NEW RESISTANCE TO VIRULENT ROOT-KNOT NEMATODES LINKED TO THE RK LOCUSOF COWPEA

Screening of cowpea [Vigna unguiculata (L.) Walp.] germplasm for addit ional resistance to the root-knot nematodes, Meloidogyne incognita (Ko foid and White) Chitwood and M. javanica (Treub) Chitwood, revealed an accession (IT84S-2049) from Africa with resistance to diverse populat ions of both root-knot species. The resistance was effective against n ematode isolates that are virulent to the resistance gene Rk present i n commercial cowpea cultivars, in addition to isolates of gene Rk-avir ulent populations. This study was conducted to elucidate the inheritan ce of a newly identified, highly effective source of resistance, and t o determine its relationship to gene Rk in existing cultivars. Egg mas s production on root systems of plants in growth pouches was used to a ssess susceptibility-resistance of F-1, F-2, F-3, BC1F1, BC1F2, and TC 1 populations generated from crosses of IT84S-2049 with California Bla ckeye no. 3 (CB3, susceptible), CB5, or CB46 (both possessing Rk). Seg regation of resistance to gene Rk-virulent M. incognita in progenies f rom IT84S-2049 x CB3 showed that resistance in IT84S-2049 is governed by one dominant nuclear gene. No susceptible recombinants were found a mong 1206 F-2 and 1144 TC1 progeny from IT84S-2049 x CB5 or CB46 and i noculated with Rk-avirulent M. incognita (used to detect both resistan ce phenotypes). Resistance in IT84S-2049 is conferred by an additional dominant allele of the Rk locus, or by another gene locus very tightl y linked to Rk within 0.17 map units. We propose the symbol Rk(2) to d esignate this new resistance factor in IT84S-2049. Thus, Rk may be a c omplex nematode resistance locus, analogous to those reported for othe r plant pathogen-host combinations.