Replacement of Rhizoctonia solani on wheat seedlings by a succession of root-rot fungi

Abstract

After tillering, Rhizoctonia solani could rarely be recovered from inoculated wheat plants in a pot experiment. Rhizoctonia attack was confined to seminal roots and only rarely affected nodal roots. It predisposed wheat to a succession of secondary infections of common root-rot and minor pathogens including Fusarium equiseti, F. avenaceum, Cochliobolus sativus, Microdochium bolleyi and Alternaria alternata . An initial effect of Rhizoctonia attack was to reduce root growth. The secondary invaders continued to reduce root growth after R. solani could be isolated no longer. The course of infection in non-sterile soil was compared to that in sterilized soil for five strains of R. solani . Failure to persist on the plant was unrelated to strain virulence or to competition from the soil-borne mycoflora. Wheat plants in sterilized soil recovered rapidly from the seedling infection, but those in non-sterile soil continued to be affected by the secondary infections of bacteria and fungi which became more severe when incited by virulent strains of R. solani . An ecological succession of bacteria and fungi colonized the wheat root system during the pot experiment. Rhizoctonia attack on the cereal plant was largely confined to the juvenile tissues of seedlings. In sterilized soil, damage from all isolates was slight; thus it is not recommended that strains be tested for virulence in an otherwise axenic culture system. In natural soil, the presence of other organisms contributed significantly to damage that is usually attributed to R. solani . Their essential role in the ‘ Rhizoctonia complex’ is important to the course of the disease on cereals.