Abstract

The threat posed by soybean rust (Phakopsora pachyrhizi) on soybean production is worsened by resistance breakdown associated with single gene resistance present in most cultivars. Few studies have however been undertaken to use mapped simple sequence markers for gene pyramiding to enhance rust resistance. This study validated use of identified simple sequence repeat markers for gene pyramiding, and determined the most effective pairwise gene combination for three independent soybean rust resistance genes, Rpp2, Rpp3and Rpp4. Markers Satt460 and AF162283 were polymorphic for the three resistance genes among the parents and were therefore used in selections made in the F₂ and F₃ families. In the F₂ generation, soybean plants (homozygous dominant or heterozygous at both loci) with two gene combinations had relatively lower disease severity and sporulation than the parents, suggesting complementary epistatic gene action for resistance. Similarly, homozygous F₃ families showed lower severity, lesion density and sporulation. Gene Rpp3 contributed positively to resistance with various genetic backgrounds for most parameters measured, compared to Rpp2 and Rpp4 resistance genes. Overall, the results suggest that marker gene pyramiding is feasible and can substantially increase resistance to soybean rust through reduced severity and reduced sporulating lesions.

Key words: Epistasis, genetic background, Phakopsora pachyrhizi, simple sequence repeats.