The oak wilt disease was discussed in issue 9. If you are considering pruning oaks, it is important to know whether oak wilt is present in your area. If you prune oaks during the growing season when sap is flowing, infested beetles may be attracted to your tree, bringing with them the oak wilt fungus. Most city arborists and state foresters are aware of outbreaks. Extension horticulture specialists are also aware.
So far this year, the Plant Clinic has confirmed cases of oak wilt in DuPage, St. Clair, and Vermilion counties. In 2002, the clinic did so for Carroll, Cook, DeKalb, DuPage, Lake, Lee, Madison, McHenry, McLean, Tazewell, and Woodford counties.
This information is limited, reporting only on Plant Clinic samples. I do not mean to imply that oak wilt is not in other counties. For example, it has been found in Champaign County but not confirmed at the clinic in the past 2 years. This information still has great value. If you live in one of these counties, know that recently confirmed cases of oak wilt have occurred. Do not prune oaks during the growing season in those areas if you can help doing so.