Effect of target spot on soybean yield and factors affecting this relationship

The lack of robust estimates of soybean yield losses due to target spot led to this study. The objective was to determine whether soybean yield at stage R8 (W, expressed as kg ha⁻¹) was related to target spot severity at soybean stage R5–R6 (S, expressed as %) and to identify variables that could affect this relationship. Plot‐level estimates of mean disease severity and yield from 41 selected Uniform Fungicide Trials carried out in Brazil during 2012–2016 growing seasons were used to estimate linear regression coefficients for the relationship between yield and target spot severity through random‐coefficient mixed effects model analysis. The overall estimated mean regression intercept and slope were β0^ = 3564 kg ha⁻¹ (disease‐free yield) and β1^ = −17.1 kg ha⁻¹ %⁻¹ (W decrease per percent increase in S), respectively. The model was then refitted with different covariates to determine their effects on model parameters. β₀ was influenced by baseline yield (less than or greater than 3300 kg ha⁻¹) and β₁ was affected by yield response to fungicide treatments. Estimated yield loss at 50% target spot severity ranged from 8% to 42%. Cultivar also had a significant effect on the magnitude of yield reduction due to target spot, which ranged from 11% to 42%, depending on the cultivar.