Cold-Stressed Soybean Sensitivity to Charcoal Rot

Charcoal rot, caused by <i>Macrophomina phaseolina</i>, is an increasingly important constraint in soybean, particularly under hot and dry conditions. While heat and drought are known to favor disease development, short early-season cold spells-common in temperate regions-may predispose soybean to subsequent infection, yet this interaction remains poorly quantified. It was evaluated whether transient chilling increases charcoal rot severity and whether cultivar-specific differences modulate this predisposition. Nine commercial cultivars spanning MG 00, 0, and 0-I were grown in a controlled walk-in chamber under either optimal conditions (control) or a three-day cold spell initiated at the first fully expanded trifoliate (20-23 days after sowing, DAS). Standardized cut-stem inoculation was performed at 26 DAS, and stem lesion length was recorded every 3-4 days across five assessments at 3, 7, 10, 14, and 21 DPI. Two-way ANOVA (treatment, genotype, treatment × genotype) with Tukey's HSD tested effects. Cold stress significantly increased lesion lengths at all assessments, with the strongest divergence at the earliest measurement. Genotype and treatment × genotype were also significant, revealing differential responses among cultivars; notably, one line (G9) showed consistently small treatment-induced increases. These results indicate that brief early-season cold exposure can predispose soybean to more severe charcoal rot, with the magnitude dependent on genotype and timing. Incorporating cold-stress predisposition into screening and breeding should enhance resilience under increasing climate variability.