New Resistance to Virulent Root-Knot Nematodes Linked to the Rk Locus of Cowpea

Screening of cowpea [Vigna unguiculata (L.) Walp.] germplasm for additional resistance to the root-knot nematodes, Meloidogyne incognita (Kofoid and White) Chitwood and M. javanica (Treub) Chitwood, revealed an accession (IT84S-2049) from Africa with resistance to diverse populations of both root-knot species. The resistance was effective against nematode isolates that are virulent to the resistance gene Rk present in commercial cowpea cultivars, in addition to isolates of gene Rk-avirulent populations. This study was conducted to elucidate the inheritance of a newly identified, highly effective source of resistance, and to determine its relationship to gene Rk in existing cultivars. Egg mass production on root systems of plants in growth pouches was used to assess susceptibility-resistance of F₁, F₂, F₃, BC₁F₁, BC₁F₂, and TC₁ populations generated from crosses of IT84S-2049 with California Blackeye no. 3 (CB3, susceptible), CB5, or CB46 (both possessing Rk). Segregation of resistance to gene Rk-virulent M. incognita in progenies from IT84S-2049 × CB3 showed that resistance in IT84S-2049 is governed by one dominant nuclear gene. No susceptible recombinants were found among 1206 F₂ and 1144 TC₁ progeny from IT84S-2049 × CB5 or CB46 and inoculated with Rk-avirulent M. incognita (used to detect both resistance phenotypes). Resistance in IT84S-2049 is conferred by an additional dominant allele of the Rk locus, or by another gene locus very tightly linked to Rk within 0.17 map units. We propose the symbol Rk² to designate this new resistance factor in IT84S-2049. Thus, Rk may be a complex nematode resistance locus, analogous to those reported for other plant pathogen-host combinations. This research was supported in part by grants from the California Blackeye Council and California Dry Bean Research Advisory Board.