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Vegetable IPM Updates Archive
Cooperative Extension
Soil-borne Lettuce Pathogens (Sep. 5, 2012)

As another lettuce growing season begins in Arizona, so does the threat of diseases caused by soil-borne fungal pathogens. Soil-borne diseases of concern for lettuce now being planted include Fusarium wilt, Sclerotinia drop, and bottom rot. Symptoms of Fusarium wilt, caused by Fusarium oxysporum f. sp. lactucae, can appear on lettuce anytime after thinning. Effective fungicides generally are not available to manage Fusarium wilt, although fumigation with Vapam did provide some disease suppression in an earlier field trial. This disease is best avoided by not planting susceptible types of lettuce (virtually all head lettuce varieties) in ground known to contain the pathogen, especially during September or October. Soil temperatures during these months favor the growth of Fusarium oxysporum and the resulting development of the wilt disease. Romaine lettuce cultivars generally are more tolerant to the Fusarium wilt pathogen and can be planted in fields harboring the pathogen in cooler months when the pathogen is less active. Sclerotinia drop and bottom rot usually do not become apparent in fields until plants are at or past the rosette stage of development. Successful management of Sclerotinia drop, caused almost exclusively by Sclerotinia minor on lettuce that will be harvested in November and December, as well as bottom rot caused by Rhizoctonia solani, require application of effective fungicides well before the appearance of disease symptoms. Sclerotinia minor and Rhizoctonia solani exist in soil as resistant structures called sclerotia. Successful management of diseases caused by these pathogens is closely tied to preventing the germination of these sclerotia; therefore, applications of fungicides are made to the soil, where the sclerotia exist. Early application of fungicides, when plants are very small, facilitates thorough coverage of the bed surface. As plants grow and cover more of the bed surface, fungicide coverage of soil and disease control decline.

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