More than 21% of the Chinese herbs tested contained substances in their aqueous extracts inhibitory to conidial germination of the powdery mildew fungus Oidium murrayae. Extracts from Chinese rhubarb and Japanese knotweed were very effective in controlling powdery mildew on cucumber, pumpkin, and eggplant. The inhibitory substance in Chinese rhubarb was soluble in polar solvents and less soluble in nonpolar solvents. The inhibitor in the aqueous extract was not dialyzable in the membrane tubing with molecular weight cut-off of 14,000, but was exchangeable by anion but not cation exchange resins, indicating that the inhibitor has a molecular weight larger than 14,000 and negative charge on its molecule.
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