Factors influencing the efficacy of the potential microbial herbicide Alternaria alternata (Fr.) Keissler on Amaranthus retroflexus (L.)

Alternaria alternata, applied to run-off 10(7) conidia ml(-1) and given a 6 h dew period, caused an 88.5% reduction in dry weight of 4 to 5 true-leaf plants of Amaranthus retroflexus, This level of control was achieved whethe r the inoculated plants were grown in the glasshouse or outside. Formulatio n of the pathogen in oilseed rape emulsion reduced its dew period requireme nt om 24 it or more to 8 h or less. For example, with 10(6) conidia ml(-1), equivalent effects were found with 8 h dew with emulsion formulation compa red to 48 h dew with the Tween formulation. Allowing spray deposits to dry on inoculated plants for ca 7 h, before exposing to dew increased the effic acy of the pathogen.