Oak Wilt in Wisconsin's Forests

Oak Wilt in Wisconsin’s Forests Biology, Symptoms and Management FOREST HEALTH FACTSHEET Wisconsin Department of Natural Resources, Division of Forestry, Forest Health Program, Revised January 2021 LOCATIONS IMPACT to nearby oak trees through ak wilt is a deadly disease Oak wilt kills thousands of trees interconnected (grafted) root Othat affects oak trees. First each year in forests, woodlots and systems, creating an expanding documented in 1944, oak wilt urban areas. Oak wilt can even pocket of dead oak trees. has since been confirmed in most attack and kill healthy trees. The of Wisconsin’s counties (Figure disease is a particularly serious IDENTIFICATION 1). Oak wilt is widespread and problem for species in the red oak Leaves will wilt and drop from the common throughout central and group such as northern red, north- upper canopy first. Symptoms southern Wisconsin; however, ern pin and black oaks. Once wilt- progress down the tree rapidly the disease is still uncommon in ing symptoms are apparent on a (Figure 4). northern Wisconsin (Figure 2). red oak, the infected tree will lose most of its leaves and die within approximately one month. Among the white oak group, bur and swamp white oaks demonstrate moderate tolerance to the disease; living several years after symptoms first appear. White akso experience even slower disease progression and may survive infection. BIOLOGY Oak wilt is caused by the fungus Bretziella fagacearum. The fungus Figure 3: Fungal mats produce oak wilt spores grows through the infected tree’s Figure 1: County distribution of oak wilt in and grow underneath the bark of trees killed by the Wisconsin water conducting system, causing disease. the tree to wilt and die. Oak wilt is introduced to an area by sap- feeding beetles that carry oak wilt spores to fresh wounds. Spore- bearing fungal mats develop under the bark in the fall or spring following the death of the infected tree (Figure 3). The sap feeding beetles are attracted to Figure 2: Known distribution of oak wilt in northern the fungal mats and can transport Wisconsin townships. Pink squares mark townships of oak wilt spores to fresh wounds or 36-square-miles where oak wilt is present in part. Oak to recently cut oak stumps. Once Figure 4: Leaves of infected trees will wilt and drop wilt is common and widespread in counties in red. in an area, the disease spreads to the ground in summer. North Zone: April 15 to July 15 Trees that have died of oak wilt South Zone: April 1 to July 15 will harbor spores for approxi- However, the guidelines’ Excep- mately 1 year after death. Once root tions and Modifications allow for grafts have been disrupted, dead stand-level flexibilities. Please read and live trees inside a pocket should the guidelines for more information be harvested to limit the spread at dnr.wi.gov, keyword “oak wilt.” of the disease by beetles. If you If oak trees need to be pruned are interested in options for active during the harvesting restricted management of oak wilt, please Figure 5. Infected leaves appear dull green/ bronze, periods, apply wound sealer imme- contact your local DNR forester or water-soaked but partially green, and will drop rapidly DNR forest health specialist. in mid-summer. diately after the wound is made to prevent the introduction of spores FIREWOOD Wilting symptoms are observed by sap-feeding beetles. from June through September, most commonly in July and August. Leaves of the infected trees turn dull green or bronze, and look water-soaked (Figure 5). Unlike normal leaf fall, the leaves of Keep firewood local. infected trees drop to the ground when they are still partially-green. Moving infected firewood can The symptoms of oak wilt are sim- spread oak wilt long-distances ilar to those of other pest, disease to a previously disease-free area. and abiotic issues. A symptomatic Keep firewood local and consider tree should be sampled and exam- covering it with 4 mil plastic until ined by a laboratory for proper the bark is loose. Wood from dead diagnosis, especially if active man- trees with loose bark, and from agement is being considered. Figure 6: Map of counties in the north and south apparently healthy trees with no Harvesting Restriction Zones. wilting symptoms, does not pose a PREVENTION risk for the spread of oak wilt. Once the disease exists in a stand, MANAGEMENT Wisconsin has developed a num- it is difficult to control. Prevention Although management to control ber of requirements pertaining to of this disease is the best approach. The the below-ground spread of oak the movement of firewood. For greatest risk of oak wilt transmis- wilt can be complicated and costly, more information about firewood sion occurs in the spring and early active management can be suc- restrictions, please visit dnr.wi.gov, summer, when oak wilt spore- cessful. Controlling below-ground keyword “firewood.” carrying beetles are abundant spread requires disrupting the and fungal mats are fresh. To help movement of the fungus through Photo credits: Wisconsin DNR reduce risk, the DNR developed root grafts from infected trees to the Oak Harvesting Guidelines healthy trees. Physical severing of to Reduce the Introduction and the root system using a vibratory Printed on Spread of Oak Wilt. These rec- plow or a trencher has proven to recycled paper. ommend harvesting only during be effective if plow lines are placed Wisconsin Department of Natural Resources the non-restricted period if your correctly. Recently, field trials have PO Box 7921, Madison, WI 53707-7921 stand is in a county that has shown some promising results to PUB-FR-622a 2021 oak wilt or is within 6 miles of a contain the below-ground spread The Wisconsin Department of Natural Resources provides equal opportunity in its employment, programs, services and functions under an county with oak wilt (Figure 1). of oak wilt by using herbicides on, Affirmative Action Plan. If you have any questions, please write to Equal Harvesting restricted periods are or by uprooting the entire root Opportunity Office, Department of Interior, Washington, DC 20240. This publication is available in alternative format (large print, Braille, dependent on your location in the mass of, healthy oaks that border audio tape, etc.) upon request. Please call the Department of Natural Resources Accessibility Coordinator at 608-267-7694 for more state (Figure 6) and are: the infected pocket. information. For more information visit dnr.wi.gov, keyword “oak wilt.”.

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