Marker-assisted Pyramiding Resistance Genes Against Angular Leaf Spot and Common Bacterial Blight Disease into Preferred Common Bean Cultivar

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 aims 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) with the aid of marker-assisted breeding method. Marker-assisted Parallel Back Crossing (MAPBC) breeding scheme with three separate parallel backcrossing streams were adopted for tracking three independent resistance loci linked to g796 ( Phg-2  for ALS resistance) and, SU91 and SAP6 genetic markers from two different donor parents to the REDWOLAITA recurrent parent. The two donor parental lines VAX6 (with known RQTLs tagged by the SAP6 and SU91 genetic markers on linkage groups 10 and 8, respectively) and MEX54 with the  Phg-2  R gene tagged by the g796 genetic marker at the linkage group 8 were 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 PNILs with different gene combination includes MNIL SAP6 , MNIL SU91  & MNIL g796 , polygenetic 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 genetic background and tested under the screening house condition. The developed lines showed high level of disease resistance to the strains of CBB and ALS present under the screening conditions. They were selected to be multiplied and tested under multiple environment, before varietal release and wider production. Developed MNILs with good agronomic background will also be used as alternative donor parent for the future gene pyramiding program.