Charcoal Rot Severity and Soybean Yield Responses to Seeding Rate and Row Spacing

Soybean (Glycine max [L.] Merr.) production is influenced by agronomic practices including seeding rate (SdRt) and row spacing (RwSp). Still, the effect of these two agronomic management practices on yield and disease severity in fields infested with Macrophomina phaseolina (Tassi) Goid, the causal agent for charcoal rot, is unknown. The objective of this study was to evaluate the effect of RwSp and SdRt on charcoal rot severity and yield performance of soybeans and to determine whether these management practices interact to influence disease severity and soybean yield. A 3-year study was conducted in M. phaseolina-infested fields to assess the effects of SdRts of 198,000, 346,000, and 494,000 ha^-1 and RwSps of 19, 38, and 76 cm on disease severity and yield performance using two genotypes: Asgrow 38X8 (susceptible) and Local Seed 4565XS (moderately resistant). There were some years and treatment combinations for which the lower RwSp (19 cm) had lower severity than the higher RwSp (76 cm). The effects of RwSp on yield were generally weak and variable; however, in the years and treatment combinations where an effect was evident, the lower RwSp consistently resulted in higher yield. The effects of SdRt on yield were also variable and mostly weak, but there were a few years and treatment combinations where evidence suggested that the highest SdRt (494,000 ha^-1) had higher yields than those of the lower SdRt treatments. Data on the effects of RwSp across the 3 years showed that disease severity for the moderately resistant genotype was consistently lower than the susceptible genotype and decreased with an increase in SdRt. It also showed a corresponding yield increase at high SdRt and low RwSp. No detectable differences in yield occurred for the susceptible cultivar, and yield remained significantly lower than the moderately resistant genotype. This suggests that growers will benefit more by using moderately resistant genotypes, using high SdRt, and using lower RwSp.