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Genetic variation of the rice blast (Pyricularia oryzae) population in Thailand was investigated based on the nucleotide sequence of three avirulence genes, AVR‐Pi9, AVR‐Pik, and AVR‐Pita1. Sixty rice blast isolates were collected from rice‐growing areas around Thailand. Gene‐specific primers were used to amplify these AVR genes and AVR‐Pi9, AVR‐Pik, and AVR‐Pita1 were detected in 60, 57, and 23 isolates, respectively. Based on the AVR‐Pi9 sequences, we identified one rice blast isolate containing an amino acid change from glutamic acid to aspartic acid. Moreover, two rice blast isolates had identical sequences to the rice blast strain originating in Japan, indicating a potential movement of this isolate from Japan to Thailand. Three AVR‐Pik alleles were found, including AVR‐PikA (3.51%), AVR‐PikD (71.93%), and isolates with two copies of AVR‐PikD and AVR‐PikF (24.56%). AVR‐PikA and AVR‐PikF are virulent to Thai rice variety Jao Hom Nin. Six haplotypes of AVR‐Pita1 were identified with one deletion and 12 amino acid substitutions. This study revealed that different AVR genes in Thai rice blast populations have different levels of genetic variation: AVR‐Pi9 and AVR‐Pik genes have a relatively low genetic diversity, while the AVR‐Pita1 gene has high genetic diversity. We found AVR‐Pi9 was not under selection pressure, while AVR‐Pita1 was under purifying selection pressure. In addition, geographic location has influenced the distribution of genetic variation of AVR‐Pita1. The information obtained from this study is valuable for the future development of breeding strategies for rice blast resistance in Thailand.

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Rice blast disease, caused by the filamentous fungus Pyricularia oryzae, is one of the most destructive diseases in rice worldwide. Breeding of resistant rice cultivars remains a cost‐effective and environment‐friendly means for controlling blast disease, but the resistance tends to break down over time because of the pathogen's rapid adaptation. In this study, AVRPiz‐t gene sequences of 46 rice blast isolates were evaluated using a Southern blot analysis. The AVRPiz‐t gene was present in 24 of 46 (52.2%) rice blast isolates. The pathogenicity assay showed that all blast isolates were avirulent against Japanese rice cv. Toride 1, which carries several rice blast resistance genes including Piz‐t, Pii, Pi37, and Pi‐ta. Screening for the Piz‐t gene in Thai rice germplasm revealed that less than 20% of rice varieties harbour the Piz‐t gene. Therefore, the Toride 1 rice variety could serve as an effective donor of rice blast resistance to be used in rice breeding programmes in Thailand. This study provides evidence for co‐evolution between the rice blast resistance gene Piz‐t and the rice blast fungal avirulence gene AVRPiz‐t. Understanding this relationship will facilitate the sustainable development of breeding for rice blast resistance in the future.

Rice blast disease, caused by Magnaporthe oryzae, is one of the most damaging diseases of rice worldwide. Cultivation of rice varieties carrying resistance genes is the most economic and successful strategy to control the disease. In this study, 451 rice varieties from around the world including 363 Thai landrace rice varieties, 21 Thai improved rice varieties, 43 Japanese rice varieties and 24 worldwide rice varieties were screened by PCR technique using gene-specific markers for 10 rice blast resistance genes: Pi9, Piz-t, Pi50, Pigm(t), Pid2, Pid3, Pia, Pik, Pi54 and Pita. The results showed that 382 (99.48%) Thai rice varieties have at least one resistance gene and two rice varieties, ‘Hom’ and ‘Bak muay’, contained eight out of ten screened rice blast resistance genes. 320 rice varieties (83.33%) contained three or more rice blast resistance genes. The frequency of the rice blast resistance gene ranges from 87.76–9.64 per cent, of which the Pid3 gene has the highest frequency and the Pi54 gene has the lowest frequency. Two major resistance genes, found in Japanese rice varieties, are the Pik gene (76.74%) and the Pi9 gene (72.09%). While two major resistance genes, found in the international rice varieties are the Pi9 gene (66.67%) and the Pi54 gene (62.50%). The disease resistance gene profile of each rice variety obtained from this study will benefit the rice blast resistant breeding programme in the future.