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What is oak wilt?
Oak wilt is an aggressive disease that affects all species of oaks (Quercus spp.) found in Minnesota. It is caused by a nonnative fungus (Ceratocystis fagacearum) that invades the water vessels of oak trees and eventually kills most infected trees. In the United States, oak wilt is found in most northeastern states and in Texas. In Minnesota, oak wilt is typically found in the southern half of the state. More information
Which trees are susceptible?
While all species of oak can be affected, oaks in the red oak group (oaks with pointed lobes on their leaves) are by far the most susceptible. Northern red and pin oaks can die within two to three months of infection. White oaks are the most resistant and may survive for years after infection. Bur oaks are intermediate and may die within two to three years of infection. More information
How does it spread?
There are two means of spread: one above ground and one below ground. Below-ground spread happens when oak roots grow into one another and become fused or graft together. The fungus can then move from tree to tree through these root grafts. Above-ground spread occurs when sap-feeding beetles pick the spores up on their bodies and travel to a fresh wound on another tree. The beetles transfer the spores to the open wound, causing another infection center. Because of the close relationship between wounds and infection, it is important not to wound oak trees during the primary infection period—April, May, and June. More information
What do I look for?
Infected trees wilt from the top down, a few branches at a time. Leaves begin to drop at a rapid rate. The fallen leaves may be brown, green, or a combination of brown and green. Infected branches may have brown streaks in the wood beneath the bark. As the disease spreads, an infection pocket is created with dead trees at the center and infected trees with wilting crowns around the edges in a bull's-eye pattern. More information
What can I do?
The most effective means of control is a combination of root graft disruption and tree removal. Root grafts can be disrupted in various ways, but it's usually done with a vibratory plow using a long, narrow plow blade. To avoid above-ground insect spread of the disease, it is important to remove recently killed trees before April 1, when the fungus begins to fruit and the beetles begin to feed. To avoid below-ground rapid disease spread, infected trees should be removed only after root grafts have been disrupted. More information