Survey of <i>Magnaporthe oryzae</i> Avirulence Genes in Arkansas Commercial Rice Fields

Rice blast, caused by the ascomycete Magnaporthe oryzae, remains a major threat to rice production. Although major resistance (R) genes have been identified and deployed, their effectiveness depends on the presence of matching avirulence (AVR) genes in the pathogen. In this study, we surveyed Arkansas commercial rice fields to assess the frequency and distribution of eight AVR genes: AVR-Pita1, AVR-Pik, ACE1, AVR-Pi9, AVR-Pib, AVR-Pii, AVR-Pizt, and AVR-Pia. A total of 227 M. oryzae isolates were collected during the 2024 and 2025 growing seasons from 15 counties and five cultivars. Eight AVR genes were tested using PCR with gene-specific primers. AVR-Pita1 and AVR-Pizt were present in all isolates, while AVR-Pib, AVR-Pi9, and ACE1 were detected in 226 isolates, and AVR-Pii was detected in 225 isolates. AVR-Pik and AVR-Pia were absent. Three distinct AVR profile groups were identified: A (AVR-Pita1 and AVR-Pizt), B (AVR-Pita1, AVR-Pizt, AVR-Pib, AVR-Pi9, and ACE1), and C (AVR-Pita1, AVR-Pizt, AVR-Pib, AVR-Pii, AVR-Pi9, and ACE1), with profile C representing the predominant combination of AVR genes in over 99% of the isolates. No significant variation in the AVR profiles was associated with the host genotype. These results reveal a highly uniform AVR gene composition in the Arkansas M. oryzae population and provide insights into the potential effectiveness of deployed R genes, supporting breeding efforts focused on durable resistance and economically sustainable blast management in the state.