In issue no. 12, the University of Illinois Plant Clinic reported laboratory-confirmed cases of oak wilt in many Illinois counties. Since that time, we have added two more for DuPage and McHenry counties, and one positive each for Knox, Lake, and Putnam counties. This does not mean there is an epidemic in Illinois. In fact, we find positive cases of oak wilt every year. It should remind us of an important pruning consideration. Please hold off pruning oaks in these areas until the dormant season. Why take a chance of attracting beetles that may be carrying the oak wilt fungus to your oak? The disease kills even mature trees.
Not all oak samples sent to the Plant Clinic are positive. Many oaks showing distress are not infected with oak wilt. It is possible that drought stress, compaction, construction injury, and many other factors could be involved in tree decline. If you send a sample to a lab for oak wilt testing, remember that, to obtain a reliable diagnosis, the wood must be alive, about thumb thickness, and showing vascular streaking. If the sample does not meet these criteria, then a negative lab test is an indication of the poor quality of the sample and not the absence of the disease.