Fitness of Virulent Meloidogyne incognita Isolates on Susceptible and Resistant Cowpea.

A study of life-history traits was made to determine factors associated with the fitness of Meloidogyne incognita isolates virulent to resistance gene Rk in cowpea. Egg hatch, root penetration, egg mass production, and fecundity (eggs per egg mass) of avirulent and virulent phenotypes were compared among M. incognita isolates, isofemale lines, and single descent lines over multiple generations on resistant and susceptible cowpea. Variation (P </= 0.05) in both hatch and root penetration rates was found among isolates at a given generation. However, this variation was not consistent within nematode lines among generations, and there was no correlation with level of virulence, except for penetration and virulence on resistant cowpea at generation 20. Resistant and susceptible cowpea roots were penetrated at similar levels. Differences in reproductive factors on resistant plants were correlated with levels of virulence expression. In some isofemale lines, single descent lines, and isolates, lower (P </= 0.05) rates of egg mass production and fecundity on susceptible cowpea were associated with virulence to Rk, indicating a trade-off between reproductive fitness and virulence. Other virulent nematode lines from the same isolates did not have reduced reproductive ability on susceptible cowpea over 27 generations. Thus, virulent lineages varied in reproductive ability on susceptible cowpea, contributing to adaptation and maintenance of virulence within M. incognita populations under stabilizing selection.