Abstract

Angular leaf spot (ALS) caused by Pseudocercospora griseola and common bacterial blight (CBB) caused by Xanthomonas campestris pv phaseoli X. campestris pv. phaseoli var. fuscans are the most economically important diseases of common bean production in Ethiopia. This research was aimed at pyramiding the Phg-2 R gene for angular leaf spot resistance and two CBB major resistance quantitative trait loci (RQTLs) into the background of the most popular and susceptible common bean cultivar “REDWOLAITA” (RW). Marker-assisted Parallel Back Crossing (MAPBC) breeding scheme with three separate parallel backcrossing streams were adopted. This technique accelerated tracking three independent resistance loci linked to g796, SAP6 and SU91 genetic marker from two different donor parents into the RW recurrent parent. The donor parental line VAX-6 with known RQTLs for CBB and tagged by the SAP6 and SU91 genetic markers on linkage groups 10 and 8, respectively was used. The other donor parent MEX-54 with known Phg-2 R gene tagged by the g796 genetic marker at the linkage group 8 was used in the gene pyramiding program. After the BC₄ generation, progenies that combined SAP6 and g796 genetic markers were created and selected from the BC₄ inter-crossing of progenies. Then, further inter-crossing was made between selected progenies that combined the SAP6 and g796 genetic markers with selected progenies with the SU91 genetic marker. Finally, from this study we developed Monogenic Near Isogenic Lines (MNILs) with R genes tagged by the SAP6, g796, and SU91 molecular markers and Polygenic Near Isogenic Lines (PNILs) with different gene combination. The developed lines include MNIL^SAP6, MNIL^SU91 and MNIL^g796, PNILs^SAP6/g796, PNILs^SU91/g796, PNILs^SAP6/SU, PNILs^{SAP6/g796/SAP6}, with more than 97% genome recovered from the RW genetic background. Marker-assisted backcrossing facilitated selection of progenies that combined good agronomic traits with resistance loci were constructed from the RW common bean cultivar. The developed lines showed high level of disease resistance to the strains of CBB and ALS under the screening conditions. The developed lines will be evaluated under multiple environment under natural epidemics, before varietal release for wider production. We also recommend the developed NILs with RW back ground and known sources of R genes and good agronomic performance could be used as alternative donor parent for the future gene pyramiding program.

Keywords: Gene pyramiding; Parallel backcrossing; RQTLs; Inter-crossing; Isogenic lines