What Causes Oak Wilt?
Oak wilt is a lethal disease caused by the fungus Ceratocystis fagacearum.
The fungus invades and disables the water-conducting system in white, red and other oak species. Different species of oaks vary in susceptibility to the disease. Red oaks typically die within 4 to 6 weeks of initial symptom development, while white oaks may survive or take 1 to 6 months to defoliate and die. Oak wilt is most often spread via root grafts between interconnected and grafted root systems.
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How does oak wilt spread?
Sap-feeding beetles (Nitidulidae) are the most common insects that carry and spread this disease, but bark beetles (Scolytidae) may also be carriers. They feed on spore mats that form between the bark and the wood of the oak, and carry oak wilt spores to wounds on uninfected trees. In Minnesota this transmission takes place throughout the spring and early summer.
Because beetle vectors (carriers) are attracted to fresh wounds it is important not to prune oaks during the spring and summer when spore mats are present. In the north, prune only during the dormant season (approx. November through March).
Root Graft Transmission
Root graft transmission is the most common mode of infection. Over 90% of all new oak wilt infections are transmitted in this manner. A root graft is formed when the roots of two trees of the same species meet and fuse together. The disease is then able to move from an infected tree into an uninfected tree.