Intraspecific Diversity and Virulence of Phytophthora sojae Races Infecting Soybean Under Waterlogged Soil Conditions

Abstract

Phytophthora root and stem rot caused by Phytophthora sojae is a destructive soilborne disease of soybean, particularly severe under waterlogged and poorly drained soil conditions. Field and greenhouse studies were conducted to assess intraspecific diversity, race structure, and virulence variability of P. sojae isolates collected from major soybean-growing regions. Diseased plant samples showing root decay, stem lesions, damping-off, and plant collapse were used for pathogen isolation and characterization. Morphological observations revealed variation in colony growth patterns, sporangial production, and oospore formation among isolates. Pathogenicity assays conducted on differential soybean cultivars carrying known Rps resistance genes demonstrated significant variation in virulence, indicating the presence of multiple physiological races within the pathogen population. Certain isolates exhibited broad virulence spectra capable of overcoming multiple resistance genes, while others showed restricted pathogenicity profiles. Waterlogged soil conditions significantly enhanced zoospore release, motility, and infection efficiency, leading to increased disease severity under experimental and field conditions. Molecular characterization using conserved gene regions supported genetic diversity among isolates and confirmed their placement within P. sojae lineages. Phylogenetic analysis revealed clustering patterns corresponding to virulence phenotypes, suggesting a relationship between genetic variation and pathogenic aggressiveness. Continuous soybean cultivation under poor drainage conditions favored selection of highly virulent and adaptive pathogen populations. The observed intraspecific diversity highlights the evolutionary potential of P. sojae and the risk of resistance breakdown in widely deployed soybean cultivars. Accurate identification of virulence profiles is essential for effective resistance breeding and strategic deployment of Rps genes in soybean production systems.