West Virginia University Agricultural Experiment Station 119.