Interactions of VirulentMeloidogyne incognitaand Fusarium Wilt on Resistant Cowpea Genotypes

Two isolates of Meloidogyne incognita from separate field sites in central California were virulent to five cowpea (Vigna unguiculata) genotypes carrying resistance gene Rk and three other isolates were Rk avirulent in findings based on egg production in greenhouse experiments. Egg production and root galling at the two field sites with virulent populations and at one field site with an avirulent population confirmed the greenhouse results. The effects of nematodes on wilt disease caused by Fusarium oxysporum f. sp. tracheiphilum and on cowpea yield were examined over 3 years at the two sites infested with virulent nematodes. Main plots in the split-plot experiments were treated with a nematicide or were not treated. Cowpea genotypes planted in subplots had combined nematode (gene Rk) and wilt resistance (genotypes CB46, 7964, and 8517), nematode resistance only (CB5), wilt resistance only (CB3), or no resistance (8679). Infection by M. incognita did not predispose wilt-resistant genotypes to wilt disease on the basis of cumulative incidence of plants with visible symptoms or midseason and late-season vascular discoloration ratings. On wilt-susceptible genotypes, wilt disease occurred in nematicide-treated plots and was exacerbated by nematode infection in nontreated plots regardless of the presence of Rk. The yield of wilt-resistant genotypes was suppressed an average of 17% as a result of nematode infection in nontreated plots compared with plots treated effectively with a nematicide. Wilt-susceptible genotypes had significantly lower yield than wilt-resistant genotypes in treated plots, and in nontreated plots yield of wilt-susceptible genotypes was suppressed an average of 37 to 65% because of the combined effects of nematode and wilt infections.