A Novel Root-Knot Nematode Resistance QTL on Chromosome Vu01 in Cowpea

The root-knot nematode (RKN) speciesMeloidogyne incognitaandM. javanicacause substantial root system damage and suppress yield of susceptible cowpea cultivars. The narrow-based genetic resistance conferred by theRkgene, present in some commercial cultivars, is not effective againstRk-virulent populations found in several cowpea production areas. The dynamics of virulence within RKN populations require a broadening of the genetic base of resistance in elite cowpea cultivars. As part of this goal, F1and F2populations from the cross CB46-Null (susceptible) x FN-2-9-04 (resistant) were phenotyped forM. javanicainduced root-galling (RG) and egg-mass production (EM) in controlled growth chamber and greenhouse infection assays. In addition, F2:3families of the same cross were phenotyped for RG on field sites infested withRk-avirulentM. incognitaandM. javanica. The response of F1to RG and EM indicated that resistance to RKN in FN-2-9-04 is partially dominant, as supported by the degree of dominance in the F2and F2:3populations. Two QTLs associated with both RG and EM resistance were detected on chromosomes Vu01 and Vu04. The QTL on Vu01 was most effective against aggressiveM. javanica, whereas both QTLs were effective against avirulentM. incognita. Allelism tests with CB46 x FN-2-9-04 progeny indicated that these parents share the same RKN resistance locus on Vu04, but the strong, broad-based resistance in FN-2-9-04 is conferred by the additive effect of the novel resistance QTL on Vu01. This novel resistance in FN-2-9-04 is an important resource for broadening RKN resistance in elite cowpea cultivars.