Genetic diversity and evolution of nine avirulence genes in South Korean Phytophthora sojae populations.

INTRODUCTION: Phytophthora sojae, the causal agent of soybean root and stem rot, is a highly destructive pathogen affecting soybean production worldwide. Rapid evolution of avirulence (Avr) genes poses a major challenge to the development of durable host resistance. However, comprehensive information on the diversity, sequence variation, and evolution of multiple Avr genes in South Korean P. sojae populations remains limited. This study aimed to characterize the pathogenicity, pathotypes, and genetic diversity of nine Avr genes in P. sojae populations from the Republic of Korea. METHODS: Fifty P. sojae isolates were collected from soybean-growing regions in the Republic of Korea between 2020 and 2025. Pathogenicity was evaluated in the soybean cultivar 'Daewon', and pathotypes were determined by amplification patterns of nine race-specific Avr genes (Avr1a, Avr1b, Avr1c, Avr1d, Avr1k, Avr3a, Avr3b, Avr3c, Avr4/6). Sequence polymorphism, nucleotide and amino acid variation, and haplotype diversity were analyzed to investigate the evolutionary patterns of these Avr genes. RESULTS: Most isolates were highly virulent on the soybean cultivar 'Daewon', with 70% causing severe disease symptoms. Five distinct pathotypes were identified, with pathotype 3, characterized by the absence of Avr3a, representing the predominant group. Gene deletions occurred most frequently in Avr3a and Avr1a. Sequence analyses revealed extensive nucleotide and amino acid polymorphisms among Avr loci, particularly within the C-terminal regions of effector proteins, whereas Avr4/6 remained highly conserved across all isolates. Haplotype analysis demonstrated substantial allelic diversity, with Avr1a exhibiting the highest haplotype diversity and the greatest number of haplotypes. The coexistence of multiple pathotypes and haplotypes within the same geographic regions indicated considerable genetic diversity within local P. sojae populations. DISCUSSION: This study provides the first comprehensive characterization of the diversity and evolution of nine Avr genes in South Korean P. sojae populations. The results demonstrate that both gene loss and sequence diversification contribute to Avr gene evolution and pathogen adaptation. These findings improve our understanding of the evolutionary dynamics of P. sojae populations and highlight the importance of continuous pathogen surveillance to support durable resistance breeding and effective management of soybean root and stem rot.