Abstract: The present study was undertaken to determine the pathogenic and the genetic diversity of the isolates of M. oryzae collected from the wet, intermediate and dry zones of Sri Lanka with a view to develop rice varieties conferring durable resistance to rice blast. No significant morphological or growth variations existed amongst the isolates studied. The genetic diversity of isolates determined by carrying out Pot2 transposable element based on repetitive-PCR revealed that the majority of isolates (92%) clustered into a single group with 45.4% similarity. The existence of nine pathotypes was identified by observing the reaction type of isolates on 16 different rice lines. Pathotype 1 which was distributed in all zones affected only one differential line. Pathotype 2 which was able to infect six lines was restricted only to the dry zone of Sri Lanka. Of the 16 rice lines, seven lines, K3 (Pik-h), C101A51 (Piz5), K1 (Pita), C105TTP2L9 (Pita), K59 (Pit), Shin (Pish) and WHD-1S-75-1-127 (Pi9) had highly effective blast resistance. None of the isolates of the fungus showed any virulence against the seven lines. These seven blast resistant lines can be used in the breeding programmes in Sri Lanka for development of lines conferring durable resistance to rice blast.

Key words: rice blast, genetic diversity, pathotype