Successful management of any plant disease is achieved by focusing efforts on one or more vulnerable stages in the disease development cycle of the pathogen. For lettuce drop caused by the fungi Sclerotinia minor and S. sclerotiorum, this point of attack centers on the fungal bodies called sclerotia. At crop maturity, sclerotia produced on infected plants will be incorporated into the soil along with crop debris as the land is prepared for planting of the next crop. For the Sclerotinia fungi, sclerotia serve the same purpose that seeds do for plants; that is, they allow the organism to carry over in soil in a dormant state until conditions become favorable for germination and growth. Over the past several years of research trials, the traditional application of fungicides to the lettuce bed surface after thinning has provided at best about a 50 to 60% reduction in dead plants, compared to plots not receiving a fungicide treatment. In a 4-year comparison of fungicide efficacy, when soil contained Sclerotinia minor, the products Contans, Endura, Rovral, and Switch provided the same statistical level of disease control. When soil contained Sclerotinia sclerotiorum, the product providing the highest statistical level of disease control was Contans. Statistically lower but equivalent disease reduction was provided by Endura, Rovral, and Switch. Application of fungicides to the bed surface prevents germination of sclerotia at or near the soil surface, but has little effect on sclerotia deeper in the soil profile. Ongoing research is focused on examining new chemistries and methods of application to soil with the goal of consistently increasing the level of Sclerotinia drop control above the 50 to 60% now achieved.

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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.