|WANG, XUEYAN - University Of Arkansas|
|WAMISHE, YESHI - University Of Arkansas|
|VALENT, BARBARA - Kansas State University|
Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 1/19/2018
Publication Date: 10/16/2018
Citation: Jia, Y., Wang, X., Bianco, T.A., Jia, M.H., Wamishe, Y., Valent, B. 2018. Dynamic changes of rice blast fungus in the USA through six decades. Proceedings of 37th Rice Technical Working Group Meeting, February 19-22, 2018, Long Beach, California. p 91. Electronic Publication.
Technical Abstract: Rice blast disease caused by the fungus Magnaporthe oryzae is a serious rice disease in the USA and worldwide. M. oryzae is highly adaptive and changeable due to the instability of its genome and resistance genes which are effective only when M. oryzae isolates contain the cognate avirulence (AVR) genes. Analysis of AVR genes, genetic identity, population structure, and pathogenicity in M. oryzae strains help identify effective blast resistance (R) genes for deployment to prevent blast disease. A total of 1022 M. oryzae isolates were collected from the southern USA from 1959 to 2015 for this study. The AVR genes in M. oryzae were determined based on the presence and absence of amplicons by Polymerase chain reaction (PCR) with AVR gene specific primers with the fungal genomic DNA directly from filter papers from storage. Genetic identity and population structure of M. oryzae was determined with 10 neutral polymorphic microsatellite markers. Pathogen virulence was first evaluated with the international rice differential rice varieties, Raminad Str. 3, Zenith, NP125, Usen, Dular, Kanto 51, Sha-tiaotsao, and Caloro, along with Katy and M202 as resistant and susceptible controls, respectively. The presence of AVR genes in M. oryzae isolates was then verified with expected disease reactions using International Rice Research Institute monogenic rice lines carrying the corresponding blast resistance genes. Through these analyses we showed that M. oryzae isolates have become more diverse and virulent over a period of six decades. However, we found AVR-Pib, AVR-Pi9, ACE1 (AVR-Pi33), and AVR-Pizt in over 90%, AVR-Pita1 in over 80 %, and AVR-Pik in 37% of the isolates. DNA sequence analysis revealed that AVR-Pita and AVR-Pik were rapidly evolved under positive selection whereas few changes in AVR-Pi9, AVR-Pi33, AVR-Pib, and AVR-Pizt were found in sequenced US isolates. Overall, frequencies of mutations at AVR-Pita and AVR-Pik genes were found to correlate with the frequencies of the deployment of the cognate R genes in rice varieties in the Southern USA. These findings suggest that blast R genes, Pib, Pi9, Pi33, Pizt, and Pi-ta, are effective R genes to reduce damage by rice blast disease in the southern USA. A strategy for future R gene deployment in different southern US states will be presented.