Identification of a major QTL for resistance to Cercospora leaf spot disease in cowpea (Vigna unguiculata (L.) Walp.) revealed common genomic region with that for the resistance to angular leaf spot in common bean (Phaseolus vulgaris L.)

Yardlong bean (Vigna unguiculata ssp. unguiculata cv-gr. sesquipedalis) is a type of cowpea (Vigna unguiculata). It is an important vegetable legume crop of Asia. One of the problems in yardlong bean production is the devastation of Cercospora leaf spot (CLS) disease caused by the fungi Cercospora canescens and Pseudocercospora cruenta which can cause yield loss from 35 to 40 %. Breeding for resistance cultivar(s) takes a long time and yet evaluating for the resistance is difficult and laborious. The objective of this research is to locate major gene/quantitative trait locus (QTL) controlling CLS resistance in yardlong bean with the ultimate goal to identify SSR markers closely linked to the resistance gene for marker-assisted selection. An F-2 population of 190 plants was developed from a cross between yardlong bean breeding line "CSR12906" (susceptible) and cowpea (Vigna unguiculata ssp. unguiculata) breeding line "IT90K-59-120" (resistant). The F-2 plants and F-2:3 families were evaluated for the CLS resistance caused by C. canescens and/or P. cruenta using a visual scoring method. Bulked segregant analysis (BSA) was conducted using simple sequence repeat (SSR) markers. Location of the QTL controlling the resistance was determined by inclusive composite interval mapping (ICIM). Disease evaluation revealed that the disease scores were continuously distributed, suggested that the resistance in IT90K-59-120 is a quantitative trait. BSA showed that SSR marker CEDG304 on linkage group 9 (LG9) of yardlong bean associated with the resistance. ICIM identified a major QTL, coined as qCLS9.1, for the resistance on LG9. qCLS9.1 accounted for 25.40-89.26 % of the disease score variation depending on environment and population. Linkage and genome comparison revealed that qCLS9.1 is likely the same as QTL ALS10.1 conferring resistance to angular leaf spot disease caused by P. cruenta in common bean (Phaseolus vulgaris).