Nematode management decisions should be based, in part, on knowledge of the damage threshold population density, and this value is likely to vary based on the level of resistance of the cotton cultivar. The damage functions of two cotton cultivars resistant to the root-knot nematode (Meloidogyne incognita) were compared with that of a susceptible cultivar in field microplots using a 3 x 7 factorial design with three cultivars (susceptible Paymaster HS 260; resistant Acala NemX and Stoneville LA 887) and seven initial nematode population densities (Pi). Final nematode population densities and root gall indices were greater on the susceptible HS 26 than on the resistant cultivars (P ≤ 0.05). Seed cotton yields of all cultivars decreased with increasing nematode Pi (r2 = 0.53 to 0.81), with relative yield losses being the greatest for HS 26. When yield responses to Pi were fitted to the Seinhorst model [Y = m + (1-m)Z P-T], the damage thresholds for each cultivar were similar (T = 0.1 to 9 nematodes/500cm3 soil). The minimum yield parameter was lower for the susceptible HS 26 (m = 0.71 to 0.8) than for the resistant cultivars (m = 0.8 to 0.93), but these differences were not significant (P ≥ 0.05). Based on both the regression and the Seinhorst models, the resistant cultivars have greater tolerance than the susceptible cultivar. The increased tolerance of the resistant cultivars increased the nematode density at which an action threshold for management is acheived.
