Symptoms and Signs
Infected plants may initially be stunted. Outer leaves exhibit marginal and interveinal browning, followed by eventual collapse. Inner leaves remain green. This last symptom sometimes helps to distinguish this disease from Phytophthora crown rot.
Comments on the Disease
The fungus is not host-specific and infects many weed species and crops worldwide. It is especially destructive in semi-arid areas where soils are irrigated. Inoculum densities may be high following planting of susceptible crops (e.g., lettuce). Disease severity is greater when high levels of nitrogen are used.
Preplant fumigation is an important component of managing Verticillium wilt in strawberry fields. If fumigation is not desirable, select fields isolated from established growing areas, avoiding any fields with detectable levels of the pathogen or with a history of susceptible crops. Crop rotation with broccoli and incorporating residue during warm parts of the year has been shown as an effective way to reduce Verticillium in the soil. Solarization of formed beds may be used to reduce pathogen levels in areas that get adequate amounts of sunshine and warm weather during summer months, although the usefulness of this technique for reducing Verticillium wilt in strawberries is unknown.
If infested fields cannot be avoided and fumigation is not feasible, either solarize the soil or implement a crop rotation program. Cover crops of cereal rye or ryegrass can help to reduce soil levels of Verticillium. Use relatively tolerant strawberry cultivars when practical; Camino Real, Petaluma, Albion, and San Andreas are among the least susceptible UC cultivars.
In warmer areas of the state, solarization has been shown to be effective for the control of soilborne pathogens and weeds. Solarization is carried out after the beds are formed and can be effective if weather conditions are ideal (30-45 days of hot weather that promotes soil temperatures of at least 122°F). The effectiveness of solarization can be increased by solarizing after incorporating the residue of a cruciferous crop, in particular broccoli or mustards, into the soil or following an application of metam sodium (40 gal/acre). For more details on how to effectively solarize soil, see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds, UC ANR Publication 21377.
Rotating strawberries with broccoli can significantly reduce levels of the Verticillium pathogen in the soil and has been shown to be an economically viable option under moderate levels of Verticillium wilt disease pressure.
Organically Acceptable Methods
Select fields isolated from established growing areas. If fields have detectable levels of the pathogen or a history of susceptible crops, plan to solarize the soil, preferably after incorporating the crop residue from broccoli or mustards. Use drip irrigation and resistant cultivars (e.g., Camino Real, Petaluma, San Andreas or Albion). Practice crop rotation with a nonsusceptible crop, such as broccoli, and avoid high nitrogen fertilizers.
Drip fumigation of pre-plant fumigants may not effectively control soilborne pathogens in the entire bed; field research has demonstrated pathogen survival at the bed shoulders and in soil profiles deeper than 12 inches. Use of this application method should be considered carefully. Growers may improve fumigant distribution by using more than two drip lines per bed and by applying larger volumes of water to deliver the fumigants. Bed fumigation will not control these pathogens in the untreated furrows.
|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.|
|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.|
|(Tri-Con 50/50)||300–400 lb||See label||0|
|COMMENTS: Methyl bromide use is allowed only in strawberry nurseries through the quarantine and pre-shipment (QPS) exemption. 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.|
|First, apply one of the following|
|(Telone C35)||Label rates||See label||0|
|(InLine)||Label rates (drip)||See label||0|
|COMMENTS: Effective for control of nematodes, soilborne fungal pathogens, and insects. InLine requires a plastic tarp. Use higher rates or impermeable films to improve weed and nematode control. One gallon of product weighs 11.2 lb.|
|. . . or . . .|
|(Tri-Clor)||150–350 lb (shank)||See label||0|
|(Tri-Clor EC)||200–300 lb (drip)||See label||0|
|COMMENTS: A liquid that diffuses as a gas through soil. Very effective for control of soilborne fungal pathogens and insects. Drip irrigation requires an emulsifier. For shank fumigation, use higher rates or impermeable films to improve weed and nematode control. For drip fumigation the use of TIF will improve both nematode and weed control. Tri-Clor: One gallon of product weighs 13.88 lb; Tri-Clor EC: One gallon of product weighs 13.46 lb.|
|Then, 5-7 days after fumigation apply one of the following|
|(Vapam HL, Sectagon 42)||37.5–75 gal||See label||0|
|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)||30–62 gal||See label||0|
|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.|
|**||Rates are per treated acre; for bed applications, the rate per acre may be lower.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|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.|
|§||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. 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.|