Lettuce Drop

Two species of Sclerotinia infect lettuce in California and cause the lettuce drop disease: in coastal growing areas Sclerotinia minor is the primary species of importance, while in other areas S. sclerotiorum is more prevalent.

Sclerotinia minor only infects the stems and leaves in contact with the soil. Once infection takes place, the fungus will cause a brown, soft decay that eventually destroys the plant crown tissue. Older leaves then wilt and later the entire plant will wilt and collapse, making it unharvestable. Plant collapse usually occurs when lettuce is near maturity. Profuse amounts of white mycelia and small [up to 0.125 inch (3 mm)], black, hard, resting bodies (sclerotia) form on the outside of the decayed crown.

Sclerotinia sclerotiorum can also infect lower leaves and stems, causing symptoms similar to those of S. minor. In addition, S. sclerotiorum has an aerial spore that can infect any of the upper leaves. Spores usually infect damaged or senescent tissue when the weather is cool and moist. Infection results in a watery, soft rot that is accompanied by white mycelial growth and formation of sclerotia. Sclerotinia sclerotiorum forms sclerotia that are larger (0.25–0.50 inch) than those of S. minor.

Sclerotia of both species enable the pathogens to survive in the soil for 2 to 3 years without susceptible hosts. Wet soil conditions favor disease development of both species. For S. sclerotiorum, cool and moist conditions are necessary for development of the fruiting structure (apothecium) that produces the airborne spores. In California, S. minor does not have a spore-producing stage. Symptoms caused by Sclerotinia species could resemble Verticillium wilt, Botrytis crown rot, and Phoma basal rot symptoms.

The use of wider, 80-inch beds for lettuce production may cause lettuce drop from S. minor to be more severe because of increased bed moisture. In addition, the use of wider beds may be allowing S. sclerotiorum to increase in importance in the Salinas and other coastal valleys.

For S. minor, crop rotations and deep inversion plowing may be helpful in reducing incidence of lettuce drop.

• Avoid known crop hosts including beans, cauliflower, celery, corn salad, endive, escarole, fennel, pepper, radicchio, and tomato. Cover crop hosts to avoid include Austrian winter pea, mustards, phacelia, and vetch.
• Avoid overly wet soils and keep the bed surface as dry as possible with careful irrigation.
• Level land to provide even distribution of water and assure good drainage with beds as high as possible.
• Collect and remove infected plants. Though costly and time consuming, it can effectively reduce inoculum in the field.

Cultural practices are not as helpful for controlling lettuce drop caused by S. sclerotiorum because the primary inoculum is windborne spores. This species also has an extremely broad host range.

Use cultural controls in an organically certified crop.

In coastal fields with a history of lettuce drop, protectant fungicides applied after thinning the crop can reduce lettuce drop caused by S. minor. Fungicides must be directed to the base of the young plants and be applied before plants become too large. Such post-thinning applications are not effective against S. sclerotiorum. For this species, apply fungicides beginning with the rosette stage when conditions are favorable for the development of the disease. A number of applications may be needed for satisfactory control.