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 up 1 inch diameter are needed. The fungus cannot be isolated from dead or dried branches. Diagnosing vascular pathogens is challenging see the vascular wilt article for more information on how to collect and submit samples to our clinic.
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. 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.
Verticillium wilt sampling checklist
please follow these instructions as much as possible.
· Select 3 to 6 living branches from the symptomatic area in a tree.
· Select branches with leaves, sending symptomatic leaves (bronzing) is a bonus, we do not test the leaves, but it gives important diagnostic information.
· Select branches 1-2 inch in diameter (we need branches, not twigs) and 6 to 12 inches in length.
· Check for sapwood discoloration see picture on the right hand side
· If the samples are left in the truck or by the house porch for a couple of hours the chances of recovering the pathogen significantly decrease.
· If possible refrigerate the sample, have a styrofoam chest with cold packs or ice bags.
· Collect the sample only when you can either deliver it directly to the PID Clinic right away or to send it overnight (never on a Friday!).
· Follow our submission instructions on the Clinic website http://www.ipm.iastate.edu/ipm/info/submit/plant
· Place your completed submission form in a bag, this will prevent from getting wet.
· How to drop off or mail the sample visit http://www.ipm.iastate.edu/ipm/info/submit/plant
· Submission form and current fees, visit: http://www.ipm.iastate.edu/ipm/info/contact
For more information see this bulletin for more information about Verticillium wilt.