Oak wilt is a fatal disease of oaks. The fungal pathogen is capable of killing oaks in the red oak group in one growing season. Oaks in the white oak group are also susceptible, but decline and death are slower, sometimes prolonged over many years. No oak species grown in Illinois is immune to the fungus. Details about oak wilt are in issue no. 8 of this newsletter or in Report on Plant Disease, no. 618, “Oak Wilt and its Control,” from Illinois Extension offices or on the Web at www.ag.uiuc.edu/%7Evista/horticul.htm.

Many fear that oak wilt will spread throughout Illinois oaks much as Dutch elm disease killed elms in the early 50s. This is very unlikely because the oak wilt fungus cannot spread quickly over great distances. It is spread from tree to tree by underground root grafts or aboveground by way of beetles that feed on fungal mats. The fungal mats do not appear on all oak species.Also, the fungal mats are only exposed to insect feeding after the bark begins to fall from the tree, exposing the mats. As long as we are able to remove dead oaks in a timely manner, above-ground spread will be reduced.

The table shows the number of oak wilt samples cultured at the University of Illinois Plant Clinic in the last 7 years. The positive diagnosis can be made only when the oak wilt fungus is isolated from the sample. The Plant Clinic handles only oak samples submitted by clients, so these samples cannot be considered a survey of what is appearing in the state. Still, these numbers do not show in increase in oak wilt isolations over the last 7 years.