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 Oaks Affected

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.

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.

Preventing

Oak wilt causes wilting and eventually death of oak trees. Trees are initially infected by sap-feeding beetles like the picnic beetle. Wounds created by storms or pruning serve as a feeding site for these beetles and as an entry point for the fungus. Once the tree has been infected, the fungus moves through grafted roots to healthy trees nearby. As the tree dies, the fungus reproduces. The fruity smell of fungal mats attracts picnic beetles, which carry fungal spores to fresh wounds.

To prevent infection, avoid wounding trees during April, May, and June when beetles are active. If trees are wounded during these months prevent beetles from interacting with fresh wounds by painting them. Apply a water-based (latex) paint or shellac within minutes. These materials act as a protective barrier; they do not help the tree recover. Since the chance for infection still exists during July, August, September, and October, the optimum time for pruning trees is when they are dormant.

To manage existing infections, use a vibratory plow with a five-foot blade to sever below ground root grafts and remove infected trees. Infected trees should be destroyed or covered with heavy plastic through June of the following year. The systemic fungicide Alamo has been used as a preventative treatment in highly valued red and white oaks, and as a curative treatment in white oaks.

{For More Help}

Choosing the right tree for the right site:

Tough Trees and Shrubs for Tough Sites

Recommended trees for southeast Minnesota

Trees and shrubs for clay soil

Proper tree care:

Tree Owners Manual

Fertilizing

Article courtesy of the Minnesota Extension Service
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