Optimizing fungicide timing for the management of white mold in processing snap bean in New York

White mold (Sclerotinia sclerotiorum) is the most important fungal disease affecting processing snap bean production in New York (USA). The most important tactic for in-season disease management is fungicide application with the aim of protecting flowers from infection by S. sclerotiorum ascospores. The most popular fungicide is thiophanate-methyl but for rotational purposes, boscalid and fluazinam are common although more expensive alternatives. Despite the application of fungicides, white mold often occurs. In the absence of knowledge on optimal fungicide timing, up to two applications may be made with the first occurring at 10% of plants having at least one open flower, and the second approximately seven days thereafter coinciding with full (100%) flowering and pin-pod. Six small plot, replicated field trials were conducted over three years (2015-2017) to identify optimal timings for the most commonly used fungicides (thiophanate-methyl, boscalid, and fluazinam) to reduce the incidence of white mold in processing snap bean cvs. Huntington and Denver. Fungicides were applied at either early (10%) and/or late (100%) flowering and the effect on white mold incidence in pods and plants, green leaf area measured using canopy reflectance at 830 nm, and pod yield was evaluated. Application of thiophanate-methyl at early flowering was optimal for reducing the incidence of white mold and a second application at late flowering was not beneficial. In contrast, if thiophanate-methyl application was delayed until late flowering, white mold incidence was high and in some cases not significantly different from nontreated plots. Timing of fluazinam or boscalid was more flexible and not significantly different between a single application at early or late flowering. Two applications of fluazinam or boscalid did not significantly reduce white mold compared to a single application. Delayed application of thiophanate-methyl to later in flowering may be a contributing factor in the suboptimal management of white mold in processing snap bean. Optimal timing of fluazinam or boscalid appears less critical and offers growers flexibility when conditions are not optimal for application at early flowering. In addition, these products allow for rotation to different modes of action, but with trade-offs in cost.