Landscape - Verticillium Wilt

As drier weather sets in, evidence of wilting in some tree species may be seen. One cause of wilting is Verticillium wilt, a fungus disease. The following is more information.

Verticillium wilt of woody plants is caused by the fungus Verticillium dahliae, or in some cases by V. albo-atrum. The fungus is capable of causing a serious vascular wilt of a wide range of woody plants. Several of our common landscape trees such as ash, katsura tree, magnolia, maple, redbud, and tuliptree are susceptible to Verticillium wilt.

Symptoms. By invading the xylem tissues of the tree, Verticillium disrupts the movement of water from the roots to the leaves. As a consequence, leaves wilt and branches die back. This often occurs one branch at a time or on one side of the tree over a period of several years, but sometimes in only a matter of months or a year. Sometimes, branches simply fail to leaf out in the spring - the result of infection the previous year. Verticillium wilt may also cause marginal browning and leaf scorch, abnormally large seed crops, small leaves, stunting, poor annual growth, and sparse foliage. However, some or all of these symptoms may also be caused by girdling roots, construction injury, bacterial leaf scorch and drought.

In the landscape and nursery, one should try to observe additional diagnostic symptoms. Usually, there is staining of xylem and cambial tissue, visible as streaks if you cut into the wood. The color of this staining will vary for different trees often being greenish black in maple, yellowish green in smoke tree, dark brown in redbud, and brown in ash and catalpa. Be aware that often young twigs and branches and some tree species simply don’t show the streaks of stained xylem tissue under the bark and that other fungi and other factors can cause staining. For a positive laboratory diagnosis of Verticillium wilt, stained vascular tissue is essential.

Disease biology. The Verticillium fungus survives as resistant, dormant microsclerotia for many years in soil, making effective crop rotation in the nursery or landscape difficult. The fungus infects plant roots through wounds, or in some cases, direct penetration of susceptible root tissue. In the nursery, the Verticillium fungus could also be transmitted from plant to plant by grafting and budding. From the root infections, the fungus spreads into the plant through the xylem. Xylem tissues become blocked so that stems and leaves no longer are supplied with adequate water and mineral elements. After the tree dies, the fungus is returned to the soil as tiny resistant fungal microsclerotia. Microsclerotia can also be spread by wind, in soil, and on equipment. Many herbaceous and weed hosts are also susceptible so it is hard to avoid contaminated soil. Verticillium wilt is favored by landscape stresses such as wounding and drought. It is possible that much of the Verticillium observed now relates back to stresses imposed by the drought last summer.

Reprinted from "Verticillium Wilt is Active in Catalpa and Smoke Tree" By John Hartman in the current edition of the Kentucky Pest News. For the full article with pictures go to http://www.uky.edu/Ag/kpn/kpn_09/pn_090714.html