Parasitic weed density: drivers and management using cultural controls.

Among the most pressing challenges facing agriculture today is the problem of parasitic weeds. A small number of species belonging to the family Orobanchaceae result in huge annual losses globally. The magnitude of this problem is likely to increase with climate change and increasing connectedness of global production systems. Research into the ecology of these weeds is therefore urgently needed. Striga asiatica is one of the most serious parasitic agricultural weeds, disproportionately affecting subsistence farming in Sub-Saharan Africa, exacerbating food insecurity. Farmers frequently lack access to novel technologies, while herbicide is largely ineffective as a control. In addition, there is a paucity of detailed information on distribution, which is required to understand current drivers, better target control efforts, as well as to predict future risks. To address this, we developed a methodology to enable rapid, large-scale monitoring of Striga populations. We used this approach to uncover the factors that currently drive the abundance and distribution of Striga asiatica in Madagascar. Two long-distance transects were established across the middle-west region of Madagascar, over which S. asiatica abundance in fields was estimated. The resulting dataset indicated the importance of crop variety and legumes in driving Striga density. Moreover, the dataset revealed significant effect of precipitation seasonality, mean temperature and altitude in determining abundance. A composite management index indicated the effect of a range of cultural practices on changes in Striga abundance. The findings support the assertion that single measures are not sufficient for the effective, long-term management of Striga. Furthermore, the composite score has potential as a significant guide of integrated Striga management beyond the geographic range of this study. Finally, I undertook a meta-analysis of available studies studying the effects of intercropping and rotation cropping on parasitic weed density and crop yields. The meta-analysis comprised 1,525 paired observations from 67 studies across 24 countries. It revealed significant effects of both spatial and temporal crop diversification on parasitic weed density reduction. Furthermore, our results show effects of spatial diversification are stronger in suppressing parasitic weeds than temporal effects. Furthermore, the analysis indicates intercrops, which alter both microclimate and soil chemistry such as Crotalaria, Stylosanthes, Berseem clover and Desmodium are most effective in parasitic weed management. This thesis overall serves to underline the importance of a range of management controls in the control of S. asiatica. Most importantly the study showed the effects of resistant host crops, legume intercrops, crop rotation and combined management in reducing Striga density. The meta-analysis largely supported the findings of the field survey on Striga and further indicated the viability of crop diversification as an important tool in parasitic weed management. In addition, the data from both fieldwork and the meta-analysis indicated the important role of climate in determining parasitic weed densities and possible implications for changing future climate