LAUSR

Heterodera glycines, the soybean cyst nematode (SCN) and fungal pathogen Macrophomina phaseolina, are economically important soybean pathogens that may co-infest fields. Resistance remains the most effective management tactic for SCN and the rhg1-b resistance allele derived from plant introduction (PI) 88788, is most commonly deployed in the northern U.S. The concomitant effects of SCN and M. phaseolina on soybean performance, as well as the effect of the rhg1-b allele in two different genetic backgrounds, were evaluated in 3 environments (during 2013-2015) and a greenhouse bioassay. Within two soybean populations, half of the lines had the rhg1-b allele and the other half the susceptible allele in the backgrounds of the cultivars IA3023 (A) and LD00-3309 (B). Significant interaction between soybean rhg1-b allele and M. phaseolina-infested plots was observed in 2014. In all experiments, initial SCN populations (Pi) and M. phaseolina in roots were associated with reduced soybean yield. SCN reproduction factor (RF = final population/Pi) was affected by SCN Pi, rhg1-b, and genetic background. A background-by-genotype interaction on yield was observed only in 2015 with a stronger rhg1-b effect in the LD00-3309 background, which suggested that the susceptible parent IA3023 is tolerant to SCN. SCN female index (FI) from greenhouse experiments was compared with field RF and Lin's concordance and Pearson's correlation coefficients decreased with increasing field SCN Pi in soil. In this study both SCN and M. phaseolina reduced soybean yield asymptomatically, and the impact of SCN rhg1-b resistance was dependent on SCN virulence but also population density.