Effects of Meloidogyne incognita on Nitrogen Fixation in Soybean

The effects of a North Carolina population of Meloidogyne incognita on N fixation on root-knot-susceptible 'Lee 68' and moderately resistant 'Forrest' soybean were evaluated 50, 75, I00, and 135 days after inoculation with nematodes. Nematodes stimulated N fixation in Lee 68 by 50 days and in Forrest by 75 days. At all other intervals, N fixation was either depressed or unaffected by nematodes. Additional observations indicate that the susceptibility of Lee 68 is associated with greater rates of penetration by larvae and more favorable responses of host tissues to nematodes than occur in Forrest. With time, however, the histological reactions of both hosts became less favorable for nematode development. Resistant or hypersensitive responses became common in Forrest by 75 days but not in Lee 68 until 90 days after inoculation. This population of M. incognita may stimulate N fixation at a specific time interval and depress it at others; therefore, disease of susceptible soybeans caused by this nematode is probably not primarily due to a net loss of fixed nitrogen but to pathogenicity similar to that which occurs on nonlegume hosts.