Interactions between the beet cyst nematode, Heterodera schachtii (Schmidt), sugar beet and Brassica trap crops

The beet cyst nematode (BCN), Heterodera schachtii (Schmidt), is a commonly occurring and widespread pest of sugar beet (Beta vulgaris ssp. vulgaris) which is found in all regions of the world where sugar beet is grown. The nematode feeds on the fibrous roots of suitable host plants, restricting their growth through impeded water and nutrient uptake and can reduce yield by up to 50%. BCN infestations require careful long term management to limit their spread and reduce yield losses. This Thesis outlines research which aimed to develop a greater understanding of how varieties of sugar beet interact with BCN and also investigate the use of brassica trap crops for BCN population control. To achieve these aims, a series of glasshouse, micro-plot and field experiments were conducted to understand more about how sugar beet interact with the nematode. Resistant sugar beet varieties were shown to be most effective at population management. However, the use of these resistant varieties is hampered by poorer yield potentials. This research project showed that yield increases of resistant varieties can be obtained by establishing higher plant population densities. Varieties of sugar beet which are declared tolerant to BCN infestations have also been evaluated and appear to be partially resistant to BCN. This trait also explains their ability to yield well in infested fields due to reduced feeding and removal of nutrients from the sugar beet. The tolerant varieties exhibited physiological resilience to infestation in relation to chlorophyll content, with initial reductions in chlorophyll in response to nematode infestation but then later a recovery, while in susceptible varieties the reduction in chlorophyll content persisted. The greater understanding of these varieties, and methods to assess them, will enable them to be used to their full potential to reduce yield losses as a consequence of BCN infestations in sugar beet and other host crops. The use of BCN resistant brassica trap crops has also been tested in field experiments. BCN population reductions were obtained from some brassica varieties, a class one radish (Raphanus sativus) and a class two mustard (Sinapis alba), but not all trap crops caused significant population reductions. This response is likely related to different hatching stimuli produced by root leachates from the different trap crop varieties and was further investigated using laboratory bioassays. Some varieties of trap crop showed distinct peaks in their ability to stimulate BCN hatch, whereas, other varieties had a more sustained impact on hatch. This stresses the importance in timing of sowing of trap crops to optimise their ability to control BCN infestations. However, further research is required to understand whether, with the use of BCN tolerant varieties of sugar beet, whether the use of brassica trap crops is justifiable for BCN control alone. As a whole, the findings contained within this PhD thesis will help to develop future control strategies to manage BCN infestations and protect yields of sugar beet