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Abstract
Rhizopus seed and seedling rot is a highly destructive peanut disease, causing rapid seed decay in 36-96 hours and rendering seeds and pre-emerged seedlings indistinguishable from the soil. The seed vigor of thirteen peanut genotypes, molecular identification, pathogenicity, in vitro fungicide sensitivity, and temperature response of Rhizopus spp. isolated from peanut seeds in Georgia were assessed in this study. Molecular analysis identified three Rhizopus spp. (R. delemar, R. arrhizus, and R. stolonifer with 26, 16, and 3 isolates, respectively) based on the TEF-1α gene. A novel pathogenicity assay for Rhizopus spp. on peanut seeds revealed that all tested peanut genotypes were highly susceptible to Rhizopus spp. Mycelial growth assays at temperatures of 15-35°C revealed variability in the thermotolerance across the three species. Rhizopus delemar and R. arrhizus exhibited rapid mycelial growth and high virulence on peanut seeds at all tested temperatures. The highest germination found at all temperatures was only 12% in inoculated seeds. Rhizopus stolonifer exhibited significantly slower mycelial growth rate and no growth at 30°C and 35°C, respectively. Due to the reduced growth, some inoculated seeds managed to germinate (40.2% at 30°C and 72.2% at 35°C). Seed treatment fungicides fludioxonil, carboxin, and pydiflumetofen consistently provided the lowest mean EC₅₀ (µg/mL) values across species (< 0.05 µg/mL), while azoxystrobin, mefenoxam, and ipconazole did not inhibit growth at the highest tested concentration (>10 µg/mL). Overall, these findings enhance the understanding of Rhizopus pathogenicity, thermotolerance, and fungicide sensitivity, offering valuable insights for managing this destructive peanut disease.