Verticillium Wilt of Shade Trees
Verticillium wilt, caused by the fungus Verticillium albo-atrum, can infect over 300 known plant hosts. It is a pathogen on many vegetable and fruit crops as well as ornamental trees. The fungus enters plants through the roots and spreads systemically throughout the water conducting vessels. This plugs the vascular system, resulting in the familiar wilt symptom. Depending upon how much of the root system is infected and where the fungus spreads in the tree, the disease may be very mild or it may kill the tree in one year. Highly susceptible trees include maple, catalpa, elm, ash, black locust, lilac, Russian olive, horse-chestnut, stone fruits, and the golden rain tree. Linden may be susceptible under some conditions.
Symptoms are usually most conspicuous in midsummer during hot, dry periods. Leaves suddenly wilt and dry up. This may occur on the whole tree, but frequently only one side or just a few branches are damaged. If affected branches are cut, a ring of discolored wood can be seen. If the bark is peeled back, longitudinal streaks on the sapwood may be evident. These can be gray, brown, or greenish, depending on the tree species. They may be hard to see sometimes, and should not be confused with streaks found around wounds in otherwise healthy trees. Confirmation of Verticillium can only be obtained by culturing the pathogen in a diagnostic laboratory.
Verticillium wilt may kill a small tree in one season, but larger trees may take several years to die or may recover completely. Several strains of Verticillium albo-atrum exist; some are more virulent than others. If a tree is affected over several successive years, it may not die but will be very stunted in growth. Sometimes a tree may have a mild case of the disease and not show symptoms until it is stressed. An infected tree may wall off the fungus from further growth an grow normally if it is properly cared for.
Verticillium is a naturally occurring organism. It is very difficult to eradicate it from the soil. The best prevention is to keep trees healthy and vigorous with proper fertilization and infrequent, deep watering during the growing season. Do not plant susceptible trees in old vegetable gardens because of high populations of Verticillium will probably be present in the soil. With infected trees, the first step is to prune out all affected branches at least one foot below any discoloration in the wood. Cuts should be clean and tools disinfected after each cut with a 10% solution of bleach. Pruned branches should be destroyed. If a tree is so severely affected that it is obviously going to die or is already dead, remove it and do not plant another susceptible tree in the same spot. When transplanting, avoid injury to roots and plant resistant species.
|Resistant Trees (but not immune)|