Fluopyram, a succinate dehydrogenase inhibitor fungicide, has shown potential in controlling Meloidogyne incognita and Rotylenchus reniformis in tomato. The effectiveness of this compound for the control of Ditylenchus dipsaci in sugar beet was evaluated. In this study, laboratory, growth chamber, glasshouse, and field experiments were conducted. In a motility bioassay, the EC 50 value was determined with 3.00 μ g/ml a.i. after 72 h exposure to fluopyram. The growth chamber experiment did not show any effects on D. dipsaci penetration rate; however, field experiments revealed a positive effect of fluopyram applied at planting in reducing D. dipsaci infectivity. The glasshouse experiment confirmed a limited effect of fluopyram on D. dipsaci population development. Under field conditions, despite a reduction of D. dipsaci penetration rates in spring, fluopyram was not effective in reducing the population development until harvest. Consequently, D. dipsaci densities in plant tissue and soil were high at harvest and not different among treatments. However, root-rot symptoms were significantly reduced at harvest. Fluopyram applied at planting showed good potential to reduce root-rot symptoms caused by D. dipsaci in sugar beet. However, for the long-term reduction of nematode populations in soil, further integrated control measures are needed to reduce the risks of substantial yield losses by D. dipsaci.

Summary The stem nematode, Ditylenchus dipsaci, causes severe damage in sugar beet. To date, nematode inoculation through the leaf axil has been used as the standard method to investigate D. dipsaci interaction with sugar beet under in vivo conditions. To get as close as possible to field conditions, we established a new screening mechanism to perform soil inoculation. The most suitable inoculation time point, inoculum level and positioning on sugar beet, as well as rearing process on carrots, were determined. At a 15:8°C day:night temperature regime, penetration rates of D. dipsaci were at maximum following soil inoculation at plant emergence. Up to 115 nematodes penetrated sugar beet seedlings 22 days post-planting with an inoculum level of 1000 nematodes into the soil at plant emergence. Ditylenchus dipsaci penetration rate was higher in plants with soil inoculation than with inoculation on to the leaf axil. High soil moisture increased nematode migration into seedlings when D. dipsaci inoculation was carried out in four holes 1 cm from the plant base. Rearing the nematodes for 35 days at 20°C on carrot discs resulted in an infective inoculum containing up to 50% eggs. We recommend a soil inoculation of 1000 freshly extracted nematodes per pot at plant emergence. The nematode suspension has to be previously reared for 35 days on carrot discs to obtain active D. dipsaci inoculum. This system will allow for the selection of suitable sugar beet genotypes that suppress nematode penetration, in support of breeding for resistance against D. dipsaci.

The stem nematode, Ditylenchus dipsaci, is a severe pest in European sugar beet production. In France, Germany, and Switzerland, D. dipsaci damage in sugar beet varies among specific geographic areas. In this study, the reproduction potential of four geographically distinct D. dipsaci populations was determined using sterile carrot disc cultures. In addition, virulence and pathogenicity were investigated in-vivo using sugar beet. No difference was found in the reproduction potential on carrot discs, as well as penetration rate in sugar beet seedlings. The reproduction rate in sugar beet tissue was significantly affected by the D. dipsaci population used. The population from Seeland (CH) showed the highest number of nematodes per plant at 60 dpi (21,071.8 ± 5340.0), compared to the three other populations contained 3588.6 ± 3858.3, 5136.9 ± 4950.8, and 3579.7 ± 5174.2, respectively. Furthermore, the reproduction rate of D. dipsaci was negatively correlated with fresh biomass of sugar beets at 60 dpi. Based on these results, the D. dipsaci population “Seeland” is suitable for breeding programs to detect resistance in sugar beet. After selecting candidate genotypes/varieties, these should be further evaluated for their field resistance in their targeted growing regions.

Ditylenchus dipsaci is an economically important plant-parasitic nematode affecting European sugar beets. To date, no sugar beet cultivars carrying resistance against D. dipsaci are available to farmers. To find potentially resistant sugar beet lines restricting reproduction and penetration of D. dipsaci, three consecutive in vivo bioassays were carried out. The first experiment determined the penetration rate of D. dipsaci in 79 breeding lines and 14 pre-breeding populations. Based on these results, D. dipsaci penetration and reproduction resistance of eight genotypes was intensively investigated. It could be demonstrated that none of the genotypes showed resistance towards D. dipsaci. However, a high variation of the penetration rate by D. dipsaci was observed among the genotypes. The breeding line ‘DIT_119’ effectively reduced D. dipsaci penetration (34.4 ± 8.8 nematodes/plant at 22 days post-planting) compared to the susceptible control (109.0 ± 16.9) while ensuring a yield comparable to non-inoculated plants. However, the breeding line ‘DIT_119’ did not reduce D. dipsaci reproduction. The paternal line of the cultivar BERETTA KWS, demonstrating a high tolerance to D. dipsaci crown rot symptoms, did not reduce penetration and reproduction. Thus, no correlation can be established between reduced penetration rates, reproduction, and tolerance to D. dipsaci. This study provides an essential basis for the development of resistant sugar beet cultivars to D. dipsaci. The variations observed among genotypes now need to be confirmed with larger-scale screenings.

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.