Oak wilt is a fungal disease caused by the organism Bretziella fagacearum that threatens Quercus spp. The disease is limited to the eastern half of the United States, Central Texas, and Southern Ontario. First described in the 1940s in the Upper Mississippi River Valley, the pathogen penetrates xylem tissue, preventing water transport and causing disease symptoms. Symptoms generally consist of leaf discoloration, wilt, defoliation, and death. The disease is dispersed by insect vectors and to adjacent trees through underground root networks. However, human spread is the most consequential dispersal method. Moving firewood long distances can potentially transport diseases and invasive species.
Disease
Oak wilt is a devastating exotic disease, killing some trees rapidly in a single season. Oak wilt is an important disease in urban areas where trees are highly valued. The disease reduces property values because of the loss of trees and is economically costly to the property owner since they or the local government must pay for tree removal. Additionally, preventing the spread of the disease to healthy trees is costly and requires vigilance. Oak wilt is also an important disease in a forest setting, as entire forest stands can die within a few years. There is a narrow window to salvage diseased trees for hardwood lumber and often the disease is not discovered within that logging window. The ecological impacts to forests is also a concern. The disease is currently restricted to North America, but is potentially a serious threat to oaks worldwide. All Quercus spp. appear susceptible to the disease, with 33 oak species confirmed to be susceptible; including three species of Castanea, one species of Castanopsis, one species of Lithocarpus, and some oaks native to Europe (Q. petraea, Q. pubescens, and Q. robur). Generally, red oaks (subsection Lobatae) display more severe symptoms with rapid and frequent mortality (particularly Q. velutina, Q. rubra, Q. ellipsoidalis and Q. coccinea). White oaks (subsection Quercus) develop symptoms more slowly, rarely die, and can recover from the pathogen with damage limited to a few branches (particularly Q. alba and Q. macrocarpa). Live oaks (Q. fusiformis and Q. virginiana) display intermediate symptoms compared to red or white oaks. However, live oaks are semi-evergreen, can propagate vegetatively by root suckering, and cohabitate with other live oaks in dense stands enabling interconnected root systems. These traits are favorable for local spread of the disease in an oak-grassland savanna.
History Oak wilt is one of three devastating North American vascular wilt diseases that appeared in the early 20th century. The other two vascular wilts are chestnut blight (1900–1950) and Dutch elm disease (1928–1980). Each of these diseases have depopulated their respective host tree populations. Chestnut trees were the dominant overstory tree species before chestnut blight and elms were an iconic landscape tree that bordered streets before Dutch elm disease. Oak wilt and the newly emerging emerald ash borer have the potential to devastate other important North American tree species with large geographical and cultural significance. Range expansion of oak wilt to the Western United States (or to other continents) is a major concern. The asexual stage of oak wilt was first described in 1942 in Wisconsin. Soon thereafter the disease was reported throughout the Upper Midwest and Central forests. The early 20th century coincided with oak regeneration after significant logging in the Mid-Atlantic and Great Lakes region. Deforestation and fire suppression in this region altered the ecology to favor oak–hickory forests, instead of coniferous forests and grasslands. Subsequently, the host for the disease became more prevalent and promoted oak wilt infections. Moving timber during the late 19th century and early 20th century, in the period of railroad expansion, coincides with the discovery of the oak wilt disease in the United States. The origin of the disease is unknown, but probably emerged from Mexico, Central America, or South America. Mexico is the global center of oak diversity, supporting it as the endemic range of oak wilt. Any biocontrol or disease resistance for this pathogen will likely come from Mexico. The disease currently affects much of the eastern and central US, from northern New York to Central Texas. It is particularly common in the Midwest where conditions are usually favorable for spore production and beetle activity during spring and early summer. Oak wilt is a major problem in Illinois, Iowa, Michigan, Minnesota, Texas, and Wisconsin.
Disease cycle
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