Phenolic compounds in conducive and suppressive soils on clubroot disease of crucifers

Three conducive and three suppresive soils were examined for differences in physical and chemical properties, as well as qualitative and quantitative content of phenolic compounds. The three suppressive soils were slightly alkaline, whereas the three conducive soils were slightly acid. The concentrations of Ca and Mg in suppressive soils were 3-15 times those in conducive soils. The total concentrations of phenolics and humic acids in conducive soils were 2-4 and 2-10 times those in suppressive soils, respectively, whereas the concentrations of gentisic (2,5-dihydroxybenzoic) acid in suppressive soils was higher than that in conducive soils. In bioassay, the radicle growth of Brassica pekinensis was inhibited more by extracts from the conducive soils than those from the suppressive soils. Clubroot disease was significantly suppressed by gentisic acid applied to conducive soil at the rate of 800 mg kg^-1. When seeds were soaked in of gentisic acid solution (800 mg kg^-1, pH 5.5) 24 h before planting, the incidence of the disease dropped from 78 to 41%.