Winter Pruning

Prune in Winter, Reduce the Spread of Oak Wilt

Winter is a food time for tree pruning, according to state forestry specialists, who say winter pruning greatly reduces the likelihood of spreading oak wilt and other tree diseases, and it also minimizes pruning stress on trees.

"The best time to prune trees in Wisconsin is during winter when a tree is dormant," according to Don Kissinger, an urban forester with the Department of Natural Resources. "Insects and diseases that could attack an open wound on a pruned tree aren't active in winter. And without leaves, broken, cracked or hanging limbs and branch structure are easy to see and prune."

Timing is especially critical for pruning oak trees in order to limit the spread of oak wilt, a devastating fungal disease of oaks that has been present in the state for probably a century or longer. Oak wilt fungus spreads from tree to tree by "hitchhiking" on sap-feeding beetles that are attracted to freshly pruned or injured trees and root grafts between neighboring trees.

"Oak wilt causes the water-and nutrient conducting channels in the tree to plug up and fail," explains Kyoko Scanlon, DNR forest health specialists. "Once a tree is infected, water and stored nutrients can't move upward from the root system, and that causes the tree's leaves to wilt and fall.  The tree dies shortly afterward in some species of oak.

"Red oaks, which include red, pin, and black oak, are particularly vulnerable to oak wilt. Once wilting symptoms appear, trees in the red oak group die very quickly, often within a month."

Oak wilt is found mainly in the southern two-thirds of Wisconsin. It has also been found in Barron, Burnett, Florence, Langlade, Marinette, and Polk Counties.

Both Kissinger and Scanlon said prevention is the best defense against this disease.  The only other treatment options are costly fungicide applications or trenching between healthy and infested trees in order to sever connected roots.

DNR foresters recommend people stop pruning, wounding, or cutting oak trees in urban setting from April through July. A more cautious approach limits pruning in urban areas until October 1.

"The most critical time for oak wilt infection being spread by insects is the spring and early summer," Scanlon said. "On some years, spring comes much earlier than we expect.  If daytime temperatures begin to reach the 50-degree Fahrenheit mark, stop pruning oaks at that time, even if it's still the middle of March."

Co,,unities where oak wilt disease is a problem include Adams, Baraboo, Black River Falls, Durand, Eau Claire, Fort McCoy, Green Bay, La Crosse, Madison, Menomonee, Mosinee, Onalasks, Richland Center, Shawano, Stevens Point, and Waupaca.

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