Effects of Pyramiding Quantitative Resistance Genes pi21, Pi34, and Pi35 on Rice Leaf Blast Disease.

Development of resistant cultivars has been an effective method for controlling rice blast disease caused by Magnaporthe oryzae. Quantitative blast resistance genes may offer durable resistance because the selection pressure on M. oryzae to overcome resistance is low as a result of the genes' moderate susceptibility. Because the effects of individual resistance genes are relatively small, pyramiding these genes in rice cultivars is a promising strategy. Here, we used near-isogenic and backcross lines of rice cultivar Koshihikari with single- or two-gene combinations of blast resistance genes (pi21, Pi34, and Pi35) to evaluate the suppression of leaf blast. The severity of the disease was assessed throughout the infection process. Resistance varied among the lines: Pi35 conferred the strongest resistance, while Pi34 showed the weakest effects. Two types of combined-gene interactions were observed, and they varied on the basis of gene combination and characteristic of the infection: (i) the combination of two resistance genes was more effective than either of the genes individually or (ii) the combination of two resistance genes was similar to the level of the most effective resistance gene in the pair. The most effective gene combination for the suppression of leaf blast was pi21 + Pi35.