Investigation of resistance against Ditylenchus dipsaci on sugar beet

The stem and bulb nematode Ditylenchus dipsaci (Kuhn 1857) Filipjev 1936 is a migratory endoparasite ranked in the top ten plant-parasitic nematodes worldwide. Ditylenchus dipsaci has emerged as an economically threatening pest in the European sugar beet (Beta vulgaris L.) production. In Germany and Switzerland, some major sugar beet growing regions are particularly affected by D. dipsaci. The nematode migrates into the plant in the spring and reproduces in the hypocotyl during the growing season. Soil-borne pathogens introduced by D. dipsaci leads to crown root rotting and plant death.
The broad range of host plants of D. dipsaci hinders crop rotation strategies for a successful management of this nematode. To date, no sugar beet cultivars carrying resistance towards D. dipsaci are available for sugar beet producers, depriving them of effective measures against this nematode. The lack of control measures and the growing public demand for sustainable sugar production have encouraged breeders to develop resistant cultivars. For this reason, this thesis aimed to investigate resistance against D. dipsaci on sugar beet.
Before investigating the interaction between sugar beet and the nematode, the development of an in vivo test system was required. It aimed to replace above-ground D. dipsaci inoculation with a soil inoculation more closely related to field conditions. The most suitable inoculation time point, inoculum level, and positioning on sugar beets, as well as rearing process on carrots, were determined. At a 15:8°C day:night temperature regime, penetration rates of D. dipsaci into sugar beet seedlings were at maximum following soil inoculation at plant emergence. High soil moisture increased nematode migration into seedlings when D. dipsaci inoculation was carried out in four holes 1 cm from the plant base. The nematode suspension was previously reared for 35 days on carrot discs to obtain active D. dipsaci inoculum.
To find potentially resistant sugar beet restricting reproduction and penetration of D. dipsaci, in vivo bioassays were carried out with 15 pre‑breeding populations and 79 breeding lines. It could be demonstrated that none of the genotypes showed complete resistance towards D. dipsaci. However, a high variation of the penetration rate by D. dipsaci was observed among the genotypes. They also responded differently to the fresh biomass reduction caused by the nematode combined with soil-borne pathogens.
Based on these results, candidates for partial resistance were further investigated in microplot experiments conducted in the Rhineland (DE) and Seeland (CH) regions. The sugar beet genotype effect on D. dipsaci penetration could not be validated. The genotypes did not show a sufficient tolerance towards the rotting of the plant tissue. Nematode pathogenicity and virulence differed depending on experiment locations and years.
Finally, virulence and pathogenicity of four D. dipsaci populations were investigated under in vivo conditions. No difference was found in D. dipsaci penetration rate into sugar beet seedlings. However, Seeland (CH) population showed a significantly higher reproduction on sugar beets than the others populations, validating observations obtained in microplot experiments.