Plants infected by Verticillium Wilt develop in response to water stress. Symptoms range from sudden wilt of individual branches to basic leaf scorch, stunted leaves, and branch die-back that occurs over a period of years. In many instances, symptoms develop only on one side of an infected tree. Larger infected branches, and occasionally small ones, exhibit brown, black or greenish streaks in the sap wood.
The fungus is an excellent soil inhabitant an can survive in the soil for many years. The fungus enters plants through the root system and colonizes the water conducting tissue (xylem) of susceptible hosts. Water transport becomes blocked by the fungal growth as well as gums created as defense mechanisms of the host plant itself. Some of the most highly affected species will fail within the first year of infection while the majority of species will show die-back over a period of years.
The fungus will remain in the soil even it the infected tree is removed. As far as a cure goes, there isn’t one. The progression of the infection may be slowed by maintaining the tree vigor through regular maintenance. (pruning, watering, and fertilization) In fact this may help delay the eventual death of the tree for several years. If a tree is found to have succumbed to the fungus, remove the tree and replant with a variety that is resistant to Verticillium Wilt. The wood debris for the failed tree should not be used as mulch unless it is composted properly.
Trees and shrubs susceptible to Verticillium
- Barberry, Japanese
- Boxwood, Korean
- Russian olive
- Buckeye, Ohio
- Smoke tree
- Cherry, other stone fruits
- Locust, black
- Coffee tree, Kentucky
- Cork tree
- Currant and gooseberry
- Oak, pin and red (rare)
Trees and shrubs resistant or immune to Verticillium
- Oak, white and bur
- Mountain ash
*Some plant species are listed in both lists (linden, dogwood).
The resistance or susceptibility will depend on the cultivar and the strain of Verticillium present in the soils.