Evaluation of the Combined Effect of <i>Heterodera glycines</i> and <i>Macrophomina phaseolina</i> on Soybean Yield in Naturally Infested Fields with Spatial Regression Analysis and in Greenhouse Studies

Abstract

<i>Heterodera glycines</i>, the soybean cyst nematode, and <i>Macrophomina phaseolina</i>, causal agent of charcoal rot, are economically important soybean pathogens. The impact and effect of these pathogens on soybean yield in coinfested fields in the Midwest production region is not known. Both pathogens are soilborne, with spatially aggregated distribution and effects. Spatial regression analysis, therefore, is an appropriate method to account for the spatial dependency in either the dependent variable or regression error term from data produced in fields naturally infested with <i>H. glycines</i> and <i>M. phaseolina</i>. The objectives of this study were twofold: to evaluate the combined effect of <i>H. glycines</i> and <i>M. phaseolina</i> on soybean yield in naturally infested commercial fields with ordinary least squares and spatial regression models; and to evaluate, under environmentally controlled conditions, the combined effect of <i>H. glycines</i> and <i>M. phaseolina</i> through nematode reproduction and plant tissue fungal colonization. Six trials were conducted in fields naturally infested with <i>H. glycines</i> and <i>M. phaseolina</i> in Ohio. Systematic-grid sampling was used to determine the population densities of <i>H. glycines</i> and <i>M. phaseolina</i>, and soybean yield estimates. Though not used in any statistical analysis, <i>M. phaseolina</i> colony forming units from plant tissue, charcoal rot severity, and <i>H. glycines</i> type were also recorded and summarized. In two greenhouse experiments, treatments consisted of <i>H. glycines</i> alone, <i>M. phaseolina</i> alone, and coinfestation of soybean with both pathogens. Moran's <i>I</i> test indicated that the yield from five fields was spatially correlated (<i>P</i> < 0.05) and aggregated. In these fields, to account for spatial dependence, spatial regression models were fitted to the data. Spatial regression analyses revealed a significant interaction effect between <i>H. glycines</i> and <i>M. phaseolina</i> on soybean yield for fields with high initial population densities of both pathogens. In the greenhouse experiments, <i>H. glycines</i> reproduction was significantly (<i>P</i> < 0.05) reduced in the presence of <i>M. phaseolina</i>; however, soybean tissue fungal colonization was not affected by the presence of <i>H. glycines</i>. The direct mechanisms by which <i>H. glycines</i> and <i>M. phaseolina</i> interact were not demonstrated in this study. Future studies must be conducted in the field and greenhouse to better understand this interaction effect.