Identifying Oak Wilt
Oak wilt is a fungal disease that infects the vascular tissue of oak trees. As the fungus infects the tree, the tree tries to defend itself by producing defense compounds. These compounds plug the water-conducting tissue causing the tree to wilt and eventually die.
All species of oak trees are susceptible to oak wilt. Trees in the red oak group, including red and pin oaks, are highly susceptible and usually die within a few weeks. Trees in the white oak group, including white and bur oaks, are more resistant and may survive for one or more years following infection.
Oak wilt is identifiable by the rapid pattern of wilting starting from the top of the tree and progressively dying down to the bottom, and on specific leaves, wilting from the edges to the base.
Oaks with oak wilt stand out with their dead crown compared to a green canopy in the summer, so much so that oak wilt infections can be spotted from the air. A new infection via beetles instead of root grafts can kill a tree somewhat more slowly, if a branch is infected instead of the trunk.
The Wikipedia entry on Oak Wilt has additional information.
To identify trees infected with oak wilt, watch for wilting leaves in the upper canopy. Wilting leaves may develop yellow margins, while the interior portion of the leaf remains green. As the tree continues to wilt, leaves turn brown and fall from the tree.
Since oak wilt causes drought-like symptoms, it may be easily confused with other stress-related factors. These factors include construction damage, drought stress, or insect colonization.
In addition, oak wilt is often confused with the common springtime disease, anthracnose. In contrast to oak wilt, anthracnose causes spotting, curling and browning of the leaves in the lower canopy. In rare cases oak wilt may cause brown streaking of the inner sapwood. This streaking is a good diagnostic symptom for detecting infected trees.
To obtain positive identification for questionable trees, contact the University of Minnesota Plant Disease Clinic or the Minnesota Department of Agriculture for sampling guidelines.