MEDICINAL-PLANTS FOR SUPPRESSING SOIL-BORNE PLANT-DISEASES II - SUPPRESSIVE EFFECT OF GERANIUM PRATENSE L. ON COMMON SCAB OF POTATO AND IDENTIFICATION OF THE ACTIVE COMPOUND

The possibility of controlling common scab of potato with Geranium pra tense, root of which show an antimicrobial activity, was investigated, Application of dried root powder or powdered methanol extract from G. pratense roots to the soil tended to decrease the area with scab lesi ons on tubers sampled from the soil with a high inoculum level. Mixed- cropping of potato with G. pratense also decreased the incidence of th e disease in the soil with a low inoculum level. We isolated an antimi crobial substance against the pathogen, Streptomyces scabies, from a m ethanol extract of G. pratense roots and identified it as geraniin by MS and H-1-and C-13-NMR, Geraniin accounted for as much as 15% of the root dry weight based on quantitative analysis by HPLC, and its antimi crobial activity corresponded to 1.25% of that of streptomycin based o n the paper disk method. These findings suggest that G. pratense could be used as organic amendment or companion crop for the control of com mon scab of potato.