Isolate-by-cultivar interaction in root-knot nematodes Meloidogyne hapla, M. chitwoodi, and M. fallax on potato

Abstract

To study virulence and aggressiveness in root-knot nematodes on cultivars of potato (Solanum tuberosum L., four isolates of Meloidogyne hapla Chitwood race A, one of M. hapla race B, three of Meloidogyne chitwoodi Golden et al., and two of Meloidogyne fallax Karssen were evaluated on 10 commercial potato cultivars under semisterile conditions in Petri dishes. Virulence and aggressiveness were assessed in terms of nematode reproduction by egg-mass index and reproduction factor, estimated by the number of egg masses and juveniles produced, respectively, divided by the number of juveniles inoculated. Significant interaction for both parameters was revealed between species of Meloidogyne and potato cultivars. Only M. hapla showed significant isolate-by-cultivar interaction, which was predominantly caused by the M. hapla race B isolate Hh. This indicates variation in virulence and suggests the occurrence of different genetic factors for virulence and resistance in M. hapla isolates and potato cultivars, respectively. Despite large differences, the observed levels of resistance were too low to be of practical meaning for breeding, with the exception of resistance to isolate Hh. No significant differences were obtained between isolates of M. chitwoodi and M. fallax or isolate-by-cultivar interaction, indicating neither variation in aggressiveness nor in virulence in the isolates used. A comparative greenhouse experiment gave comparable results for egg-mass index but contradicting results for reproduction factor, most likely because of differences in life cycle, which appeared to be shorter in M. fallax than in M. hapla and M. chitwoodi. The Petri-dish method proved to be accurate for virulence studies of root-knot nematodes on potato culitvars. Meloidogyne fallax was most aggressive on potato, followed by M. chitwoodi, M. hapla race A, and finally by M. hapla race B. Key words: aggressiveness, biadditive model, biplot, plant breeding, resistance, virulence.