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Verticillium wilt can infect more than 300 different plants. It attacks many vegetables, fruits, flowers and shade trees. The fungus enters plants through the roots and spreads throughout the water conducting vessels. This plugs the vascular system, giving the familiar wilt symptom.
The disease may be very mild or it may kill the tree in one year. The severity depends on how much of the root system is infected and where the fungus spreads in the tree. Highly susceptible trees include maple, catalpa, elm, ash, black locust, lilac, Russian olive, horse chestnut, stone fruits, and golden rain tree. Lindens are susceptible under some conditions.
Wilt symptoms are usually most conspicuous in midsummer during hot, dry periods like we have just experienced. Leaves suddenly wilt and dry up. This may affect the whole tree, but frequently only one side or several branches are damaged.
If you cut affected branches, 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 recognize and should not be confused with streaks found around wounds in otherwise healthy trees. Confirmation of Verticillium is only obtained by culturing the pathogen in a laboratory.
Verticillium wilt may kill a small tree in one season, but larger trees may take several years to die or may recover completely. 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 contain the fungus and grow normally if there are no other problems.
Verticillium is a naturally occurring organism. Eradicating it from the soil is very difficult. 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)
Common Name Apple Beech Birch Crabapple Fir Hawthorne Honey Locust Linden Mountain Ash Mulberry Oak Pear Pines Poplar Spruce Sycamore Walnut Willow Zelkova
