Dynamics of Meloidogyne incognita Virulence to Resistance Genes Rk and Rk in Cowpea

The virulence index of three Meloidogyne incognita field isolates to the resistance gene Rk in cowpea was 0%, 75%, and 120%, with the index measured as reproduction on resistant plants as a percentage of the reproduction on susceptible plants. Continuous culture of the 75% virulent isolate on susceptible tomato for more than 5 years (about 25 generations) resulted in virulence decline to about 4%. The rate of the decline in virulence was described by exponential decay, indicating the progressive loss of virulence on a susceptible host. The 120% virulent isolate declined to 90% virulence during five generations on susceptible cowpea. Following virulence decline, the two isolates were compared over 5 years in inoculated field microplots both separately and as a mixture on susceptible, gene Rk, and gene Rk(2) cowpea plants. At infestation of the plots, the two isolates were 1.2% and 92.0% virulent, respectively, to gene Rk and 0.2% and 8.1% virulent, respectively, to gene Rk(2). Virulence to gene Rk in the two isolates and in mixture increased under 5 years of continuous Rk cowpea plants to 129% to 172% and under Rk(2) cowpea plants to 113% to 139 % by year 5. Virulence to gene Rk(2) increased during continuous cropping with Rk cowpea plants to 42% to 47% and with Rk(2) cowpea plants to 22% to 48% by year 5. Selection of Rk(2)-virulence was slower in the isolate with low itt-virulence. The virulence to both genes Rk and Rk(2) in the mixed population was not different from that in the highly virulent isolate by year 5 of all cropping combinations. Selection of Rk(2)-virulence on plants with Rk, and vice versa, indicated at least partial overlap of gene specificity between Rk and Rk(2) with respect to selection of nematode virulence. This observation should be considered when resistance is used in cowpea rotations.