Multiple Rpp-gene pyramiding confers resistance to Asian soybean rust isolates that are virulent on each of the pyramided genes

Since Asian soybean rust (ASR) isolates in South America are highly virulent, diverse, and distantly related to Japanese ones, limited numbers of resistance resources are available in soybean breeding in that region. Pyramiding of available ASR resistance genes (Rpp) in a single soybean genotype may provide wider spectrum and higher level of ASR resistance to soybean. However, the desired combinations of genes conferring adequate resistance to highly virulent or distantly related ASR isolates have not yet been studied. In this study, seven pyramided lines carrying multiple Rpp genes have been developed and evaluated for their resistance against one ASR isolate from Japan and two from Brazil. Significantly higher resistance was observed in the pyramided lines, No6-12-B (Rpp4 + Rpp5), Oy49-4 (Rpp2 + Rpp3 + Rpp4), and No6-12-1 (Rpp2 + Rpp4 + Rpp5) compared to the original resistance sources, PI 230970 (Rpp2), Hyuuga (Rpp3), PI 459025 (Rpp4), and Kinoshita (Rpp5) carrying single Rpp genes. Although infection of the resistance sources with the highly virulent Brazilian ASR isolates resulted in susceptible phenotypes with moderate to abundant sporulation, highly resistant phenotypes with almost no sporulation were observed in the three Rpp-pyramided lines. Therefore, pyramided lines carrying these Rpp gene combinations are useful in soybean breeding for conferring broad spectrum, high resistance to ASR isolates that are virulent to the varieties carrying single resistance genes.