The reason to avoid pruning oaks now is that you donít want to invite sap-feeding beetles in the Nitidulidae family to your trees. When you prune during the growing season, the oak branch produces sap where it is cut, and that sap is inviting to these beetles. The beetles themselves are not harmful to healthy trees, but they may have visited an oak infested with oak wilt spores. If they did, they are bringing the spores to your tree.
If oak wilt is not known to be a problem in your area, you donít have to abide by this advice. On the other hand, we have found oak wilt on trees as far south in Illinois as Marion and as far north as the Wisconsin border. Our samples are not a survey of the state. They generally reflect the more populated areas of the state. This advice would probably be good for all of Illinois.
Oak wilt is a devastating fungal wilt disease that may kill a mature oak in a matter of weeks. Oaks in the red oak group are most susceptible, and those in the white oak group less susceptible. None of the oaks is resistant. The picture shows a red oak in Urbana, Illinois, that died rapidly from oak wilt a few years ago.
Symptoms may include scorching of the foliage near the top of the tree, entire branch death or areas of a tree that die, overall off-color foliage, and tree death. Vascular tissue is stained brown in infected trees. Scattered leaf spots are not part of this disease.
You canít save a tree that is severely infected with the oak wilt fungus, but fungicides (Alamo and Savvi) can be used to prevent infection in nearby oaks. If you would like your oak to be tested for the oak wilt fungus, send a sample of live wood from a symptomatic area of the tree to the Plant Clinic, along with the $12.50 fee. The wood will be cultured in the lab, and within 7 to 10 days cultures will tell us whether the oak wilt fungus is present. For the most accurate testing, take samples that are 8 to 10 inches long, thumb thick, and exhibiting vascular streaking. The image shows a sample with vascular streaking.
If you would like more details about oak wilt, consult Report on Plant Disease, no. 618, ďOak Wilt and Its Control,Ē at http://www.ag.uiuc.edu/~vista/abstracts/a618.html.