Oak Wilt is a disease caused by the fungus Bretiziella fagacearum that is specific to oaks (Quercus spp.).
The fungus is spread through root grafts between neighboring trees and by insects. Red Oaks are particularly susceptible to oak wilt. The infection causes leaf discoloration, defoliation and death in a very short period of time (from two months to one year). Fungal mats will form under the bark and force outwards, cracking the bark of the tree. White oaks are more tolerant of oak wilt infection. Fungal mats will not form and it will take much longer for the tree to succumb to the disease. White oaks will show infected annual rings when viewed in cross section.
Initial symptoms of Oak Wilt will be browning leaves, beginning at the leaf tip and moving downward and inward toward the stem. As the disease progresses, limbs will die off. Fungal mats may develop under the bark, pushing the bark out and causing cracks. Untreated, the tree will die, sometimes within a matter of months.
We recommend a trunk injection of Propizol. Propiconazole is a systemic fungicide that will suppress Bretiziella fagacearum. Because Oak Wilt is spread through root grafts and insect carriers, We recommend the treatment of non-infected oaks in close proximity to the infected trees to slow the spread of the disease. When treating multiple trees, it is recommended to disinfect drill bits and injection equipment between trees.
Generally, the best seasons for injection are fall and spring, as uptake occurs when trees are transpiring. The environmental conditions that favor uptake are adequate soil moisture and relatively high humidity. Soil temperature should be above 40 degrees fahrenheit for trunk injection. Hot weather or dry soil conditions will result in a reduced rate of uptake, so trees should be watered if applications are made when soil is extremely dry.
Tree recovery with Propizol will be proportional to the severity of the infection at the time of treatment. Trunk injection of propiconazole will kill and suppress Bretiziella fagacearum and allow the tree to refoliate. Trees should be re-evaluated for retreatment every 12-36 months.
Main photo taken by Minnesota Department of Natural Resources Archive, Minnesota Department of Natural Resources, Bugwood.org
Fungal mat taken by USDA Forest Service Archive, USDA Forest Service, Bugwood.org