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Another lettuce growing season is starting in southwestern Arizona, so it is a good time to review the kinds of diseases that can occur on this crop at this time of year. Diseases of most concern in the autumn include Fusarium wilt, Sclerotinia drop, and bottom rot, all caused by soil-borne fungal pathogens. Symptoms of Fusarium wilt, caused byFusarium oxysporum f. sp. lactucae, can appear on lettuce anytime, sometimes as early as after thinning. Effective fungicides generally are not available to manage this disease. Susceptible types of lettuce (most head lettuce varieties, for example) should not be planted in ground known to contain the pathogen, especially during September or October, as soil temperatures during these months favor the growth of Fusarium oxysporum and the resulting development of Fusarium wilt. Variety trials from 2015 to 2018 have shown that the romaine varieties Del Sol, Valley Heart, and Duquesne sustained minimal losses (1-2%) due to Fusarium wilt, compared to 60-100% plant loss on susceptible head lettuce varieties. Also, in the same trials, losses due to Fusarium wilt on head lettuce varieties Meridian and Oracle were 10-11%, again much lower than disease losses on most other head lettuce varieties. 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 mostly 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 germination of sclerotia and subsequent infection of plants; therefore, applications of fungicides are made to the soil where sclerotia are found. Early fungicide application, 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 is reduced and disease management is less effective.
To contact Mike Matheron go to: matheron@ag.arizona.edu.
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For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
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