How to Manage Pests
UC Pest Management Guidelines
Pathogen: Verticillium dahliae
(Reviewed 3/09, updated 3/09)
In this Guideline:
SYMPTOMS AND SIGNS
Leaves on one or more branches, often on only one side of the tree, will turn yellow or wilt early in the growing season. The symptoms progress until the affected shoots die and dry up later in the season. Affected young shoots often resemble a shepherd's hook. When shoot, branch, or trunk tissue of infected trees is cut in cross section, the vascular ring and often much of the heartwood will display dark discoloration. Foliar symptoms usually appear only on young trees (first to fifth or sixth leaf). Older trees do not normally exhibit symptoms of Verticillium wilt.
COMMENTS ON THE DISEASE
The causal fungus survives from season to season in soil, in debris of previous susceptible crops, and probably in roots and the lower trunk of infected trees. Often the fungus can be isolated from living portions of infected tissue year-round in the Central Valley. Research has shown that tree yields can be reduced by Verticillium even when foliar symptoms are not readily apparent. Specific rootstock or scion varieties may vary in susceptibility. Second-to fourth-year trees are usually the most susceptible to Verticillium wilt.
Orchards can be adversely affected by this disease even when low pathogen numbers in soil (two to three propagules per gram) are present. Avoid interplanting young orchards with susceptible cover plants, such as cotton, tomatoes, melons, etc. When replanting in an area where susceptible perennials were previously grown, try to remove as many roots of the previous crop as possible.
To solarize the soil before planting, cover the moistened soil with clear, UV-inhibited plastic sheeting in late spring. Leave the plastic in place during summer months. To solarize the soil after trees have been planted, cover the soil around trees with black plastic sheeting. Leave in place for one to two growing seasons.
Orchards may also be fumigated before trees are planted.
UC IPM Pest Management Guidelines:
J. E. Adaskaveg, Plant Pathology, UC Riverside