Oak Wilt is a disease of oak trees caused by a non-native fungus known as Ceratocystis fagacearum. The fungus invades the vascular system, or xylem, of a tree, which include the water conducting vessels that transport water up to the canopy of the tree from the roots.
As a defense mechanism, an infected tree produces substances in the xylem in an attempt to block the fungus from spreading throughout the tree. In reality, these substances actually clog the water-conducting vessels, preventing water from reaching the canopy (leaves) of the tree. Eventually, most infected trees die as a result of an oak wilt infection.
All types of oak trees found in Minnesota are affected by oak wilt. However, some oak species are more resistant to the disease than others. Generally, oaks can be placed into two groups, the red oak group (leaves with pointed lobes) and the white oak group (leaves with rounded lobes).
The red oak group includes both red oak (Quercus rubra) and northern pin oak (Quercus ellipsoidalis) and is highly susceptible to oak wilt. Trees in this group can succumb to the disease in as little as a few weeks to a few months. Once a tree in the red oak group is infected, no current treatment is available to save or “cure” it.
The white oak group, which includes bur oak (Quercus macrocarpa), white oak (Quercus alba), and bicolor oak (Quercus bicolor), is susceptible to oak wilt, but these species are able to withstand the disease for longer periods of time (usually one to several years). Furthermore, if an infected white oak is identified and chemically treated early enough, it can often be preserved.
The following images offer comparisons of oak leaves.