Genetic Variation of Rice Blast (Pyricularia oryzae) Isolates in Laos

Abstract

The pathogenicity of 192 blast isolates collected from regions across the whole of Laos from 2007 to 2009 showed a wide variation in terms of the frequencies of virulence toward differential varieties (DVs) and the susceptible control cultivar Lijiangxintuanheigu. High frequencies of virulence (>50%) were found in the reactions of DVs for Pit, Pia, Pi3, Pi5(t), Pik-s, Piz-t, Pi19(t), and Pi20(t); intermediate values (from 10 to 50%) were found in DVs for Pib, Pii, Pik-m, Pi1, Pik-h, Pik, Pik-p, Pi7(t), Piz-5, Pita (two lines), Pita-2 (two lines), and Pi12(t); and low frequencies (<10%) were found in DVs for Pish, Piz, and Pi9(t). The blast isolates were classified into three cluster groups: Ia and Ib (low virulence) and II (high virulence), based on the patterns of reaction to them. The blast isolates in cluster group Ia were dominant in rainfed lowland areas, those in Ib were dominant in irrigated lowland areas, and those in II were dominant in upland areas. Cluster groups Ia, Ib, and II were dominant in the Southern, Central, and Northern regions, respectively. Blast races in Laos were distributed according to ecosystems for rice cultivation and geographical regions from south to north with different virulence. These isolates were categorized into 156 races, and the numbers of blast isolates were few in each race. A total of 15 representative isolates were selected from among them as standard differential blast isolates, to develop a differential system.