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Vegetable IPM Updates Archive
Cooperative Extension
Managing Sclerotinia Drop of Lettuce with Fungicides (September 30, 2015)
Successful management of any plant disease is achieved by targeting one or more vulnerable stages in the disease development cycle. For Sclerotinia drop of lettuce, 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 soil along with crop debris as the land is prepared for planting 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 their germination and growth. Over the past several years of fungicide evaluation trials, the traditional fungicide application to the lettuce bed surface beginning 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 6-year comparison of fungicide efficacy in plots containing Sclerotinia minor, the average reduction of disease was 61, 53, 44, 43, 43, and 38%, respectively, for plots treated with fluazinam (Omega), boscalid (Endura), iprodione (Rovral), penthiopyrad (Fontelis), Coniothyrium minitans (Contans), and fludioxonil (Cannonball). In soil infested with Sclerotinia sclerotiorum, the mean reduction of disease in the same six trials was 73, 56, 49, 47, 44, and 23%, respectively, for plots treated with Contans, Omega, Endura, Cannonball, Rovral, and Fontelis. Application of fungicides to the bed surface prevents germination of sclerotia at or near the soil surface, thus reducing the incidence of Sclerotinia drop. Ongoing research is focused on examining new active ingredients and methods of application to soil with the goal of consistently increasing the degree of Sclerotinia drop control above levels now achieved.

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