Effect of Oriental Medicinal Plant Extracts on Spore Germination of Alternaria brassicicola and Nature of Inhibitory Substances from Speedweed.

Among 65 species of oriental medicinal plants tested, 39 contained substances inhibitory to conidial germination of Alternaria brassicicola, with the most inhibitory extract from speedweed (Polygonum perfoliatum). The inhibitory substances in dried speedweed were insoluble in water. However, the inhibitors were readily extracted with ethanol or methanol, but not with acetone, ether, or chloroform. The ethanol extract was very effective in controlling black leaf spot of spoon cabbage, Brassica campestris subsp. chinensis, caused by A. brassicicola. The inhibitory effect of the extract was not affected by treatment with anion exchange resins, but was partially reduced by cation exchange resins, indicating the presence of two inhibitory substances in the extract, one with a positive charge and the other with no charge on its molecule. The inhibitory substances in the extract were dialyzable with molecular weight minimums of 10,000 or 1,000, but not 500 or 100, suggesting that both inhibitors have molecular weights between 500 and 1,000.