Induction of Suppressiveness toRhizoctonia solaniin Soil

Abstract

When soil was infested with large (> 589 .mu.m) propagules of R. solani, soil suppressiveness to R. solani was generated only in acidified or naturally acidic soils after replanting radish [Raphanus sativus], alfalfa [Medicago sativa] or sugar beet [Beta vulgaris] at weekly intervals. As suppressiveness increased, the propagule density of Trichoderma harzianum also increased. Acidic pH levels enhanced in vitro growth of T. harzianum more than that of R. solani and stimulated its conidiophore formation and spore germination. The mechanism of antagonism was parasitism followed by lysis rather than antibiosis; the fungus released active .beta.-(1-3) glucanase and chitinase when grown on R. solani cell walls and lysed both living mycelium and cell walls of that fungus. In raw soil, T. harzianum also controlled damping-off of radish.