Oak wilt is a fungal disease affecting oak trees caused by the fungus known as Bretziella Fagacearum. It was first recognized in 1944 when over half of the oaks in affected areas of Wisconsin were infected and died as a result. The most recent evidence suggests oak wilt to be an exotic disease which arrived in the United States as early as 1900. Its exact origin is still unknown, possibly from Central or South America, or Mexico.
The disease currently affects much of the eastern and central US, As far north as New York and from Virginia to Minnesota to Arkansas, with infected areas as far south as the Hill Country of central Texas. It is particularly common in the Midwest. In Michigan where oak trees comprise about 10 percent of Michigan forests, it has been confirmed in 56 counties. This disease has the potential to impact the estimated 149 million red oak trees throughout 3.9 million acres of Michigan’s forest land. Red Oaks are particularly at risk, but this disease affects white oaks as well as other varieties. White oaks are a bit less susceptible when infected and sometimes live several years after infection.
The fungus is spread from diseased to healthy trees by insects as well as connections between tree roots. It can also be spread during the warmer months as a result of tree pruning, hanging lanterns from trees, climbing spikes, using nails to attach items to trees, tree barking, and storm damage. One such instance of storm-related transference was recorded in Minnesota and Northern Wisconsin in 2012 and 2013 where trees appeared to become infected as a result of a mesoscale convective vortex (MCV) windstorm which showed wind speeds of over 100 miles per hour. These winds damaged trees and carried the oak wilt spores which then infected other oaks. The areas affected included Wisconsin, Minnesota and the Keweenaw Peninsula in Michigan.
Symptoms vary by tree species but generally consist of leaf discoloration, wilt, defoliation, and death. You’ll notice a sudden drop or browning of leaves in the summer months. Some of which may be brown or green with partially brown areas while the leaf base remains green. Since other pests and pathogens may cause similar symptoms, it’s best to confirm your suspicions with lab verification.
Source: Wikipedia, Michigan State University Extension