Many fungal diseases affect oaks, but oak wilt is the one that strikes fear in the heart of most landscapers and homeowners. It has the ability to kill a mighty oak in as little as a few weeks. This disease was discussed in issue No. 7 of this newsletter. Here is some important information about oak pruning that will help discourage oak wilt.

Because of the many cankers, wood rots, and injuries that occur on oak, periodic removal of dead wood is necessary. With many trees, we can do this as the dead wood appears, but oaks should be pruned only in the dormant season. Most references say to prune oaks in Illinois from September through March, while others suggest any time after July. Still others just say to prune in the dormant season.

Oak wilt is spread in two ways. One method is by root graft from oak to oak. This happens underground and is not influenced by pruning. The other method is by insects that have acquired the oak wilt fungus. The fungus must enter the tree through a wound, and the insect provides transportation to that wound site. These insects are attracted to sap, and they are attracted to your tree when sap is exposed to the air. When trees are pruned in the growing season, they exude sap from the wound, which could attract the insects carrying the oak wilt fungus. For this reason, oak pruning is not recommended during the growing season. City arborists do not have the luxury of waiting for a specific time period. They often have their work force during the growing season, so they may push the limit and prune in late summer, especially in areas that do not have a known oak wilt problem. Homeowners can and should be more conservative. For more information on oak wilt, consult Report on Plant Disease No. 618.