Optimizing Fungicide Inputs for Management of Lettuce Drop Caused by Sclerotinia minor and S. sclerotiorum

Lettuce drop, caused by the soilborne fungi Sclerotinia minor and S. sclerotiorum, continues to be an important disease on this crop in Arizona. Trials were conducted over a 5-year period to compare different fungicides as well as the number, timing, and method of application. Compared with nontreated plots, disease reduction ranging from 49.6 to 61.0% was achieved on lettuce beds containing S. minor and treated with fluopyram + trifloxystrobin, fluazinam, fluxapyroxad + pyraclostrobin, and boscalid. Treatment of beds containing S. sclerotiorum with Coniothyrium minitans, iprodione, and boscalid reduced lettuce drop from 50.6 to 71.5%. No difference in disease control was noted between one and two applications of boscalid in plots containing either pathogen. In the presence of S. minor, beginning the first of two applications of boscalid after seeding did not differ from starting after thinning; however, in plots containing S. sclerotiorum, starting application after seeding was superior to beginning after thinning. Physical incorporation of soil treated with boscalid to a depth of 5.0 cm did not differ from soil treatment without incorporation in plots containing S. minor or S. sclerotiorum. On the other hand, in beds containing S. sclerotiorum, incorporation of soil treated with iprodione improved disease control compared with no incorporation.