Isolation and Identification of a Phytotoxic Substance from Echinochloa crus-galli Infected with Leaf Blight for the Development of Bioherbicides

The infestation of Echinochloa crus-galli (L.) P.Beauv. in crop fields results in significant yield loss in many agricultural systems. Currently, the most effective strategy for controlling E. crus-galli is the application of synthetic herbicides. However, biotypes of E. crus-galli that are resistant to different modes of herbicide action often emerge. Thus, it is necessary to develop alternative control methods and address ecological concerns about synthetic herbicides. During the field survey, we observed diseased E. crus-galli exhibiting symptoms of leaf blight. These symptoms indicate a potential pathogen infection and subsequent phytotoxin production during the pathogenesis. Therefore, we aimed to isolate and identify the phytotoxic substances present in the diseased leaves. Aqueous extracts of the diseased leaves exhibited phytotoxicity, suppressing the growth of Echinochloa crus-galli seedlings in a concentration-dependent manner. A phytotoxic substance was isolated from the leaf extracts through a bioassay-guided separation process using the E. crus-galli bioassay. Spectrum analysis revealed that the phytotoxic substance was monocerin. Monocerin inhibited the growth of coleoptiles and roots of E. crus-galli seedlings at concentrations greater than 30 and 10 μM, respectively, and inhibited germination at concentrations greater than 100 μM. Therefore, monocerin may be involved in the phytotoxic activity exhibited by the extracts of E. crus-galli leaves with blight symptoms. Creating bioherbicides based on the monocerin structure could be an environmentally friendly approach to weed management.