Rice blast caused by Pyricularia grisea Sacc. is the main production constraint in rice worldwide. Development of resistant cultivars has been the preferred means of controlling this disease; however, resistance is defeated by the pathogen shortly after cultivar release. The blast pathogen population in Colombia's rice growing areas has been grouped into six genetically different families named SRL-1 to SRL-6 using DNA fingerprinting. The spectrum of virulence of isolates within each family is highly similar, differing mainly in single virulences. Although the six genetic families of the fungus share a high number of virulence factors, high specific interaction between some avirulence/virulence factors in the pathogen and resistance genes in the host has been observed. This specific interaction is the basis for selecting the progenitors to be included in a breeding programme aimed at obtaining more durable blast resistance. Combinations of genes showing complementary resistance to different genetic families of the fungus should exclude any compatible interaction with a blast isolate. Identification of complementary resistance genes is based on detecting those virulence factors whose combinations in individual isolates within the pathogen population have a frequency near zero. It is assumed that certain virulence combinations in the blast pathogen may confer a low fitness or have a deletereous effect on the fungus, reducing its frequency in nature. The frequency of the virulence factors to the resistance genes Pi-1 and Pi-2 present independently in two different near isogenic lines is high in the blast fungus population of genetic families SRL-5 and SRL-1, respectively. The two genes show complementary resistance that excludes all the genetic families of the fungus, and no isolate with a combination of the two virulence genes infecting both isogenic lines has been detected. Induced mutations can be a useful technique for producing rice lines with specific resistance genes to different genetic lineages. These lines can be incorporated into breeding programmes to combine such genes for developing durable resistance to blast.
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