No abstract

A series of pot and laboratory experiments was carried out to assess the effects of N status of sorghum roots and timing of N application (as NH 4 NO 3 ) on the germination of Striga hermonthica seeds. Root N concentrations varied from 10 to 26 mg N g 21 . The cut root and the root exudates technique used in assaying S. hermonthica seed germination gave similar results. However, the cut root technique was easier to handle and more discriminating at low germination levels. S. hermonthica seed germination per unit sorghum root mass followed a brokenstick model. It decreased with increasing root N concentrations, reaching lowest levels at a root N concentration of 19.5 mg N g 21 , after which no further reduction occurred. It was not possible to reduce S. hermonthica seed germination to a zero level. Timing of N application influenced the time a higher N concentration is reached, not the S. hermonthica seed germination. Both timing and rate of N application are important in maintaining root N concentrations above 19.5 mg N g 21 , thereby potentially reducing S. hermonthica germination in the field. Translation of results to reductions in infection levels and yield losses is hampered by density-dependent relations after the S. hermonthica germination stage.

a b s t r a c tSurveys of Striga (S. hermonthica (Del.) Benth.) infestation in northern Cameroon over the period 1987-2005 assessed Striga dynamics and evaluated its control strategies. In that period the percentage of Striga-infested fields increased in North and Far-North Provinces. Striga incidence increased more in maize fields than in the already heavily infested sorghum fields, where it remained almost constant. During the study period increased land pressure led to a reduction in the use of fallow and a higher frequency of cereal (mono-) cropping. Yields from farmers' fields did not correlate with Striga incidence, confirming farmers' prioritization of soil fertility, weeds, and labour for weeding as production constraints, rather than Striga. We discuss how conceptualization of Striga as a weed in the research arena may have led to a misunderstanding of farmers' constraints. The decline of the cotton industry reduced farmers' access to fertilizers, while access to organic manure remained limited, increasing the soil fertility constraint. We conclude that two decades of emphasis on Striga were unsuccessful. Enhanced crop yield through soil fertility management should be the entry point to tackle low yields and further worsening of the Striga situation.