Genetic structure and virulence diversity of Pyricularia grisea for breeding for rice blast resistance

Rice blast caused by Pyricularia grisea Sacc. is the main production constraint in rice worldwide. Development of resistant cultivars has been the preferred mean to control 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 families genetically different named SRL-1 to SRL-6 by the use of DNA-fingerprinting. The spectrum of virulence of isolates within each family is highly similar differing mainly by single virulences. Although the six genetic families of the fungus share a high number of virulence factors, a high specific interaction between some avirulence/virulence factors in the pathogen and resistance genes in the host is observed. This specific interaction is the base for the selection of progenitors to be included in a breeding program 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. The identification of complementary resistance genes is based on the detection of 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 deletereous effect on the fungus reducing its frequency in nature. Frequency of virulence factors to the resistance genes Pi-1 and Pi-2 present independently in two different near isogenic iines is high in the blast fungus population of Colombian genetic families SRL-5 and SRL-1, respectively. The two genes show complementary resistance that exclude all the genetic families of the fungus in Colombia and no isolate with the combination of the two virulence genes infecting both isogenic lines has been detected. Induced mutations can be a useful technique to produce rice lines with specific resistance genes to different genetic lineages. These lines can be incorporated within breeding programs to combine such genes for the development of durable resistance to blast.