Distribution of Pathogenic Races of Rice Blast Fungus in Vietnam

Abstract

One hundred and twenty-nine monoconidial isolates of Pyricularia grisea from rice were collected in Vietnam, mainly in the Mekong River Delta (MRD), in 1995-1996 to characterize their virulence. These isolates were classified into 12 pathogenic groups (races) based on their virulence to 12 Japanese differential rice varieties (modified Kiyosawa's differentials). Race 002.4 was predominant and was found in 10 provinces of MRD, followed by races 106.4, 006.4, 102.4 and 002.0. Differential varieties Aichi Asahi (which carries resistance gene Pia) and K59 (Pit) were susceptible to 93.8% and 86.0% of the isolates, respectively. On the other hand, none of the isolates were virulent to differentials Shin 2 (Pik-s, Pish), Kusabue (Pik, Pish), Fukunishiki (Piz, Pish), Pi No.4 (Pita-2, Pish), Toride 1 (Piz-t), K60 (Pik-p) and BL1 (Pib, Pish). The reference rice variety AA/S2-3, which carries only one resistance gene Pish, was also resistant to all the isolates, but the variety AA/S2-75, carrying only Pik-s, was susceptible to 95.3% of the isolates. Therefore, the resistance in Shin 2, with Pik-s and Pish, might be due to Pish or an unknown gene(s). Reaction patterns of 45 Vietnamese rice varieties to four isolates belonging to different races, based on their virulence to the Japanese differential varieties, could be classified into eight groups. These results suggested that the pathogenicity of the Vietnamese isolates was complex and that the pathogenic specialization of Vietnamese isolates may be difficult to classify in detail using the Japanese differential varieties.