Activity of boscalid, fenhexamid, fluazinam, fludioxonil, and vinclozolin on growth of Sclerotinia minor and S. sclerotiorum and development of lettuce drop

Sclerotinia drop is a major disease of lettuce caused by two soilborne fungi, Sclerotinia minor and S. sclerotiorum. Fungicides such as dicloran (Botran), iprodione (Rovral), and vinclozolin (Ronilan) are currently available in the United States to manage this disease. Studies were conducted to investigate the relative effect of some new fungicides, including boscalid, fenhexamid, fluazinam, and fludioxonil, in comparison with vinclozolin, on growth of S. minor and S. sclerotiorum in agar plate tests as well as control of lettuce drop in the field. At a rate of 0.001 mug/ml, all tested compounds only suppressed mycelial growth of either pathogen from 0 to 20%. At 0.01 mug/ml, mycelial growth of S. minor was reduced 82 to 84% by fludioxonil and fluazinam and only 1 to 16% by boscalid, fenhexamid, and vinclozolin. At the same rate, mycelial growth of S. sclerotiorum was reduced 78% by fluazinam and from 0 to 12% by boscalid, fludioxonil, fenhexamid, and vinclozolin. At 0.1 mug/ml, all tested chemistries except vinclozolin inhibited mycelial growth of S. minor from 70 to 98%, whereas growth of S. sclerotiorum was suppressed 95 to 99% by fludioxonil and fluazinam, significantly less (40 to 47%) by boscalid and fenhexamid, and not at all by vinclozolin. At a rate of 1.0 mug/ml, all tested fungicides reduced mycelial growth of S. minor and S. sclerotiorum from 87 to 100% and 77 to 100%, respectively. Mycelial growth emerging from sclerotia of S. minor was reduced from 98 to 100% by all fungicides tested at a rate of 1.0 mug/ml, whereas growth from sclerotia of S. sclerotiorum was suppressed from 90 to 96% by fenhexamid, fludioxonil, fluazinam, and vinclozolin. In lettuce plots infested with S. minor, boscalid and fluazinam provided the highest level of disease control, significantly greater than that achieved with fenhexamid, fludioxonil, and vinclozolin. In the presence of S. sclerotiorum, the highest degree of disease suppression occurred with application of fluazinam, fludioxonil, and vinclozolin, whereas the least effective compound was fenhexamid. Boscalid and fluazinam were more effective against lettuce drop caused by S. minor than disease caused by S. sclerotiorum.