Interaction of Soil Type and Elevated Temperature Drives the Emergence of Macrophomina phaseolina in Soybean Fields

Charcoal rot (Macrophomina phaseolina) is an emerging threat to Korean soybean production under climate warming. This study examined how soil type and temperature influence the composition and dominance of soilborne pathogens. Blighted soybeans were collected from upland and paddy fields nationwide (2020-2022) and isolated at 25°C and 35°C. Isolation frequency was significantly affected by soil type, temperature, and their interaction. M. phaseolina predominated in upland soils but was rarely recovered from paddy fields. In contrast, Fusarium spp. associated with soybean blight, Diaporthe longicolla causing pod and stem blight, and Calonectria ilicicola causing red crown rot were frequently isolated but showed reduced recovery or pathogenicity at 35°C. Growth and pathogenicity assays revealed a temperature-driven shift in dominance: various pathogens were virulent at 25°C, while M. phaseolina showed enhanced growth and severity at 35°C. Greenhouse experiments demonstrated that moisture stress (reduced irrigation) significantly increased charcoal rot severity, strengthening the competitive advantage of M. phaseolina. These findings suggest that soil type and temperature interact to restructure pathogen communities. M. phaseolina is emerging primarily in upland systems where high temperature coincides with moisture stress, necessitating targeted management under climate change. While culture-based recovery reflects a pathogen's potential under specific thermal conditions, it may not directly represent actual field density.