Symptoms and Signs
Symptoms vary somewhat with the kind of plant and the environment, but some symptoms are common to most situations. The leaves may wilt and turn yellow, first at the margins and between the veins, then they turn tan or brown and die starting from the base to the tip of the plant or branch. Dead leaves can either fall off or remain attached to the plant. Woody plants often are affected first on one side, and affected branches usually die. The water-conducting tissues (xylem) of infected plants are often discolored with dark brown to black streaks. In some plants, including olive, ash, and roses, there is little or no discoloration.
Comments on the Disease
Verticillium wilt, one of the most widespread and destructive soilborne diseases of plants, affects a large number of herbaceous and woody species throughout the world. The causal fungus, Verticillium dahliae, infects susceptible plants through the roots and plugs the water conducting tissues.
Susceptible flower crops include China aster, chrysanthemum, cineraria, dahlia, geranium, gerbera, heather, marigold, peony, pelargonium, rose, snapdragon, statice, stock, and strawflower. The V. dahliae fungus forms microscopic black resting structures (microsclerotia) capable of surviving in soil for many years in the absence of a susceptible plant. When a susceptible plant is planted in infested soil, the microsclerotia germinate and infect the plant. Long rotations with nonsusceptible plants are not effective in controlling the fungus.
The fungus also produces conidia that can be transported in irrigation water; however, they are not long-lived. The fungus can be disseminated by leaves dropping from infected plants and being blown around by the wind.
Many horticultural crop plants have been selected or bred for resistance to the fungus. Use resistant cultivars and pathogen-free plants whenever possible.
Steam (at 140°F for 30 minutes), solarize (double-tent at 160°F for 30 minutes or 140°F for 1 hour), or chemically treat growing medium. For outdoor cut flower or nursery production, avoid fields previously used for susceptible crops (e.g., tomato, cotton, potatoes, strawberries, as well as the ornamentals listed above) unless disinfected. Soil fumigation or soil solarization (in warmer climatic areas) can be useful. During the growing season, remove and destroy any plants that exhibit symptoms of Verticillium wilt.
|Common name||Amount to use||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.|
|A.||CHLOROPICRIN*||Label rates||See label|
|Allowable for use under a Critical Use Exemption only. Fumigants are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.|
|B.||Sequential application of:|
|(Note: Fumigants such as 1,3-dichloropropene and metam products are a source of volatile organic compounds (VOCs) but minimally reactive with other air contaminants that form ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.)|
|(Pic-Clor60)||300–332 lb (shank)||See label|
|(Pic-Clor60 EC)||200–300 lb||See label|
|COMMENTS: Very effective for control of soilborne fungal pathogens and insects. Drip irrigation requires an emulsifier. For shank fumigation, using higher rates or plastic mulch, especially virtually impermeable film (VIF), improves weed control. For drip fumigation the use of VIF will improve both nematode and weed control. According to state permit conditions, the maximum application rate of 1,3-dichloropropene is 332 pounds active ingredient per acre. Pic-Clor60: One gallon of weighs 12.1 lb; Pic-Clor60 EC: One gallon of weighs 11.8 lb.|
|Following 5 to 7 days after fumigation:|
|(Vapam HL, Sectagon 42)||37.5–75 gal||See label|
|COMMENTS: Water-soluble liquid that decomposes to a gaseous fumigant (methyl isothiocyanate). Efficacy affected by soil texture, moisture, temperature, and percent organic matter. One gallon of product contains 4.26 lb of metam sodium.|
|. . . or . . .|
|(K-Pam HL, Sectagon–K54)||30–45 gal||See label|
|COMMENTS: Water-soluble liquid that decomposes to a gaseous fumigant (methyl isothiocyanate). Efficacy affected by soil texture, moisture, temperature, and percent organic matter. One gallon of product contains 5.8 lb of metam potassium.|
|(Basamid)||200 lb||See label|
|COMMENTS: Powder incorporated into the soil, followed by irrigation or tarping. It decomposes to a gaseous fumigant (methyl isothiocyanate).|
|(Regalia CG)#||Label rates||4|
|MODE OF ACTION GROUP NAME (NUMBER1): anthraquinone elicitor (P 05)|
|*||Permit required from county agricultural commissioner for purchase or use.|
|§||Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Materials Use Requirements, which may be lower than maximum label rates.|
|‡||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.|