| Summary: | The soil-borne pathogen Rhizoctonia solani and the potato cyst nematode Globodera rostochiensis are detrimental to the growth and productivity of potato crops in the UK. Previous work has shown that plant parasitic nematodes and fungal pathogens can occasionally interact synergistically to form destructive disease complexes. Furthermore, there is some evidence to suggest that an interaction might exist between G. rostochiensis and R. solani on potato. In order to investigate interactions between G. rostochiensis and R. solani, a series of glasshouse and field experiments were undertaken (2000-2001). In field experiments, plots infested with similar population densities of G. rostochiensis were either uninoculated or inoculated with R. solani. A series of potato plant harvests were undertaken to investigate the effects of nematode infestation on the incidence and severity of R. solani diseases and the associated development of plants. In both experiments, a positive relationship was found between the density of G. rostochiensis juveniles present in potato roots and the incidence of stolons infected by R. solani, 6 weeks after planting. Weaker relationships were found between G. rostochiensis densities and stolon infections at 4 and 8 weeks after planting. In addition, a number of relationships were found between G. rostochiensis infestations and other R. solani disease symptoms, although these were less consistent across the harvest dates. In 2002, the experimental plot design was modified to further investigate the effect of G. rostochiensis on black scurf disease caused by R. solani. However, no relationship was observed between G. rostochiensis infestations and the later development of black scurf on daughter tubersThe glasshouse experiments of this project did not show a direct interaction between G. rostochiensis and R. solani diseases. However, both glasshouse experiments were limited by the method used to infest potting medium with G. rostochiensis. On the basis of results obtained from the field experiment, two controlled environment experiments (2002) were undertaken to investigate the growth rate of R. solani in response to root leachates from potato plants uninfested or infested with G. rostochiensis, at different time intervals after the introduction of nematodes. In addition, the concentration of carbohydrates and nitrogen was determined from the root leachate samples. In both experiments, R. solani isolates were found to have a significantly higher radial growth on media amended with root leachates from G. rostochiensis plants compared to uninfested plants at 12 days after the introduction of nematode treatments. Furthermore, a higher sucrose concentration was detected in root leachates from G. rostochiensis infested plants compared to root leachates from uninfested plants. Since exogenous carbohydrates are known to influence the growth and attraction of R. solani, these results may go some way to explain the interaction found
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