Abstract/Summary

The supply of faba bean in UK is unreliable. This may be linked to many factors, including biotic pressure especially from pathogens. The study investigated the interaction between a biotrophic nematode (Ditylenchus gigas) and a necrotrophic fungus (Botrytis fabae). In faba bean, B. fabae causes chocolate spot disease while D. gigas causes stem lesions. D. gigas is the dominant species of Ditylenchus on faba bean in the UK. Its spread is controlled in part by certification schemes. To understand how fast isolated infections of D. gigas evading certification will spread spatially, field experiments building on the work in other species of Ditylenchus were conducted over two seasons. These suggest that spatial spread of the nematode on beans through the growing season is dependent on distance of inoculum from the plant, the orientation of the inoculum from the plant and environmental factors in the field. Nematodes spread from a centrally pre-inoculated plant to a distance of 1.6 m within a season. B. fabae often co-exists in the presence of D. gigas and therefore glasshouse investigations were designed to test the hypothesis that B. fabae (as a necrotroph) will decrease susceptibility to D. gigas (a biotroph) and together they will reciprocally affect each other’s population density and reduce the productivity of faba bean. An initial experiment involved nematodes alone, while a second experiment involved D. gigas and B. fabae co-inoculations. Similar data from the two experiments were pooled together for analysis using the mixed model for multiple experiments in GenStat. Antagonistic interaction on the population of D. gigas was observed, as the number of nematodes extracted from lone inoculation of D. gigas was more than those from co-inoculation of the two organisms, suggesting an induced response. However, a synergistic interaction was observed on grain yield, as the reduction in grain yield was more with co-inoculation than lone inoculation of either organism. In the light of work on induced defence mechanisms to plant pathogens and pests, the hypothesis that the order of infection (i.e., whether D. gigas infects before B. fabae or vice versa) will have an effect on induced resistance and affect crop production was then tested. Potted faba bean (cv. Fuego) were first inoculated with either B. fabae or D. gigas at 2 weeks after planting. A second inoculation was done 2 weeks after the first one. The second B. fabae inoculation was applied on the upper leaves with protection to prevent spill over to the previously inoculated leaves. Inoculation of B. fabae did not induced defence response v against D. gigas. However, pre-inoculation of B. fabae reduced subsequent B. fabae infection, suggesting an induced defence response specific to this species. Two successive field trials were conducted to determine whether the glasshouse results obtained previously were applicable to the field situation and tested the hypothesis that plant nutrition (application of fertilizers) and/or plant variety affect crop production. None of the results were significant at the 5 % level. The results from the first season were consistent with those from the controlled environment experiment but those from second season differed. The trend indicated that cv. Babylon was more resistant to chocolate spot than cv. Fuego. Also, potassium sulphate fertilizer very slightly but consistently reduced the severity of chocolate spot and rust compared to potassium chloride. It became increasingly apparent that the methods of monitoring and quantifying D. gigas in the studies were time consuming. Therefore, a qPCR assay was developed for the simultaneous quantification and identification of D. gigas and B. fabae in faba bean, although specificity of the assay at species level was not tested.