Verticillium Wilt

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.

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.

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.