Oak wilt is a tree disease affecting oak trees caused by the fungus Ceratocystic fagacearum. The disease has been found in 21 states causing the most damage in the midwest. It was first found in Wisconsin in 1944 where localized pockets of mortality were found; nearly half the oaks an area of less than 100 acres were killed. No species of oak is immune or especially resistant to this disease, infections have been found on 16 native species including those of economic and environmental value. Red oak species do seem more susceptible to oak wilt infection and succumb more often than white oaks.
Symptoms of oak wilt may first appear on leaves which will turn dull green or bronze and appear water-soaked and wilted prior to turning yellow and brown. Damage begins along the outer edges and moves toward the leaf’s mid-rib. Wilting leaves curl toward the mid-rib and the delineation between live, green tissue and chlorotic, dying tissue is very distinct. These symptoms appear and spread quickly throughout the crown, this can occur within just a few weeks and results in premature leaf drop at the ends of branches. Heavy defoliation continues, with leaves in all stages of distress falling. Some branches may have persistent green leaves until Autumn, crowns of affected oak trees are rarely uniform.
The high-risk period for oak wilt infection begins soon, this means that the potential overland transmission of the disease is very high. Fungal mats composed of oak wilt which have been killed the previous year but are still present on the tree when sap-feeding beetles (nitidulids) become active put trees at an increased risk of infection. Risk is further increase as variable weather conditions persist at the edges of the southern range of oak species. The USDA and Forest Service advise against pruning or wounding oak trees during the months of April, May and June. Open wounds are very attractive to spore-carrying nitidulids, when they visit wounds to feed on sap they can deposit oak wilt spores infecting the tree. Any open wounds apparent on oak trees in the spring should be treated, covered or dressed if it is at all possible. New oak disease infections will display visible symptoms come July and August.
Unfortunately control of oak wilt once it is already present and causing damage is difficult. Prevention is the best control, this includes cultural practices to keep trees and landscape plants healthy such as proper planting and watering, maintaining adequate pH and nutrient applications. If you suspect or have concerns about oak trees on your landscape contact your arborist and visit: http://www.savatree.com/tree-disease-treatment.html.