Q. An oak tree in our yard has a low-hanging branch that whaps me in the face when I mow the lawn. The first time it happened, I went to get my pruning saw to remove the obstacle, but my wife stopped me because she had heard that oaks should be pruned only in the winter. Is this true, and, if so, why?
A. Oak wilt is a serious fungal disease that affects all oaks. Texas live oak (Quercus fusiformis) and species in the red oak group, such as northern pin oak (Q ellipsoidalis) and northern red oak (Q rubra), tend to decline rapidly when infected by oak wilt and may die within a matter of weeks to a few months. White oak (Q. alba), bur oak (Q. macrocarpa), and some live oaks may live with the fungal infection for one to several years, but most trees eventually succumb to the disease.
The two main ways in which the fungus spreads from tree to tree are by naturally occurring root grafts that form between oaks of the same species growing in proximity to one another and by insectborne fungi. The insects that transmit the disease are sap beetles and oak bark beetles that are attracted to fresh wounds in the bark of trees. A primary cause of widespread oak wilt infection across the eastern United States is believed to be injury to oaks during construction in wooded areas.
According to The Organic Gardener's Handbook of Natural Pest and Disease Control, pruning oaks only during dormancy (when insect vectors are inactive) reduces the chances of oak wilt fungi entering through the resulting wounds. Once oak wilt begins, there is no control.