Relative susceptibilities of eleven potato cultivars and breeders' clones toGlobodera pallida pathotype Pa 3, with a discussion of the interpretation of data from pot experiments
Curves according to the equationPf=M′ (1-e−aPi/M′) fitted well to the relations according to Seinhorst's (1993) modelPf=yeyhM(1-e−aPi/M) between initial egg densities of potato cyst nematodes (Globodera rostochiensis, G. pallida)Pi up to 5Th (Th=the tolerance limit of haulm weight) andPf at the end of the growing season (a=maximum rate of reproduction,M, M′=different theoretical maximum egg densities). Variation of estimates ofM′, due to variation of the parameters of the submodelsye andyh for the effect of weight reduction of haulms (and, therefore, of roots) on cyst production and damage to root tissue on egg production, respectively, was small enough to be ignored relative to experimental error. Therefore, ratios of values ofM′, determined in simultaneous pot experiments with different potato cultivars, are reliable measures of the relative host status of these cultivars at initial egg densitiesPi of these nematodes up to about 5Th. Variation between potato cultivars of growth reduction and damage to root tissue by the nematodes reduces the reliability of ratios of rates of relative susceptibility of these cultivars.The ratios between the maximum rates of reproduction ofG. pallida, pathotype Pa 3, on 8 out of 9 cultivars and one breeder's clone of potatoes and this rate on the susceptible cvs Bintje and Irene (relative susceptibilitiesrsa) could be considered to be equal to the ratios of maximum population densitiesM′ on these cultivars (relative susceptibilitiesrsM′) (relative susceptibilities independent of initial egg density). The latter ratios were 0.59 times the first (relative susceptibilities negatively correlated with initial egg density) in one cultivar and one breeder's clone. Relative susceptibilitiesrsa andrsM′ of the tested cultivars and breeders clones suggest the existence of continuous ranges of both relative susceptibilities between 0.50 and 0.15 with, in a great majority of cases,rsa=rsM′.
Host-parasite interaction between the potato tuber rot nematode (Ditylenchus destructor), stem nematode (Ditylenchus dipsaci) and potato
Nematodes are among important pest constraints influencing potato production worldwide. The tuber rot nematode, Ditylenchus destructor Thorne 1945, and the stem nematode, Ditylenchus dipsaci (Kuhn, 1857) Filipjev, 1936, cause lesions on potato tubers degrading their quality and market value. These nematodes are difficult to control due to their wide host range and therefore are listed as quarantine nematodes in many countries. In this PhD thesis, experiments were conducted to investigate the interaction between potato and each of these nematode species. 
 
Molecular and morphometric characterization of different populations of D. destructor and D. dipsaci were studied. Sequence analysis of the cytochrome oxidase subunit I (COI) gene located on the mitochondrial DNA (mtDNA) was used to develop a phylogenetic relationship of the studied populations. The results demonstrated two highly supported clades containing D. destructor populations and the other D. dipsaci populations. The discriminant function analysis (DFA) of the morphometric data of males and females of D. destructor and D. dipsaci populations revealed that these species could be separated using the a-ratio and the highest body width. The combination of both methods molecular and morphometric methods complemented the identity of the species under study. 
 
Two greenhouse experiments were performed to evaluate sources of resistance and tolerance to D. destructor and D. dipsaci in 25 cultivated potato varieties. A standard screening protocol for resistance and tolerance to D. destructor and D. dipsaci was developed. Resistance and tolerance was evaluated based on the current definition of these terms in nematology. Relative susceptibility (RS) and external potato tuber damage were found to be the best methods for resistance and tolerance evaluation respectively. Potato varieties tested were not resistant or tolerant against D. destructor or D. dipsaci. However, some varieties were more tolerant than others. 
 
Pre-plant densities of D. destructor and D. dipsaci and their impact on yield loss were also assessed. Initial population densities of D. destructor and D. dipsaci had significant influence on potato tuber damage and nematodes reproduction factor under greenhouse conditions. Damage caused by D. destructor started at a lower initial population density compared to that caused by D. dipsaci. 
 
Influence of temperature on D. destructor and D. dipsaci population density and their impact on tuber damage was studied under climate chambers. Temperature influenced the nematodes population dynamics and consequently levels of potato tuber damage caused by D. destructor and D. dipsaci. Temperature of 26°C was optimal for both D. destructor and D. dipsaci multiplication compared to 16°C. Although D. destructor and D. dipsaci are reported to have different temperature requirements, both species caused external potato tuber damage at similar temperature ranges. 
 
Beauveria bassiana is a cosmopolitan fungus used mainly in the management of insect pests in potato production. Dual infestation of potatoes with spore suspensions of B. bassiana in the soil together with D. destructor or D. dipsaci benefited the nematodes, thus leading to increased nematodes reproduction and tuber damage. B. bassiana on its own was not harmful to potato. It was hypothesized that B. bassiana played an indirect role in the nematode-plant interaction. In order to add value, experiments are suggested which might help to give detailed mechanisms involved during the nematode-B. bassiana-plant interactions
The Influence of Volunteer Potato Plants in Oats On the Population Density of Heterodera Rostochiensis
The relation between initial and final densities Pi and Pf in plots with 4 and 16 volunteer plants per m2 under oats was in reasonable agreement with the equation: Pf = a (— lnq)-1 (1 — qPi) + bPi, in which a → (Pf — bPi)Pi-1 when Pi → 0, b = the proportion of the eggs of the initial population, that did not hatch during the experiment and q a constant < 1. For both volunteer plant densities, b, as derived from the relation between Pi and Pf at medium and high values of Pi was 0.65. It is probably entirely due to spontaneous hatching. The high value of b suggests that the volunteer plants caused very few larvae to hatch. It is concluded that only larvae that hatched spontaneously, attacked and multiplied on the volunteer plants. It is also concluded, that high nematode densities are not influenced by the presence or absence of volunteer plants in cereals. Lower nematode densities may remain higher under cereals with volunteer plants than under those without. The lower the density of volunteer plants the lower the highest nematode density that is still markedly affected.