Abstract:Redroot pigweed, Amaranthus retroflexus, is a notorious weed worldwide. To evaluate the potential of Alternaria amaranthi-3 as a mycoherbicide for redroot pigweed control. The effect of inoculum potential, dew period and formulation of emulsions in water (EW) on the pathogenicity of A.amaranthi-3 was studied using pot experiment. The result showed that the inoculum potential significantly affected the pathogenic efficiency. When the redroot pigweed was inoculated with 1×105 spore/mL, the strain spore in aqueous formulations inhibited the growth of redroot pigweed at 35.55%. The inhibition of the aqueous formulation against redroot pigweed growth arrived 75.25% at 1×107 spore/mL concentration. The pathogenicity of the strain was dependent on the dew period. Without a dew period, aqueous formulation of strain provided the inhibition rate of 26.43%. In 48h dew period, the aqueous formulation of strain can provide the inhibition rate of 77.96%. The EW significantly enhanced the pathogenicity of the strain and reduced the requirement of dew period for maximum weed control. In the absence of dew, the biocontrol potential of the EW formulation is better than that of aqueous formulation, the inhibition rate was 88.35% and 26.43% respectively. In 48h dew period, EW formulation and aqueous formulation inhibited the growth of redroot pigweed at 90.59% and 77.96% respectively. These results indicated that when A.amaranthi-3 was properly formulated, A.amaranthi-3 can be effective for controlling redroot pigweed.