Development of a Weather-Based Advisory Program for Scheduling Fungicide Applications for Control of White Rust of Spinach.

Weather-based advisory programs were developed and evaluated for timing of fungicide applications to control white rust of spinach (Spinacia oleracea) in three field trials using a susceptible cultivar. The advisory programs were based on previous studies that defined periods of temperature (T) and wetness (relative humidity ≥90%, W) that favored disease development. The protectant fungicides mancozeb or maneb (ethylene bisdithiocarbamates; EBDCs) and the systemic fungicide azoxystrobin were applied after 3, 6, 12, 24, and 36 cumulative hours of wetness weighted by temperature (T*W). T*W programs were compared with an advisory program based on 12 h of continuous wetness (12-h W), a 7-day calendar program, and a nonsprayed control treatment. Except for the 3-h T*W program, the number of fungicide applications per trial was reduced for all advisory programs compared with the 7-day program. Averaged over the three trials, applications were reduced from 2.7 per trial for the 6-h T*W program to 3.7 per trial for the 36-h T*W and the 12-h W programs. For the EBDC fungicides, all advisory programs except the 36-h T*W and 12-h W programs reduced incidence and severity of white rust compared with the nonsprayed control. Disease incidence and severity for the 3-h and 6-h T*W programs did not differ from the 7-day program. Compared with the EBDC fungicides, azoxystrobin resulted in reduced disease incidence for each of the spray programs and reduced disease severity for all spray programs except the 7-day program. For azoxystrobin, all advisory programs reduced disease incidence and severity compared with the nonsprayed control. Incidence and severity of white rust did not differ from the 7-day program for the 3-h, 6-h, and 12-h T*W programs. Based on reductions in the number of fungicide applications and the level of disease control, the 6-h and 12-h T*W programs were most efficient advisory programs for the EBDC fungicides and azoxystrobin, respectively.