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 (TW). TW 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 TW 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 TW program to 3.7 per trial for the 36-h TW and the 12-h W programs. For the EBDC fungicides, all advisory programs except the 36-h TW 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 TW 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 TW 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.