Verticillium Wilt of Shade Trees

Verticillium Wilt of Shade Trees

Verticillium wilt is a serious fungal disease caused by the fungus Verticillium albo-atrum. It can infect over 300 different plant hosts. It attacks many vegetable and fruit crops as well as ornamental trees.

The fungus enters plants through the roots and spreads systemically throughout the vascular or water conducting vessels. As it multiplies, it plugs the vascular system making the plants wilt, 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 ranges from mild to so severe that 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 worst 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.

To check for verticillium wilt, cut the affected branches and look at the wood. A ring of discolored wood will be visible. Cut the wood lengthwise and you will see longitudinal streaks on the sapwood. These are be gray, brown, or greenish, depending on the tree species.

They are hard to see so do not confuse them with streaks found around wounds in otherwise healthy trees. Verticillium can only confirmed by culturing the pathogen in a diagnostic laboratory.

Verticillium wilt can kill a small tree in one season, but larger trees can take several years to die. Some strains of Verticillium are more virulent than others. Affected trees may not die but are very stunted in growth.

Sometimes trees have a mild case of the disease and do not show symptoms until they are stressed. Infected trees can wall off the fungus from the new growth an grow normally if it is properly cared for.

Verticillium is a naturally occurring organism and is almost impossible to eradicate it from the soil. 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.

Prune out infected branches at least one foot below the discoloration in the wood. Destroy pruned branches and remove dead plants . same spot. When transplanting, avoid injury to roots and plant resistant species.

Resistant Trees (but not immune)

Apple, Beech, Birch, Crabapple, Fir, Hawthorne, Honey Locust, Mountain Ash, Mulberry,

Oak, Pear, Pines, Poplar, Spruce, Sycamore, Walnut, Willow, Zelkova