Oak Wilt: A Threat to Red Oaks
Oak wilt, caused by Ceratocystis fagacearum, is a lethal disease of oak (Quercus sp.). The disease is most serious on members of the red oak family, including red oak, scarlet oak, black oak, pin oak, etc. Members of the white oak family are generally not nearly as susceptible: white oak, swamp white oak, burr oak, etc. The disease is analogous to and potentially as serious as Dutch Elm Disease on American Elm. Both are introduced, exotic diseases caused by vascular wilt fungi; both diseases are transmitted by insects; both diseases can be transmitted through root grafts, and both fungi kill their host plants rather quickly.
Once a red oak becomes infected with the oak wilt fungus, it usually dies within several months. The most common predisposing factors are injury; and human activity is among the leading causes of infection. Symptoms in recently infected trees include the following: leaf margin browning progressing inward and from the leaf tips to the leaf bases; leaves on branches may appear to be wilting: some leaves may defoliate green or brown while others remain attached; vascular discoloration in the sap may or may not be present. These symptoms may be descriptive of other problems and hence misdiagnosis is not uncommon. Fungal mats, called pressure pads, may develop beneath the bark by the following spring; the fungal mats serve as sources of infection for transmission by the picnic beetles to healthy trees, thus completing the cycle.
Once established in a tree, the oak wilt fungus becomes systemic within the tree. Hence neighboring trees which happen to be root grafted to the diseased tree are now at risk. In this manner, the disease may affect many trees in an area, moving radially from the point source. Urban and rural/native forests can be affected from these point source infections.
The Pruning Connection
The health of oaks becomes high risk during the warm season months when they are injured through human practices, especially pruning, and through storm damage. April, May and June are the months posing the greatest risk but other warm season months are also suspect. Several species of picnic beetles are attracted to the fresh wounds caused by trimming and storm damage, and their activity transmits the fungus to healthy, recently pruned oaks from nearby diseased trees. Trees which come into contact with the fungus in the Spring are often dead by August or September.
Archived Press Release
Detroit Free Press: 2/18/00 "Something is killing the majestic oak..."
Associated Press: 9/25/00 "Experts worry oak wilt ...spreading"
Although not widely known, oak wilt is widely distributed throughout Michigan and can be found in any areas where oaks are found or propagated. I have often been asked whether oak wilt exists in a county, township or region of the state. My common reply has become, “prune a red oak in May and wait”. Anyone who prunes oaks during April, May and June is definitely taking some serious risks, even though such practices might have been acceptable previously.
There are very few funds available for oak wilt research largely due to the fact that urban oak forests do not represent a traditional agricultural commodity. The information presented herein, has been gathered from other states’ research and through our own limited research here in Michigan. In 1999, limited funds have been competitively obtained by the author through the GREEEN Project at MSU. This research would not have been possible however, without the assistance of Mayor Mark Steenburgh, City of Warren, and George R. Lee of Branch Tree Service. If individuals or organizations wish to contribute to our ongoing oak wilt research, they may do so by contacting me.