Lettuce

by Sherrie Smith and Jason Pavel

Lettuce Drop – Sclerotinia minor

Photo by Sherrie Smith, University of Arkansas Cooperative Extension

Sclerotinia minor and Sclerotinia sclerotiorum both cause a disease in lettuce known as Lettuce Drop. Sclerotinia minor infects lettuce leaves and stems in contact with the soil. The fungus causes a soft, brown, watery rot of the lower leaves and crown. First, the outer leaves wilt, and then the entire plant collapses. White fungal mycelium and black irregularly shaped (3-5mm) sclerotia form on the decaying crown.

Lettuce Drop caused by Sclerotinia sclerotiorum is considered one of the most economically important diseases of lettuce. Seventy-five percent or more of the crop may be lost when the disease is severe. This fungal pathogen produces the same symptoms as Sclerotinia minor but also makes aerial spores that can infect the upper portions of the plant. The sclerotia are generally larger (5-10mm) than those produced by Sclerotinia minor. Additionally, Sclerotinia sclerotiorum produces fruiting bodies called apothecia that resemble tiny clusters of mushrooms.

Rotating to non-host crops, deep plowing to bury sclerotia, and applying fungicides after thinning help control Lettuce drop caused by Sclerotinia minor. The same controls apply for Lettuce Drop caused by Sclerotinia sclerotiorum, but fungicides should be started at the rosette stage. Wilted plants will not recover. Fungicides at early plant growth can be effective. Rovral®, Cannonball®, Endura®, and Botran®, are labeled.

Lettuce Drop Sclerotia – Sclerotinia minor

Photo by Keiddy Urrea, formerly University of Arkansas Cooperative Extension

Avoid known crop hosts including beans, cauliflower, endive, escarole, fennel, pepper, radicchio, and tomato. Avoid cover crop hosts such as winter pea, mustards, phacelia, and vetch. Avoid overly wet soils and water only when dry. Plant on raised beds. Remove infected plants.

Take Aways

•  Fungicides at early plant growth
• Avoid known crop hosts
• Avoid overly wet soils
• Collect and remove infected plants

This work is supported by the Crop Protection and Pest Management Program [grant no. 2017-70006- 27279/project accession no. 1013890] from the USDA National Institute of Food and Agriculture.