Another lettuce growing season has begun in Arizona, so it is appropriate to review the kinds of lettuce diseases that can occur this time of year. Diseases of most concern during early autumn, such as Fusarium wilt, Sclerotinia drop, and bottom rot, are caused by soil-borne fungal pathogens. Symptoms of Fusarium wilt, caused by Fusarium oxysporum f. sp. lactucae, can appear on lettuce anytime, sometimes as early as during thinning. Effective fungicides generally are not available to manage this disease, although fumigation with Vapam did suppress Fusarium wilt severity in an earlier field trial. Susceptible types of lettuce (virtually all 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. In 2016 field trials, the romaine cultivars Del Sol, King Henry, Valley Heart and Duquesne were minimally affected by Fusarium wilt and could be considered for fields containing Fusarium oxysporum f. sp. lactucae. 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 carry over in soil as small resistant structures called sclerotia. Successful management of the diseases caused by these two 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.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.