Symptoms and Signs
Verticillium wilt becomes apparent when leaves on one or more branches, often on only one side of the tree, turn yellow and/or wilt early in the growing season. The symptoms progress until the infected shoots die and dry up later in the season. When shoot, branch, or trunk tissue of infected trees is dissected, the vascular ring and often much of the heartwood will display dark discoloration. Foliar symptoms usually appear only on young trees (2nd to 4th leaf). Older trees do not normally present symptoms of Verticillium wilt.
Comments on the Disease
The causal fungus, Verticillium dahliae, survives from season to season in soil, debris of previous, susceptible crops, and probably in the roots and lower trunk of infected trees. Often the fungus can be isolated from living portions of infected tissue year around in the Central Valley. Tree yields can be reduced by Verticillium even when foliar symptoms are not readily apparent. Specific rootstock/scion varieties may vary in susceptibility and are not well known.
Management
Orchards can be adversely affected by the disease even when low pathogen numbers in soil (2–3 propagules per gram) are present. Avoid interplanting young orchards with susceptible crop plants, such as cotton, tomatoes, melons, etc. Verticillium dahliae is usually present in these soils. Inoculum levels can be reduced by fumigating the soil, flooding fallow fields in summer, solarizing the soil, growing several seasons of grass cover crops (especially sudangrass or ryegrass), or a combination of these methods. When replanting in an area where susceptible perennials were previously grown, try to remove as many roots of the previous crop as possible.
Soil Solarization
Preplant. Beginning in late spring, cover the moistened soil with clear, UV-inhibited plastic sheeting. Leave in place during the summer months.
At planting. Cover soil around trees with black plastic sheeting. Leave in place for one to two growing seasons.
| Common name | Amount per acre** | REI‡ | |
|---|---|---|---|
| (Example trade name) | (hours) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | |||
| PREPLANT | |||
| A. | METHYL BROMIDE*+CHLOROPICRIN* | Label rates | See label |
| COMMENTS: Use allowed under a Critical Use Exemption only. Fumigants such as methyl bromide are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone; methyl bromide depletes ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available. | |||
| * | Permit required from county agricultural commissioner for purchase or use. |
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. |