Oak trees (Quercus sp.) have long been prized for their strength, longevity and excellent wood properties. Oak trees adapt well in the natural forest, the suburban landscape and the parks of inner cities. Oak is a favorite wood used for hundreds of manufactured forest products so it is also a favored crop grown tree. Oaks are the state trees of Maryland, Connecticut, Illinois, Georgia, New Jersey and Iowa. The mighty oak is also the official tree of the capitol of the United States, Washington, D.C.
Some oak trees live for more than 1,000 years. The Major Oak is an 800- to 1000-year-old tree located in Sherwood Forest, Nottinghamshire. According to folklore, it was used by Robin Hood for shelter. The Emancipation Oak is designated one of the 10 Great Trees of the World by the National Geographic Society and is part of the National Historic Landmark District of Hampton University.
Oak Wilt: A Threat to Red Oaks
In spite of their reputation for strength and longevity, ‘Oak Wilt’ has become a serious threat to oak trees in Michigan and elsewhere in the USA. The disease is most serious in members of the red oak family, including red oak, scarlet oak, black oak and pin oak. Members of the white oak family (white oak, swamp white oak, burr oak, etc.) are generally not as susceptible. The disease is as serious as Dutch Elm disease on American Elms. Both diseases are caused by vascular wilt fungi, transmitted by insects or root grafts, and both fungi kill their host plants rather quickly.
The health of oaks becomes high risk during the warm season months when they are injured through human activities, 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.
Once a red oak becomes infected with the oak wilt fungus, it usually dies within several months. 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; 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.
Winter Pruning: Save Our Oaks
Dormant pruning (winger pruning) enables us to correct issues which would be more risky during the summer season. The insects and fungi that cause the disease are nearly completely absent during winter.
The oak wilt fungus could potentially be transmitted via pruning utensils because of the transfer of fungal material residing in saw dust or chips during the warm season, when fungal growth is at its maximum. However, there is probably little chance of spreading oak wilt during the dormant season, but we still recommend sanitizing equipment between trees.