1.14.4
VIRULENCE IN THE ROOT-KNOT NEMATODE SPECIES MELOIDOGYNE CHITWOODI TOWARDS RESISTANCE IN THE WILD POTATO SOLANUM FENDLERI

OE SCHOLTEN1, GJW JANSSEN1,2, A VAN NOREL1 and J HOOGENDOORN1

1Centre for Plant Breeding and Reproduction Research (CPRO-DLO), PO Box 16, 6700 AA Wageningen, The Netherlands; 2Present address, Novartis Seeds AB, PO Box 302, 261 23 Landskrona, Sweden

Background and objectives
The root-knot nematodes Meloidogyne chitwoodi, M. fallax and M. hapla are a potential threat to the potato crop in north-west Europe and the USA. Resistance would be effective in controlling them, but appears to be absent in available potato cultivars. In wild tuber-bearing Solanum species, high levels of resistance to these nematode species have been identified and introgression into the cultivated potato gene pool is in progress. The durability of these resistances will depend on the frequency of using the resistance genes in potato cultivation and on the presence of virulent nematode populations in the soil. In this study the possibility of selecting virulence from a virtually avirulent root-knot nematode population towards resistance in wild potato has been investigated.

Materials and methods
The plant material consisted of genotypes belonging to S. bulbocastanum, S. fendleri, S. hougasii and S. stoloniferum, having moderate to high levels of resistance to M. chitwoodi and M. fallax and the susceptible control cultivar Nicola. Nematode lines of M. chitwoodi were obtained from individually isolated egg masses, grown on S. fendleri. Resistance tests were carried out, first on S. fendleri and Nicola, and secondly on all genotypes mentioned above.

Results and conclusions
Among 185 seedlings of the S. fendleri genotype tested, 12 were found with one, and one with two egg masses. After multiplication on tomato, eight populations were obtained. A complete susceptible response of S. fendleri was found after one selection cycle with five of the eight nematode populations, indicating that virulence was already present in the population in a relatively low percentage. The other three populations remained completely avirulent towards the resistance.The virulent populations were also able to circumvent the resistance in another resistant genotype of S. fendleri. Clear differences were noticed between the virulent populations when inoculated on S. bulbocastanum and S. hougasii. Some populations showed an intermediate number of root knots compared to the susceptible control, whereas other populations were completely avirulent on these genotypes. Resistance of S. stoloniferum remained effective against the five virulent populations. Results indicate a possibly monogenic inheritance of virulence in M. chitwoodi towards resistance in S. fendleri. More virulence factors may be involved to explain the differences that were noticed between the virulent populations. Ploidy differences, sexual barriers and genetical distance between S. fendleri and S. bulbocastanum and S. hougasii mean that a close homology of the resistance genes due to introgression is highly unlikely. However, resistance in S. fendleri, S. hougasii and S. bulbocastanum is proven or suggested to be based on single dominant resistance genes [1, 2]. Resistance in S. stoloniferum is based on several resistance genes [2]. The results indicate that the threat of selecting virulence should not prohibit the introduction and use of Meloidogyne resistance into potato cultivars, but should stimulate the introduction of multiple resistant sources which differ in virulence selection.

References
1. Brown CR, Yang C-P, Mojtahedi H, Santo GS, 1996. Theoretical and Applied Genetics 92, 572-576.
2. Janssen GJW, Van Norel A, Janssen R, Hoogendoorn J, 1997. Theoretical and Applied Genetics 94, 692-700.