Oak Wilt Disease:
A tree disease caused by the fungus, Ceratocystis fagacearum. The fungus infects the vascular system of a tree. The vascular system contains vessels which transport moisture throughout the tree. The vessels of an infected tree effectively become blocked by the infection of the fungus, and cannot transport adequate moisture to sustain a healthy or living tree. In most cases, the end result is tree mortality.
- When possible, we here at Envision Landscapes avoid trimming or pruning Live oaks and Red oaks (Spanish, Shumard, Texas Red, and Blackjack oaks) from March 1 to June 1.
- At all times and irrespective of limb size, all cuts and wounds to oak trees shall be dressed immediately using a non-phytotoxic tree wound dressing.
- Disinfection of pruning tools, saws, and related equipment is mandatory during the trimming or pruning of oak trees. Disinfection of tree removal and trimming equipment shall occur before work begins in a project area, between work in individual oak trees, and again prior to leaving a project area. Acceptable disinfectants include either aerosol disinfectant or a 10 percent bleach-water solution.
*NOTE: Although this policy would require the disinfection of pruning equipment before and between oak trees as a precaution, research does not substantiate disinfection as a means of preventing the transmission of the oak wilt disease.
Infections and Symptoms:
Oak wilt spreads in two basic ways. A transmission via root graft is the most common source of infection, as trees within as much as 15 m (50 feet) of an infected tree can be infected. The second method of infection is via sap beetles. These beetles are attracted to the bleeding sap of the oak tree, as well as the fungus in an infected or dead tree, and so can transfer the disease to healthy but injured trees. This is less common as trees are rarely infected this way unless injured, but it is the only way to jump barriers (rivers, for example) and infect trees in new areas.
Oak wilt is identifiable by the rapid pattern of wilting starting from the top of the tree and progressively dying down to the bottom, and on specific leaves, wilting from the edges to the base. Oaks with oak wilt stand out with their dead crown compared to a green canopy in the summer, so much so that oak wilt infections can be spotted from the air. A new infection via beetles instead of root grafts can kill a tree somewhat more slowly, if a branch is infected instead of the trunk.
Prevention is key, as there is no permanent cure. To prevent beetle transmission, oaks should never be pruned in the spring months; late fall and early winter are preferable. Dead oaks should be checked for fungal mats in the spring. If present, all wood from affected trees should be chipped then burned or covered with plastic sheeting to effect composting. Heat from composting should destroy or severely enervate the fungus. Logs from trees infected with oak wilt should never be moved to unaffected areas, even for firewood.
If a tree is discovered to be infected, a trench (or better yet, two generally parallel trenches at different distances from the area of risk) may be dug to a minimum depth of four feet (ca. 1.2 meters) between it and other potentially susceptible species to prevent root graft disease transmission. Such trenching will sever root grafts that may transmit the disease to neighboring trees. Injections of propiconazole can help prevent transmission as well; as many as a half dozen injections every six months may be required to protect a large red oak near infected trees. Injections every other year may also be used to treat a white oak, if it is not yet greatly affected. An infected tree should not be immediately cut down lest the oak wilt migrate down into the roots and increase the risk of passing via roots to nearby trees. Parts of and debris from downed infected trees can make neighboring trees more susceptible to oak wilt transmission by beetles. Infected trees should be cut down the following fall. The debris, including bark, should be chipped then burned or composted beneath impermeable cover (e.g., plastic sheeting) until the next spring to prevent contamination by fungal mats.