A deadly tree disease known as oak wilt was discovered for the first time on the Northern Highland-American Legion State Forest near Woodruff in October, prompting state forestry officials to take immediate action to stop its spread in the north.
Brian Schwingle, Department of Natural Resources forest health specialist, says staff cut down the diseased tree and oaks around it, ripping their stumps out of the ground with a bulldozer and burning all diseased wood. The area will be monitored next summer to determine if these efforts were successful in eradicating oak wilt at this isolated site.
“Such drastic action is necessary to prevent oak wilt from spreading from tree to tree through connected roots in the oak-dominated stands on the state forest,” Schwingle said. “Once oak wilt gets established in an oak stand, it’s like a slow-moving fire, killing all oaks in its path.”
Schwingle says oak wilt is not native to northern forests and has been moving northwards in recent years with the inadvertent help of people. Spores of the disease are most frequently brought to new areas in firewood from oaks killed the previous summer.
“New oak wilt infestations are frequently found around vacation homes or campgrounds where people bring wood from southern and central Wisconsin where the disease is common,” Schwingle says. “The first step in protecting your oaks is to get your wood locally. The nearer the wood was cut to where you plan to use it, the less likely it is to be carrying pests or diseases new to your area that can emerge to attack your trees, including oak wilt.”
The second step you can take to protect your oaks is to be sure the spores do not have a way into your trees. Oak wilt spores need a fresh wound in spring to summer to infect an oak.
“Too often, we provide an entry for the oak wilt spores when we prune oaks, nail ornaments, insert screws or bump our vehicles into oaks when opening up the cabin,” Schwingle says. “This is the other reason new oak wilt infections are so often found in lake shore developments and other vacation areas.”
Also, he says, avoid causing even small wounds to your oaks from April 15 to July 15 when spores commonly infect fresh wounds. If you accidentally damage an oak or have to prune off a branch broken in a storm, paint or spray the wound with a tree dressing or paint to prevent infection.
For more information on recognizing, preventing, and controlling oak wilt, search the DNR website, dnr.wi.gov, for keywords “oak wilt.” If you suspect oak wilt is killing oaks north of Highway 64 between Sawyer and Forest counties, contact Brian Schwingle, Merrill DNR office, at 715-536-0889.
Prune in late autumn and winter
Late autumn, starting Nov. 1, through winter is a good time for tree pruning, according to DNR tree health experts.
Pruning during winter greatly reduces the likelihood of spreading oak wilt and other tree diseases and minimizes pruning stress on trees.
“The best time to prune trees in Wisconsin isn’t in April; it’s during winter when a tree is dormant,” DNR urban forester Don Kissinger says. “Insects and diseases that could attack an open wound on a pruned tree aren’t active in winter. Also, without leaves on the trees it is easier to see and prune broken, cracked or hanging limbs.”
“Timing is especially critical when pruning oak trees,” Kyoko Scanlon, DNR forest pathologist, said. DNR recommends that people stop pruning, wounding, or cutting oak trees from April through July to limit the spread of oak wilt.
It is also important not to move infected firewood. “Many of the recent oak wilt finds in northern Wisconsin likely originated from infected firewood being transported from the areas where oak wilt is prevalent,” Schwingle said.
Infected oak firewood should remain where it was cut for one year after the tree dies before moving it to a new area. Once bark is loose, there is no longer a threat of moving oak wilt.
Oak wilt is common in the southern two-thirds of Wisconsin. In much of northern Wisconsin though, it is still a new and uncommon disease. Since 2010, oak wilt was confirmed for the first time in Lincoln, Oneida, Rusk, Sawyer, and Vilas Counties. The disease has not been confirmed in Ashland, Bayfield, Calumet, Door, Douglas, Forest, Iron, Kewaunee, Manitowoc, Price, Sheboygan, Taylor, and Washburn counties.
For additional information online, search the DNR website dnr.wi.gov for keywords “oak wilt.”
Trees should be pruned throughout their entire life. During the tree’s first 10 years prune every other year to foster strong structural or “scaffold” limbs. Once proper structure is established, prune about every five years to maintain the structure and remove larger pieces of dead wood. Certified arborists who offer pruning and other tree care services can be found at waa-isa.org/arborists/search.asp.
“Pruning should not remove more than 25 percent of the live crown of a tree. The lower third of established trunks of deciduous trees should be free of limbs,” Kissinger said.
The DNR pruning brochure offers more detailed, step by step tips for tree pruning. Find it by searching the DNR website, dnr.wi.gov, for “tree pruning.”