Inheritance of a broad-based form of root-knot nematode resistance in cowpea

Root-knot nematodes (Meloidogyne spp.) are serious pests of cowpea [Vigna unguiculata (L.) Walp.] and many other crops worldwide. Host plant resistance is the primary means for managing root-knot nematodes in cowpea, yet few resistance genes have been identified and their effectiveness can vary depending on the specific nematode isolate. Therefore, there is a need to describe new genes that confer a high level of resistance to a broad spectrum of these Rests. This study was conducted to determine the inheritance of resistance to M. incognita (Kofoid and White) Chitwood and M. javanica (Treub) Chitwood in a blackeye-type cowpea line H8-8R that is expressed at a higher level than the resistance conferred by gene Rk in current cultivars CB46 and CB88. F-1, F-2, and F-2-derived F-3 families from crosses between HS-IR and CB88 or CB46 were screened for reaction to Rk-avirulent and Rk-virulent M. incognita and Rk-aggressive M. javanica isolates in growth-pouch and pot tests. Nematode egg numbers and egg-mass production on roots gave segregation patterns in all tests that were consistent with the model that the additional resistance in H8-8R is conferred by a single recessive gene. The data indicated this recessive gene is independent of gene Rk, confers partial resistance when expressed alone, and has an additive effect of increasing resistance in the presence of gene Rk. Line TVu4552 in the pedigree of H8-8R was identified as the probable donor parent of the recessive gene, for which we propose the gene symbol rk3.