Molecular mapping of the Rcs3 gene for resistance to frogeye leaf spot in soybean

Frogeye leaf spot (PLS) (caused by Cercospora sojina Hara) is a foliar disease of soybean [Glycine max (L.) Merr.] that causes significant yield loss in southeastern USA. The Res 3 gene in soybean has been reported to condition resistance to all known races of C. sojina, Molecular mapping of the Rcs3 gene will be helpful in breeding soybean for resistance to FLS. The objective of this study was to map the Rcs(3) gene with DNA markers. The parents, 123 F-2 plants, and the F-2:3 families from the cross of 'Blackhawk' (susceptible) x 'Davis' (resistant and original source of the Rcs3 gene) were scored for FLS reaction in the greenhouse after inoculation with race 5 of C. sojina. In addition,'Wright' and the Wright(6)-Rcs 3 (near isoline of Wright) were also scored for FLS by the same protocol. A resistant and a susceptible DNA bulk were created by pooling the DNA of 15 resistant and 15 susceptible F-2 plants, respectively, from the Blackhawk x Davis population. The two bulks and resistant-susceptible near isolines (NILs) of Wright were screened with simple sequence repeat (SSR) markers from soybean linkage groups with known disease resistance gene clusters, The bulks and the NILs of Wright both showed a putative association between the Rcs 3 locus and a SSR marker Satt244 from linkage group (LG) J. A molecular map of the relevant segment of the LG a for the Blackhawk x Davis population was then created by screening the DNA of 95 Fl plants with the available polymorphic SSR and RFLP markers. We have mapped the Rcs3 gene for resistance to FLS in soybean near a previously known disease resistance gene cluster on LG J. The gene was closely linked to SSR markers Satt244 and Satt547. Close proximity of this gene with these SSR markers provides soybean breeders with an opportunity to use these markers for marker assisted selection (MAS) for FLS resistance in soybean.