Resistance to root-knot nematodes, Meloidogyne spp., in potato

Root-knot nematodes, Meloidogyne spp., are world-wide one of the most damaging pests to arable farming. In North Western Europe, the species M. chitwoodi, M. fallax and M. hapla are becoming a serious problem in potato growing areas as a result of recent changes in crop rotation, that now include highly profitable host crops, and a reduced use of nematicides in potato. The root-knot nematodes can cause yield reduction and deteriorate the quality of the tuber to an unmarketable product. Since root-knot nematodes can multiply inside the tuber, infected potato tubers are a threat to infest Meloidogyne-free fields. To prevent this way of dispersal, quarantine measures will be in force for seed tubers in the countries of the European Community from August 1997.Plant resistance is an effective, economical and environmentally safe alternative to control root-knot nematodes, but resistance is lacking in the presently used potato cultivars. The goal of the research described in this thesis was to identify and evaluate sources of resistance to M. chitwoodi, M. fallax and M. hapla and, if present, to initiate the transfer of resistance into cultivated potato.A large screening of wild Solanum spp. was performed to identify sources of resistance to M. chitwoodi, M. fallax and/or M. hapla by selecting seedlings, which showed no to hardly any reproduction of the nematodes on the roots. High levels of resistance to both M. chitwoodi and M. fallax were observed in genotypes of S. bulbocastanum, S. cardiophyllum, S. brachistotrichum, S. fendleri and S. hougasii, whereas additionally moderate resistance to M. fallax was present in S. stoloniferum and S. chacoense and to M. chitwoodi in S. gourlayi. More Solanum species were selected with resistance to M. hapla, namely S. bulbocastanum, S. brachistotrichum, S. cardiophyllum, S. arnezii, S. chacoense, S. tarijense, S. boliviense, S. gourlayi, S. microdontum, S. sparsipilum, S spegazzinii, S. sucrense, S. acaule and S. hougasii. In general, resistance to M. chitwoodi and M. fallax was restricted to Solanum species originating from Central America, whereas resistance to M. hapla was present in numerous Central- and South American Solanum species. This might indicate that co-evolution has occurred between M. chitwoodi and related M. fallax and Central American Solanum species.The following step was the evaluation of resistance with regard to the effectivity and working spectrum. In a glasshouse, resistant and susceptible Solanum genotypes were tested with two to four populations of M. chitwoodi, M. fallax and M. hapla to determine the level of resistance and to detect the presence of virulent populations within these Meloidogyne species. Resistant genotypes of S. bulbocastanum, S. hougasii, S. cardiophyllum and S. fendleri showed an almost absolute level of resistance and were able to suppress all populations of M. chitwoodi and M. fallax tested. Some genotypes of S. chacoense and S. stoloniferum showed moderate resistance to M. fallax, but not or in a lesser extent to M. chitwoodi. In contrast, large differences in virulence were observed between the four tested populations of M. hapla on resistant genotypes of S. bulbocastanum, S. hougasii, S. chacoense, S. gourlayi, S. sparsipilum and S. spegazzinii. Some genotypes with resistance to M. chitwoodi, M. fallax andlor M. hapla were also tested against isolates of the tropical and subtropical Meloidogyne species M. incognita, M. arenaria and M. javanica, but resistance was not effective to these high temperature adapted species.The effectivity of resistance of some selected wild Solanum species was also evaluated under natural field conditions. In two fields in the Netherlands, naturally infested with M. hapla or M. fallax, the level of infection of soil surrounding resistant and susceptible genotypes was followed during a growing season. From August onwards, large differences in number of second-stage juveniles were present between resistant and susceptible genotypes. At the end of the growing season, the level of infection in soil of resistant wild Solanum genotypes was equal or lower compared to the beginning, whereas soil surrounding susceptible wild and cultivated genotypes showed a 7- to 22-fold increase of nematode infection. The results were comparable with the resistance tests in glasshouse experiments.An important feature for a rapid introgression of resistance is the inheritance and this has been investigated for the resistance to M. chitwoodi and M. fallax in S.fendleri, S. hougasii and S. stoloniferum. Although these Solanum species are polyploid, a disomic genetic behaviour can be expected as earlier indicated by cytogenetic and genetic studies. Various populations were produced from crosses between resistant and susceptible plants, self- pollinations and backcrosses within the wild Solanum species and segregation patterns of progenies in resistant and susceptible plants were analysed. The progeny tests of S. fendleri clearly indicated the action of a single dominantly inherited gene, effective against both M. chitwoodi and M. fallax, and the symbol R mc2 is proposed for this gene. In the case of S. hougasii, difficulties were met in producing backcross populations, but results also indicated the presence of a simple dominant factor for both nematode species. From the results of progeny tests of S. stoloniferum, it was concluded that several additive genes are involved.The introgression of resistance from various wild Central American Solanum species into the cultivated potato has been initiated through interspecific hybridisation. Crosses were made between diploid S. tuberosum and diploid S. bulbocastanum, S. brachistotrichum and S. cardiophyllum, but no plants were obtained from these crosses. From crosses of tetraploid S. tuberosum with tetraploid S. stoloniferum and S. fendleri, and of diploid S. tuberosum with hexaploid S. hougasii few seeds leading to tetraploid hybrids were obtained, sometimes after in vitro culture of immature seeds. The hybrid status was confirmed with RAPD markers and the ploidy level was analysed using flow cytometry. These cross combinations were thought not to be possible according to the Endosperm Balance Number hypothesis and the hybrids obtained are considered to be escapes. Backcrosses were made and a variable number of seeds leading to first backcrossed genotypes (BC 1 's) was produced depending on the hybrid genotype. The introgression of resistance to root-knot nematodes from S.fendleri, S. stoloniferum and S. hougasii has now advanced to the evaluation of resistant BC 1 's for other traits before continuation of further backcrosses.In conclusion, resistance to the root-knot nematodes M. chitwoodi, M. fallax and M. hapla has been identified in various Solanum species and has the potential to become an effective tool to control these pathogens under field conditions after transfer into cultivated potato. The first steps of introgression of resistance into S. tuberosum has been made. The introduction of multiple sources of resistance in new potato cultivars will enable a resistance management based on durable exploitation of useful resistance genes from natural resources.