The Minnesota DNR is asking residents to avoid pruning oak trees through July in order to prevent disease.

Oak wilt is widespread in the southern half of Minnesota and is continuing to expand.

The disease can affect all species of oak found in the state. It's caused by a non-native, invasive fungus, which invades the water-conducting vessels of oaks, eventually killing infected trees.

It can be spread by bark beetles or below the ground through the roots.

"The first thing you're going to notice is wilting in the outer tips of the canopy," Mankato Parks and Open Space Superintendent Ashley Steevens said. "If you're noticing it more in the inside, the inner part of the canopy, that can be a sign of some other issue."

Leaf wilt and drop off is most noticeable around July and August.

The best thing to do to prevent the spread of the disease it to avoid damage and pruning during the high risk period, which lasts from April through July.

"If you want to take the most precaution, we want to wait until after the October time frame is over, otherwise we can kind of just think of it as that leaf-off period," Steevens said.

Oak wilt can be hard to identify, so if you think you have an infected tree, Steevens recommends sending samples into the University of Minnesota's plant clinic to have it identified.

"Typically they want a branch or twig that's at least a half inch in diameter, and they want to make sure that it's live," Steevens said. "They often want it refrigerated and shipped overnight to them."

If a tree does have oak wilt, contact your local DNR office to come up with a plan to properly remove the tree.

If you see oak wilt outside of the high risk zone, you're asked to take photos of the tree, record the location, and contact your local DNR forester.

--KEYC News 12