Interactions between the closely related potato cyst nematode species Globodera rostochiensis (Woll.) and G. pallida (Stone)

in integrated control of the potato cyst nematodes, Globodera rostochiensis and G. pallida , the use of resistant varieties is an important strategy, especially now the application of agrochemicals in The Netherlands is increasingly restricted. Repeated cropping of varieties with the same or similar resistance genes produces a selection pressure on nematode populations leading to an increase in virulence and a progressive loss of resistance. To prolong the efficient use of resistant varieties, alternate cropping of varieties with species specific resistance instead of totally resistant varieties might prove to be a more successful strategy. In this situation the population development of each nematode species is alternately suppressed and selection for increased virulence will be reduced at the same time. To develop rotation schemes with these objectives information is needed on the reproduction of the species on the varieties to be used, the actual species composition and distribution in the field, and the interactions between the nematode species. The latter was the subject of this thesis (chapter 1)A method has been developed to quantify species ratios in mixed populations. The method is based on the separation of species specific thermostable proteins by SDS- PAGE. Densitometric analysis of the 17 kD protein of G. pallida and the 18 kD protein of G. rostochiensis revealed a high correlation (R=0.93) with the species ratio in the mixed samples. Within the limits of 10 to 90% of each species, one can estimate with 95% reliability the species composition with 3 to 6% deviation (chapter 2).To determine whether interactions between the species takes place mixtures of G. rostochiensis (Ro1) and G. pallida (Pa3) were reared on susceptible (cv. Bingtje) and species specific resistant (cv. Elkana and clone ZB35-29) potato varieties at different nematode densities. The proportion of newly formed cysts belonging to each species was determined by way of SDS-PAGE and the relative population increase of each species, defined as the final population (P f ) divided by the initial population (P i ), was calculated. Comparisons of the relative population increase of each species in the single populations and in the mixtures revealed that interaction between the two species occurred. The relative population increase of G.rostochiensis was significantly decreased when G. pallida was highly dominant in the mixed populations. These effects were density independent which makes it unlikely that exploitation competition is responsible for this. It was suggested that interference competition for mates might be responsible for these interaction effects (chapter 3).Differences in hatching behaviour of the two species in response to the host plant could interfere in the interaction. Therefore, hatching differences of G.rostochiensis and G.pallida were assessed in potato root diffusate (PRD) of cv. Bintje, cv. Elkana and clone ZB35-29. G.pallida hatched better in the PRDs than G. rostochiensis. However, the reduced relative population increase of G.rostochiensis was found at all densities indicating that G.pallida did not reach the roots sooner and occupy all available feeding sites. Therefore, it was concluded that the differences in hatching behaviour did not play any significant role in the interactions.It was shown that the experimental test conditions strongly influenced the hatching results. The water type used in the hatching tests had a significant discriminating effect on the species; . rostochiensis had a relative hatch of 60 to 90% in demineralized and tap water, whereas the hatch of G.pallida never exceded 15%. These differences were independent of the various cyst batches that were used or the different years the tests were carried out. Silver sand percolate had an inhibiting effect on the hatching of both nematode species. The high concentration of boron may explain the latter effect (chapter 4).An investigation was conducted to study whether both species could hybridize in pots under optimal environmental conditions and to see what consequences this might have for the next generations. Pot experiments were carried out for four years with 50% mixtures of both species and single species populations, reared on the susceptible cv. Bintje. Analysis of the cyst production, the cyst contents and the relative population increase of each species show that the two species do not interbreed. They reproduce independently and are in stable coexistence in these circumstances. The fact that hybridization did not take place confirms the idea that the observed interaction is mainly a result of interference competition (chapter 5).The different reaction of the species to the initial hatching trigger, the consistently different behaviour in hatching and reproduction and the absence of hybridization indicate that the species are much more divergent than the term sibling species suggests. Findings on one of the two species cannot be uncritically translated to the othet species. A thorough study of the biology of the species on natural host plants in the country of origin will give a better understanding of the ecological differences.The results were obtained under controlled conditions with optimal chances for interspecific encounters leading to interactions. Only in one of the experiments was a significant but slight interaction found. In field situations conditions are much more heterogeneous and variable causing seggregation of the species. Additionally, nematodes infest the soil as cysts ensuring a certain amount of aggregation. It is therefore unlikely that interactions occur in the field, hence both species populations will develop independently.For control strategies, in particular for using the right resistant varieties, it is essential to know which species inhabits the soil and additionally what species composition is present. For monitoring field populations the method described here has some draw backs, a recently developed ELISA test might be of more use.To prevent underestimation of the total population increase one ought to determine the initial density of both species separately, as nematode reproduction is dependent on its own population density before planting. Predictions of the population development based the separate species are relatively straight forward because interactions between the different potato cyst nematode species in the field are of no importance (chapter 6).