Van der Beek, J. G., Poleij, L. M., Zijlstra, C., Janssen, R., and Janssen, G. J. W. 1998 The root-knot nematode species Meloidogyne chitwoodi Golden et al. and M. hapla Chitwood can cause growth and yield reductions and quality loss in a large number of crops. In potato (Solanum tuberosum L.), these root-knot nematodes are able to reproduce on roots, causing yield reduction (21). M. fallax Karssen, which was described recently (22), is allopatric with M. chitwoodi in the Netherlands, and its host range is similar to that of M. chitwoodi. M. hapla causes little quality damage due to gall formation on the outside of tubers, in contrast to M. chitwoodi (7). Recent observations indicate that M. chitwoodi and M. fallax reproduce similarly on tubers, and both result in quality loss (J. G. van der Beek, unpublished data). In addition to yield reduction and quality loss, an increase in the nematode population due to cultivation of a susceptible host makes these pathogens a potential threat to agriculture in Western Europe, because many crops in the current rotation schemes are good hosts (33). However, M. hapla does not reproduce on monocotyledonous crops, which may be used in rotation with dicotyledons as an effective control measure.The introduction of resistant potato cultivars also would improve the efficiency of crop rotation in reducing this threat. However, in current potato cultivars, no desirable level of resistance to M. chitwoodi and M. hapla has been identified (7,17).Thus far, intraspecific variation in M. chitwoodi has been described by the occurrence of three host races that can be distinguished on alfalfa cv. Thor (nonhost for race 1 and host for races 2 and 3), carrot cv. Red Cored Chantenay (host for race 1 and nonhost for races 2 and 3), and S. bulbocastanum Dun. SB22 (nonhost for races 1 and 2 and host for race 3) (26,27,36).Resistance to M. chitwoodi and M. fallax has been identified in accessions of wild tuber-bearing Solanum species, originating mainly from Central America, such as S. bulbocastanum and S. hougasii Corr. (5,6,18). Resistance to M. chitwoodi has been transferred from a resistant genotype of S. bulbocastanum to the gene pool of the cultivated potato by somatic hybridization (1,7), and a single gene has been localized on chromosome 11 that appears to control this resistance (8). Also, various levels of resistance to M. hapla have been reported (9,15,18). Recent investigations indicated the presence of variation in host responses to different M. hapla isolates on some genotypes of S. bulbocastanum, S. hougasii, S. chacoense Bitt., S. gourlayi, S. sparsipilum, and S. spegazzinii that were selected for resistance (19).In general, the efficacy of resistance depends largely on genetic variation in the virulence of the pathogen and factors involved in the durability of resistance. Often the presence of resistance is concluded from experiments with one or a few isolates. The current study focused on the variation in virulence within M. chitwoodi, M. fallax, and M. hapla by testing s...