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 trees' yields can be reduced by Verticillium wilt, 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.
Management
This disease can adversely affect orchards even when low pathogen numbers are present in the soil (two to three propagules per gram). 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.
Soil Solarization
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.
Rootstocks and Varieties
Lovell rootstock is very susceptible to Verticillium wilt. Hansen may be more susceptible than Nemaguard. Carmel is much more susceptible to wilt symptoms than most other cultivars. Atlas rootstock appears to be highly tolerant.
Fumigation
Orchards may also be fumigated with chloropicrin-containing fumigants before trees are planted.
| Common name | Amount per acre | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| 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 SOIL FUMIGATION | ||||
| A. | CHLOROPICRIN* | |||
| (Pic-clor 60) | Label rates | See label | See label | |
| COMMENTS: This restricted use product is applied only by professional fumigation companies and is a multi-purpose liquid fumigant for preplant treatment of soil to help manage certain soil-borne diseases and to control other pests (e.g., plant parasitic nematodes, symphylans) in croplands. Fumigants such as chloropicrin are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. | ||||
| B. | 1,3 DICHLOROPROPENE/CHLOROPICRIN* | |||
| (Telone C-35) | Label rates | See label | See label | |
| COMMENTS: It is effective at 39 to 46.7 gal/acre rate (labeled for shank applications) if applied to dried sandy soils or sandy loam soils with no more than 12% soil moisture content anywhere in the surface 5 feet of soil profile. In California the applications must be applied to soils having a moist surface; this task is difficult to achieve without use of sprinklers unless there is a fortunate rainfall. Do not flood irrigate prepared lands to achieve this surface moisture requirement. Broadcast apply where nematode resistance is unavailable for prevailing nematodes. Strip applications are permitted at higher treatment rates and effective where resistant rootstocks are available, the clay loam soil profile contains no more than 19% soil moisture, the field has been pre-ripped to 4- or 5-foot depth, and the delivery shank is winged to limit off-gassing. Fumigants such as 1,3-dichloropropene and chloropicrin are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. | ||||
| C. | 1,3 DICHLOROPROPENE * | |||
| (Telone II) | Label rates | See label | See label | |
| COMMENTS: This restricted use product is applied only by professional fumigation companies and is a multi-purpose liquid fumigant for preplant treatment of soil to help manage certain soil-borne diseases and to control other pests (e.g., plant parasitic nematodes, symphylans) in croplands. Fumigants such as 1,3-dichloropropene are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. | ||||
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode-of-action group number. |
| ‡ | 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. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur. |
| * | Permit required from county agricultural commissioner for purchase or use. |