Lettuce Pathogens in Soil (September 17, 2014)

Since another lettuce growing season has begun in Arizona, it is an appropriate time to think about lettuce diseases that can occur at this time. The diseases of most concern during the early autumn are caused by soil-borne fungal pathogens and would include Fusarium wilt, Sclerotinia drop, and bottom rot. Symptoms of Fusarium wilt, caused by Fusarium oxysporum f. sp. lactucae, can appear on lettuce any time after thinning. Effective fungicides generally are not available to manage Fusarium wilt, although fumigation with Vapam has suppressed disease severity in some field trials. This disease can be effectively dealt with by not planting susceptible types of lettuce (virtually all head lettuce varieties, for example) 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 small resistant structures called sclerotia. Successful management of the 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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To contact Mike Matheron go to: matheron@ag.arizona.edu.