CHARACTERIZATION OF NEW SOURCES OF RESISTANCE IN COWPEA TO THE SOUTHERN ROOT-KNOT NEMATODE

A series of greenhouse and field studies was conducted over 9 years to characterize three new sources of resistance in cowpea [Vigna unguicu lata (L.) Walp.] to the southern root-knot nematode [Meloidogyne incog nita (Kofoid & White) Chitwood] and to determine if the resistances ar e conditioned by genes allelic to the Rk root-knot nematode resistance gene in Mississippi Silver'. Three plant introductions (PI), PI 44198 7, PI 441920, and PI 468104, were evaluated for reaction to M. incogni ta in four greenhouse tests, and in every test each PI exhibited less galling, egg mass formation, or egg production than 'Mississippi Silve r'. F, populations of the crosses between 'Mississippi Silver' and eac h of the three resistant PIs were also evaluated for root-knot nematod e resistance in a greenhouse test. None of the F2 populations segregat ed for resistance, indicating that PI 441917, PI 441920, and PI 468104 each has a gene conditioning resistance that is allelic to the Rk gen e in 'Mississippi Silver'. Our observations on the superior levels of resistances exhibited by PI 441917, PI 441920, and PI 468104 suggest t hat the allele at the Rk locus in these lines may not be the Rk allele , but one or more alleles that condition a superior, dominant-type res istance. The availability of additional dominant alleles would broaden the genetic base for root-knot nematode resistance in cowpea.