Yield reduction of cassava infected with African cassava mosaic virus (ACMV) was greater when plants were infected from the outset as cuttings than later by the whitefly vectorBemisia tabacl.Early infection byB. tabacicaused greater losses than late infection. When symptoms first appeared, more than four months after planting, there was no significant yield reduction. Yield reductions in plots where diseased plants were inter‐mixed with healthy ones and subject to inter‐plant competition were much greater than when they were assessed using separate healthy and infected plots. Crop losses due to ACMV in the cultiver CB were estimated atca40%.