Induction of Soil Suppressiveness Against Rhizoctonia solani by Incorporation of Dried Plant Residues into Soil

Suppressive effects of soil amendment with residues of 12 cultivars of Brassica rapa on damping-off of sugar beet were evaluated in soils infested with Rhizoctonia solani. Residues of clover and peanut were tested as noncruciferous controls. The incidence of damping-off was significantly and consistently suppressed in the soils amended with residues of clover, peanut, and B. rapa subsp. rapifera ‘Saori’, but only the volatile substance produced from water-imbibed residue of cv. Saori exhibited a distinct inhibitory effect on mycelial growth of R. solani. Nonetheless, disease suppression in such residue-amended soils was diminished or nullified when antibacterial antibiotics were applied to the soils, suggesting that proliferation of antagonistic bacteria resident to the soils were responsible for disease suppression. When the seed (pericarps) colonized by R. solani in the infested soil without residues were replanted into the soils amended with such residues, damping-off was suppressed in all cases. In contrast, when seed that had been colonized by microorganisms in the soils containing the residues were replanted into the infested soil, damping-off was not suppressed. The evidence indicates that the laimosphere, but not the spermosphere, is the site for the antagonistic microbial interaction, which is the chief principle of soil suppressiveness against Rhizoctonia damping-off.