Oak wilt is a fungal disease that kills mature oaks in as little as one growing season. The University of Illinois Plant Clinic has been testing oak wood for the presence of the oak wilt fungus since 1976. You can learn details about this disease in the University of Illinois Report on Plant Disease, no. 618, available in Extension offices or in electronic form at http://www.ag.uiuc.edu/~vista/abstracts/a618.html.
The only way to be certain that an oak is infected with the oak wilt fungus is to culture live wood on a laboratory medium (such as potato dextrose agar), isolate the causal fungus, and observe diagnostic spores. The tree is considered as a positive case only when the fungus is isolated from the wood. If you need details on how to submit a sample for oak wilt testing, refer to issue no. 12 (2008) or no. 7 (2007) of this newsletter.
Two frequently asked questions concerning oak wilt are (1) Are we seeing more oak wilt than usual this year at the University of Illinois Plant Clinic? and (2) How have the numbers of positive cases of oak wilt changed over the past few years?
First, it is important to realize that the Plant Clinic does not survey the state of Illinois. We work on whatever samples are sent to us for testing. Comparing this season to records of oak wilt for the past eight seasons, we are not seeing any more positive cases than usual. We test over 50 oaks each year as a normal part of our service. The number of positive cases in the last 8 years follows:
So far in 2008, we have tested 28 trees, representing 16 Illinois counties. Eleven cases have been positive. Those positive oak wilt cases occurred in DuPage, Effingham, Fulton, Macoupin, Morgan, Ogle, Sangamon, and Stephenson counties. Bur (4), red (2), shingle (2), and unidentified oaks (3) were the species involved.