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. The fungus cannot be isolated from dead or dried branches. 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.