Described are new strains of Drechslera spp., which possess herbicidal effects against all varieties of barnyard grass, i.e., Echinochloa spp. as weed but do not give any influence to crops under cultivation such as rice. Weed control agents according to the present invention, which contain one or more of the novel strains, have high safety and selectivity, thereby providing practical control effects against barnyard grass. Weed control compositions according to the present invention, which contain one or more of the new strains in combination with one or more chemical herbicides, have sufficient herbicidal effects even at dosages which are too low to permit control of weeds when they are applied singly, thereby making it possible to reduce the dosage of herbicides. The weed control compositions can therefore contribute to the prevention of environmental pollution and the occurrence of weeds with acquired pesticide resistance.