Size of bare-patches in wheat caused by Rhizoctonia solani AG-8 is determined by the established mycelial network at sowing

Growth and establishment of Rhizoctonia solani AG-8 in soil was studied in specially designed pots. Pots were divided into two sections using a 25 μm mesh attached to a plastic frame fixed across the centre of the pot. One-half of the pots (section A) were infested with the pathogen and incubated for 24 d. The pathogen was allowed to grow into the other half (section B), by removing a plastic sheet placed along the mesh, 21 or 10 d before sowing, at the time of sowing or 10 d after sowing. When the fungus was allowed to grow into section B for 21 or 10 d before sowing, it caused 89 or 75% reduction in root and shoot production, respectively, compared to the plants grown in the uninfested control pots. When the pathogen was allowed to colonise the soil in section B at the time of sowing or 10 d after sowing it failed to cause significant reduction in root and shoot weights compared to uninfested control pots. Established mycelial network of the pathogen in the soil is considered to survive effectively in the hot dry summer conditions between cropping seasons. These results indicate that the disease patch area is pre-determined at the time of sowing by the area of the established network of R. solani present at that time.