Copper phosphite enhances efficacy of a strobilurin-triazole fungicide in controlling late season foliar diseases of soybean

Septoria brown spot (Septoria glycines) and Cercospora leaf blight (Cercospora kikuchii) are late season foliar diseases (LSFD) that co-occur every year in the soybean growing regions of Argentina. Repeated use of commercial, formulated mixes of strobilurin-triazole fungicides to control LSFD has prompted the need for tactics to increase the efficacy of these fungicide groups, thereby reduce number of applications and help prolong their life use. The objective of this study was to determine the effect of copper phosphite (CuPhi), tank-mixed with a strobilurin-triazole fungicide, in the fungicide efficacy in controlling LSFD and protecting soybean yield. Field experiments were conducted during the 2014/2015 and 2016/2017 growing seasons in six different locations in the Pampas region of Argentina. At each location, treatments consisted of: i) one foliar application of a fungicide formulated as a mix of picoxystrobin and cyproconazole (60 and 24 g a.i. ha−1, respectively), ii) one foliar application of the fungicide tank mixed with a CuPhi formulation, and iii) an untreated control. Treatment application timing in each trial was defined by an LSFD scoring system previously developed by the authors. All treatments were disposed to the experimental units in a randomized complete block design with four replications. Severity of LSFD was estimated visually as the percentage of diseased leaf area in 50 plants randomly selected in each plot at R6-R7. Grain yield (kg ha−1) was determined at physiological maturity in all trials. Severity and yield analysis using a linear mixed-effect model indicated that addition of CuPhi to the fungicide tank mix significantly reduced LSFD severity and protected yield (P < 0.05). The observed effect on disease control, however, was influenced by location in the 2014/2015 growing season, but not in the 2016/2017 growing season. The net economic return with the addition of CuPhi to the fungicide tank mix was 66.1 and 85.5 USD ha−1 higher than the fungicide formulation alone, in the 2014/15 and 2016/17 crop seasons respectively. This study shows that the addition of CuPhi to a strobilurin-triazole fungicide tank mix had a synergistic and/or additive effect in controlling LSFD in soybean and protecting grain yield. Enhanced efficacy of these fungicide groups with the addition of CuPhi could reduce the number of applications, and in consequence, help prolong life use of these fungicide modes of action.