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Application of 250 L ha 2 1 containing 10 7 Alternaria alternata conidia ml 2 1 caused 61% reduction in dry weight and a 45% mortality in Amaranthus retro¯exus plants growing in a wheat crop. However, only 10 -22% of the applied conidia were retained on the leaf. In one experiment, conidia remained viable for 4 days on the leaf surface until conditions were favourabl e for germination. Competition from the wheat crop, as well as its creation of a moist micro-climate, improved the control of Am. retro¯exus by A. alternata. Total control of Am. retro¯exus may be diYcult to achieve with A. alternata, but suppression to reduce or eliminate its competitive eVect on crops is possible.