A routine test for Relative Susceptibility estimation of potato genotypes with resistance to Meloidogyne chitwoodi

Several aspects of the Relative Susceptibility (RS) of eight potato genotypes, one susceptible and seven partially resistant against M. chitwoodi, were studied in four successive greenhouse experiments, in 10, 5, 3 and 2 kg pots. The cultivar Désirée was used as a susceptible control. The initial nematode densities (Pi), consisted of a 2 log series: 2x, with x ranging from - 4 till 8. The final population density (Pf) was estimated in the roots, soil and, in one experiment, also in the tubers. The proportion of nematode free tubers was estimated in all experiments. Seinhorst’s logistic model of population dynamics was fitted to the data to estimate the maximum multiplication rate (a), maximum population density (M), RSa and RSM. Apart from one genotype, resistant to G. pallida but susceptible to M. chitwoodi, the averaged estimates for RSa and RSM of all tested genotypes were smaller than 0.29% and 0.23%, respectively, with coefficients of variation of 0.21 and 0.09, respectively. The M/a ratio for six out of seven genotypes and Désirée was 1.3, indicating that for these genotypes RS is independent of Pi (RSa = RSM). RS can, therefore, be tested at one well-chosen Pi.
Although a and M were sometimes influenced by pot size, both RS and M/a were not. A choice of 24 J2 (g dry soil)-1 as a Pi and pot sizes of 2 or 3 kg for routine testing is discussed. Tuber infestation levels after 8-10 months of storage, only estimated in Exp. 2, was 0.35 J2 (g dry soil)-1 for Désirée and less than 0.0015 J2 (g dry soil)-1 for four tested genotypes, on average 2% of the total Pf.