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High Risk for Oak Wilt

M. Grabowski, UMN Extension

Minnesota is now in the high risk period for new oak wilt infections. This means that the fungus that causes oak wilt, Ceratocystis fagacearum, and the beetles that transmit the pathogen are active. The good news is that oak trees have a natural defense mechanism that is very effective in stopping beetle transmission of the pathogen - bark!

Beetles in the Nitidulidae family are attracted to the sweet fermented smell produced by spore mats of the oak wilt fungus. As they crawl over the spore mat, spores cling to their bodies. Nitidulidae beetles are sap feeding beetles. They are attracted to the sap coming from wounds or pruning cuts on oak trees. As they feed from these fresh wounds, spores from the oak wilt fungus are introduced to the tree's vascular system and disease begins.

Red oak leaves with symptoms of oak wilt. M. Grabowski
A few simple steps will help to protect oaks from beetle transmission of oak wilt. Do not prune oaks during the months of April, May or June. The best time to prune oaks in Minnesota is November through March when neither the fungus nor the beetle are active. Avoid wounding oak trees during the high risk period. Mulch around the base of oak trees to help prevent wounds from weed whips or lawn mowers. Do not allow cars or other heavy equipment to come close to oak trees. Avoid doing construction that might damage the oak trees during the high risk period. Strategies to protect trees during construction can be found at the UMN Extension Garden webpage. Avoid removing oak trees during the high risk period. If an oak must be removed, contact an arborist to cut the root grafts between the tree and neighboring oaks, as oak wilt can move through root grafts from one tree to another. If an oak tree must be pruned during the high risk period, shellac or water based paint should be applied to the wound  within 15 minutes of making the cut as the beetles arrive quickly.

Oak wilt infects all species of oaks grown in Minnesota. Leaves on infected trees brown around the edges,wilt and fall off prematurely. Red oaks can be killed in several weeks, where as bur and white oaks may take several years to die. The oak wilt pathogen is not present in every county in Minnesota. To prevent introducing the pathogen to new areas, never move firewood or lumber with bark intact out of an oak wilt infected area.

If you suspect your oak may be infected with oak wilt, contact an arborist or send a sample to the UMN Plant Diagnostic clinic.
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