GENETIC DIVERSITY OF THE RICE BLAST FUNGUS IN A DISEASE NURSERY IN COLOMBIA

We determined DNA fingerprints and pathotypes on international differe ntials for 151 isolates of the rice blast fungus Pyricularia grisea th at were collected from 15 rice cultivars at a blast disease breeding n ursery in Santa Rosa, Colombia. The pathogen population expressed an e xtraordinarily high number (39) of pathotypes. DNA fingerprinting, usi ng the P. grisea repetitive DNA sequence MGR586, identified 115 haplot ypes that were partitioned into six discretely distinct genetic lineag es. The mean genetic similarity within lineages was high, ranging from 92 to 98%. The mean similarity between lineages ranged from 37 to 85% . Each lineage was associated with a specific subset of cultivars and expressed a generally nonoverlapping subset of pathotypes. Typically, the pathotypes within a lineage had a closely related infection spectr um, differing by single compatibility differences on a particular subs et of the international differentials. The cultivar and pathotype asso ciations indicated that the lineages express historically divergent vi rulence spectra. MGR-DNA fingerprint analysis also deciphered the line age associations of 31 isolates whose pathotypes could not be scored, i.e., the isolates were avirulent on all international differentials. MGR-DNA fingerprint analysis provides the genealogical framework for e valuating pathogen variation that is essential for understanding disea se epidemiology and pathotype evolution. This kind of analysis may be particularly useful for selecting germ plasm sources for more durable blast-resistance breeding.