AbstractBACKGROUND: Striga hermonthica (Del.) Benth. (witchweed) poses the greatest biological constraint to food production in sub‐Saharan Africa (SSA). Control options for Striga are currently largely ineffective or unavailable to farmers, and other management possibilities are urgently needed. Biological control obviates some of the problems of several of the other techniques and provides a management option that is durable and environmentally responsive. The efficacy of S. hermonthica control using different formulations of three isolates of Fusarium oxysporum Schlecht. emend. Synder & Hans f. sp. strigae was tested on Striga‐resistant and Striga‐susceptible varieties of sorghum and maize under African field conditions for the first time.RESULTS: Isolates PSM197 and Foxy 2 were effective in witchweed repression, especially when applied as pesta granules. Isolate M12‐4A was less effective under the field conditions investigated. Application of the fungi was generally more beneficial in maize than in sorghum for the varieties tested. Application of the biocontrol agent caused significant decreases in the number of flowering Striga plants, and hence deposition of seeds with impact of enhancing future crop yield.CONCLUSIONS: Synergistic effects between the Striga‐resistant maize line and Fusarium oxysporum f. sp strigae led to over 90% reduction in Striga emergence. These results will further encourage the distribution of the isolates tested or selection of country‐specific relatives as viable and environmentally safe biocontrol agents to be used against Striga. Pesta was the most effective formulation, while seed coating may be more cost effective. Copyright © 2009 Society of Chemical Industry