Population structure and virulence diversity of the rice blast fungus in Colombia

More than 700 field isolates of the rice blast fungus in Colombia have been characterized for MGR-DNA fingerprint (a profile of 50-60 Eco RI RFLPs for the dispensed middle-repetitive sequence, MGR586) and for pathotype (virulence spectrum) on international differential and other rice cultivars. The isolates were collected over a multi-year period (1988-1992) and represented a total sampling of more than 75 pathotypes and more than 90 cultivars growing under a variety of disease pressure regimes. The sampling provided the opportunity to study the structure of rice blast fungus populations under the chronic epidemic conditions of a disease nursery (Santa Rosa), in 'trap nurseries' placed in a newly cultivated region (altillanura), and in typical commercial plantings. The fingerprints separate the isolates into only 14 distinct haplotype groups, each of which was diagnostic over time and across broad geographic distributions. This pattern implied that each group represented a clonal genetic lineage and that recombination between lineages was not an important feature for generating virulence diversity. Generally, each lineage was compatible with only a limited number of cultivars and most cultivars were susceptible to only one or a few lineages. Cultivar-lineage associations were generally maintained throughout Colombia, although several lineages were detected only in the sampling of the altillanura region. Each lineage expressed multiple, but generally lineage-specific pathotypes. The lineage-specific pathotypes were often organized into networks of virulence forms related by single differences in compatibility on the international differential cultivars. These features indicated that lineages have definable and, at least over several years, limited virulence spectra. Pathotype diversity per lineage was highest in the disease nursery epidemic populations and lowest in commercial fields, indicating that resistance breeding under epidemic nursery conditions is efficient but may amplify the rate of evolution of new pathotypes and resistance breakdown