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 in a curled position often called a "shepherd's crook". 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-round in the Central Valley. Tree yields can be reduced by Verticillium, even when foliar symptoms are not readily apparent. Specific rootstock and 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. Verticillium is very common when orchards are planted in soil formerly planted to susceptible row crops such as cotton, tomatoes, melons, etc. Avoid interplanting young orchards with these susceptible crop plants.
Inoculum levels can be reduced by flooding in summer, solarizing the soil, growing several seasons of grass rotational crops (especially rye or sudangrass), or a combination of these treatments. When replanting in an area where susceptible perennials were previously grown, try to remove as many roots of the previous crop as possible. Fumigating with chloropicrin before planting will reduce inoculum.
Soil Solarization
Soil can be solarized either prior to planting, or at planting:
- Preplant. Beginning in late spring, cover the moistened soil with clear, UV-stabilized plastic sheeting. Leave in place during the summer months. Remove before planting.
- At planting. Cover soil around trees with black plastic sheeting, which is less likely than clear plastic to increase soil temperatures to a level that harms new root development. Leave in place for one to two growing seasons.
| 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 | ||||
| A. | CHLOROPICRIN* | |||
| (Tri-Clor EC) | Label rates | See label | NA | |
| COMMENTS: 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 | NA | |
| COMMENTS: 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. | ||||
| ‡ | 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 two intervals is the minimum time that must elapse before harvest. |
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode-of-action group numbers 1, 4, 7, 9, 11, or 17, make no more than one application 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 fungicide with a different mode-of-action group number. |
| * | Permit required from county agricultural commissioner for purchase or use. |
| NA | Not applicable. |