INTERACTIONS OF VIRULENT MELOIDOGYNE-INCOGNITA AND FUSARIUM-WILT ON RESISTANT COWPEA GENOTYPES

Two isolates of Meloidogyne incognito from separate field sites in cen tral California were virulent to five cowpea (Vigna unguiculata) genot ypes carrying resistance gene Rk and three other isolates were Rk avir ulent in findings based on egg production in greenhouse experiments. E gg production and root galling at the two field sites with virulent po pulations and at one field site with an avirulent population confirmed the greenhouse results. The effects of nematodes on wilt disease caus ed by Fusarium oxysporum f. sp. tracheiphilum and on cow-pea yield wer e examined over 3 years at the two sites infested with virulent nemato des. Main plots in the split-plot experiments were treated with a nema ticide or were not treated. Cowpea genotypes planted in subplots had c ombined 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 i ncidence of plants with visible symptoms or midseason and late-season vascular discoloration ratings. On wilt-susceptible genotypes, wilt di sease occurred in nematicide-treated plots and was exacerbated by nema tode infection in nontreated plots regardless of the presence of Rk. T he yield of wilt-resistant genotypes was suppressed an average of 17% as a result of nematode infection in nontreated plots compared with pl ots 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 wa s suppressed an average of 37 to 65% because of the combined effects o f nematode and wilt infections.