Variability of Pyricularia oryzae Populations Causing Agents of Rice Blast in Thailand

Abstract

Rice blast, caused by Pyricularia oryzae, is an important disease of rice production in Thailand. This disease is distributed in all localities of the country. It affects the quality and quantity of rice production. The purpose of this research was to evaluate the variation of P. oryzae populations collected from major rice growing areas of the country by studying their pathogenicity towards 9 differential rice varieties that had single resistance genes (near isogenic lines; NILs): Pi-7(t), Pi9, Pi-a, Pi-b, Pi-i, Pi-kh, Pi-t, Pi-ta and Pi-sh. Results revealed that from the 80 isolates studied, the pathogen could be identified into 47 races. When all isolates were clustered by UPGMA methods Past 4.10 program by using disease index data, the classical clustering analysis showed that P. oryzae races were grouped into 13 clades with cophenetic correlation at 0.80. However, race identification and clustering were not correlated with the sources of P. oryzae, more than one race could occur in one site. The results indicated that pathogenicity of the fungus was variable within the population as well as among populations. These results were informative for the planning of improvement of blast disease resistant rice varieties.