Several shade tree species are susceptible to Verticillium wilt. Maples are quite susceptible. Ash, catalpa, golden rain tree, smoke tree, magnolia, and redbud, and others can also be affected. Susceptible shrubs include barberry, boxwood, dogwood, lilac, spirea, weigela and viburnum. Verticillium is not extremely aggressive but can be a problem on stressed trees and shrubs. Samples that have tested positive for the disease in the Plant Disease Clinic this year include green ash, maple, and catalpa.
The fungus Verticillium is found in the soil. After the roots of susceptible trees are invaded, the fungus moves into the vascular system. Trees may show yellowing, wilting, and drying of leaves in the top branches, or sometimes the entire crown. Leaves usually remain attached for a time before dropping off.
If the bark is scraped away on wilted branches, you can sometimes see a green to brown streaking, although this is not always apparent. For confirmation, laboratory isolation is necessary. For a laboratory assay, several freshly wilted branch segments about 6 inches long and approximately 1/4 to 1/2 inch diameter are needed. The fungus cannot be isolated from dead or dried branches. Submit samples to your local Extension office or directly to the Plant Disease Clinic, 323 Bessey Hall, Ames, IA 50011. There is a $5.00 fee for this test.
Verticillium may completely kill the tree or shrub in one growing season or cause damage to only a par of the plant. Sometimes an actively growing plant can wall off the invasion of the fungus and can survive for several years. Water infected trees or shrubs regularly to reduce stress. Application of fertilizer may help suppress disease development. Dead branches should be pruned, but be sure to disinfest (such as with a 10% household bleach solution) pruning tools between cuts.
Other problems which can be confused with Verticllium wilt includes girdling roots, soil-applied herbicide damage, or root injury.
This article originally appeared in the August 9, 1996 issue, p. 143.