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It is now the high risk period for oak wilt in Minnesota. Gardeners in the area of the state where oak wilt occurs should not prune oaks until next fall.
Oak wilt is a fatal disease that can affect all oak trees. Red oaks can wilt and die in as little as 4 weeks. White oaks often slowly decline over several years. Protecting trees from wounds and pruning cuts during the high risk period is critical in preventing new oak wilt infections.
At this time of year, the oak wilt fungus produces a spore mat with sticky fruity smelling spores that attract sap beetles. The spores stick to the sap beetles' bodies when they visit the spore mat. Sap beetles are also attracted to the sap from fresh pruning cuts or wounds. If the beetle visits a recently pruned oak tree after visiting a spore mat, the tree can become infected.
The good news is sap beetles can not infect an oak tree without a wound. Bark is the best protection. If an oak tree must be pruned during the high risk period, the surface of the pruning cut should be immediately covered with water based paint or shellac.
The safe period for pruning oaks typically occurs from November through March. If you missed this years safe pruning period, mark your calendar and wait until the next safe pruning period begins in November.