Pathogenicity of Rice Blast (Pyricularia oryzae) Isolates from the Ryukyu Archipelago, Okinawa, in Southern Japan

Rice ( Oryza sativa L.) is a major crop, and blast ( Pyricularia oryzae Cavara) has been the most serious disease in Japan. Ninety-seven rice blast isolates were collected from the Ryukyu Archipelago, including Okinawa Main Island (OK) and Yaeyama Islands (YA), in Japan's southern region. The pathogenicities of blast isolates were clarified by an inoculation test based on the resistance reaction patterns to differential varieties (DVs) and a susceptible control, Lijiangxintuanheigu (LTH). Virulent isolates occurred with high frequencies to LTH and DVs for Pish, Pia, Pii, Pi3, Pik-s, and Pi19(t). The DVs for Pib, Pik-m, Pi1, Pik-h, Pik, Pik-p, Pi7(t), Pi9(t), Piz, Piz-5, Piz-t, Pita-2 (two lines), Pil2(t), and Pi20(t) had low frequencies, and those for Pit, Pi5(t), and Pita (two lines) had intermediate frequencies. These isolates were classified into two groups (I and II) based on the reaction patterns to DVs by cluster analysis. The virulence of the blast isolates from cluster II against DVs for Pit, Pi5(t), Pita (two lines), and Pi20(t) was much greater than that of those in cluster I. Cluster I blast isolates were distributed dominantly in OK, and those of cluster II were in YA. This is the first report on the distribution of blast races in the Ryukyu Archipelago in Japan. This information will be useful for developing sustainable protection systems and rice breeding against blast disease.