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April 5, 2011

MADISON - People who value their oak trees should not prune them from April through July, according to the latest recommendations from state forestry officials. The reason? Spring and early summer pruning makes oak trees vulnerable to oak wilt, a serious and almost-always fatal fungal disease of red oaks.

Special care should also be taken to avoid wounding oaks from April through July, according to Kyoko Scanlon, Department of Natural Resources forest pathologist. Any action that might provide an opening into the tree such as carving initials into the tree or attaching a birdfeeder or clothes line, could provide an opportunity for the oak wilt fungus to invade and establish itself in the tree.

If an oak tree is pruned from April through July, a wound dressing or paint should be applied to the cut surface as soon as the wound is created. Even half an hour can be enough time for beetles that transmit the disease to land on a fresh wound and infect your tree, according to Scanlon. While the risk of spreading oak wilt is low after July, she said homeowners should avoid pruning or wounding oaks until autumn, to be on the safe side.

"Oak wilt can spread from a diseased tree to a healthy tree through a connected root system as well as by insects," according to Scanlon. "Very small sap beetles transport fungal spores by landing on fungal mats found beneath the cracked bark of trees that died the previous year. The spores are then transmitted from the beetle body onto the fresh wound of a healthy oak tree while the beetle is feeding at the pruned or damaged site. A beetle that transmits oak wilt disease is not capable of boring into a tree," Scanlon added.

If a wound is left unprotected, a new oak wilt pocket may develop in a location where oak wilt did not previously exist and will radiate to other oaks through the connected root systems. If no management steps are taken, Scanlon said the pocket could continue to expand year after year. Once oak wilt establishes itself in an area, control of the disease is both difficult and costly. The prevention of oak wilt is the best approach.

Scanlon said builders and developers should also be very careful as many oak wilt infections and deaths have occurred through inadvertent damage to roots, trunks, or branches during the construction process.

Oak wilt is found in all Wisconsin counties except Ashland, Bayfield, Calumet, Door, Douglas, Forest, Iron, Kewaunee, Lincoln, Manitowoc, Price, Rusk, Sawyer, Sheboygan, Taylor, Vilas, and Washburn. Oak wilt was confirmed in Oneida County for the first time last summer.

The disease kills many oaks in the state by interfering with the tree's water and nutrient-conducting systems, essentially starving the tree. Leaves begin to wilt and the tree may eventually die. Trees in the red oak group, such as northern red and northern pin oak are especially vulnerable. Once wilting symptoms become visible the tree loses most of its leaves and dies very quickly, often within weeks. Trees in the white oak group - those with rounded or lobed leaves - are more resistant to oak wilt and the disease progresses much more slowly, often one branch at a time. White oaks could live with oak wilt for many years, and some trees may recover from the disease.

"Besides oaks, pruning deciduous trees in general should be avoided in the spring, as this is the time when tree buds and leaves are growing and food reserves are low," says Don Kissinger, a DNR urban forester. "The best time to prune any deciduous tree is winter, followed by mid-summer, after leaves have completed their growth."

Anyone interested in learning more about oak wilt and other forest pests as well as tree pruning should visit the Forest Health pages of the DNR website for more information. Additional information about proper pruning techniques is available from your community forester, a University of Wisconsin-Extension agent (exit DNR), or DNR urban forestry coordinators.

FOR MORE INFORMATION, CONTACT: Kyoko Scanlon 608.275-3275 or Don Kissinger 715.359-5793.

Last Revised: Tuesday, April 05, 2011

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