University of Wisconsin - Extension

Thousands of oaks in woodland and urban settings die from oak wilt every year in Wisconsin. White and red oak trees are susceptible to oak wilt, although trees in the red oak group fall prey to the disease most often. Mark Rickenbach, University of Wisconsin-Madison/Extension forest ecology and management specialist, offers advice to woodland owners about the disease and its management.

Oak wilt is caused by the fungus Ceratocystis fagacearum. Mats of fungus develop, most often in spring, under the bark of trees that have died from oak wilt. These mats force the bark to crack open releasing a sweet odor that attracts sap-feeding beetles. The beetles pick up fungal spores and then fly to healthy oaks to feed on sap flowing from fresh wounds, infecting new trees. As fungus invades a tree¿s water conducting system, the leaves turn dull green, bronze or tan, and wilt from the top of the tree downward. Leaves fall rapidly after wilting and infected trees are bare in four to six weeks.

¿Oak wilt can also spread from infected to healthy trees through root grafts underground,¿ Rickenbach says. ¿Most root grafts form between oaks of the same species. Red oak roots graft more often than do those of white oaks. Grafts between red and white oaks are very rare.¿

¿The disease moves outward from the original infected trees through root grafts, killing more trees each year,¿ says Rickenbach. ¿This creates a pocket of dead trees in an oak stand that characterizes oak wilt infections.¿

Rickenbach recommends taking two precautions to lessen the chances of oak wilt invading your woodlands.

First, do not harvest, prune or otherwise wound oak trees from bud swell up to two or three weeks past full leaf development (generally April 15 to August 15). During this time, fungal mats are most abundant and oak trees most vulnerable to oak wilt. Infection is less common later in the summer but can occur after August 15. Take a very cautious approach¿limit any cutting activities from April 1 to October 1.

Second, do not move infected trees with the bark still attached (as firewood or logs) into your woodlands. Fungal mats may form on the transported wood, attracting the beetles that spread oak wilt.

You may need a laboratory analysis to confirm the presence of oak wilt. To obtain an analysis, collect three twigs (about a half inch in diameter and four inches long) from three different branches with wilting leaves. You must send samples that still have live tissue. Scratch the sample branch with your fingernail. If the wood under the bark is a light color (white to green), the sample is fresh. If the wood is brown or dark, it is too old to be useful. Wrap the samples in wax paper and keep them cool until you mail them. Mail samples to the Plant Disease Diagnostic Clinic, Dept. of Plant Pathology, University of Wisconsin-Madison, 1630 Linden Drive, Madison, WI 53706. For information about testing and fees, call 608-262-2863 or go online to http://www.plantpath.wisc.edu/pddc/">www.plantpath.wisc.edu/pddc.

If sampling results are positive for oak wilt, there are two management strategies from which you can choose: passive and active management.

• Passive management If you allow the disease to progress, it will spread to healthy oaks through the roots of diseased trees. In pure oak stands, the disease moves outward from the original infected trees through root grafts, killing more trees each year. In mixed stands, where oaks are scattered among other species, root grafts may occur less often than in pure oak stands; thus, spread may be slower or less conspicuous.

If you choose this approach, Rickenbach advises that you consider the impact on the health of your neighbors¿ trees. Even if root-graft spread to your neighbor¿s trees is unlikely, spread via beetles to a neighbor¿s yard or woodlot is a real possibility. Proper removal and use of infected trees reduces the hazard of this overland spread.

• Active management Since oak wilt spreads both underground and above ground, you must combat it with a two-pronged approach. To contain the disease you can install a root graft barrier and remove and properly use trees inside the barrier. Work with a forest pest specialist, forester or arborist trained in oak wilt management to plan and establish the barrier.

To learn more about these management strategies, see the UW-Extension publication ¿Lake States Woodlands: Oak Wilt Management¿What are the Options?¿ (G3590). The publication also gives guidelines for removing and using dead and infected trees, regenerating trees by planting oak seedlings or encouraging natural oak seeding in the pocket of stumps, and using herbicides as a tool to stop the root graft transmission of oak wilt.

For a copy of ¿Lake States Woodlands: Oak Wilt Management¿What are the Options?¿ (G3590), contact your county UW-Extension office. The publication is available from UW-Extension Publications and can be viewed online at http://www1.uwex.edu/ces/pubs/index.cfm"> http://www1.uwex.edu/ces/pubs/index.cfm.