CONTRIBUTION OF RHIZOCTONIA TO REDUCED SEEDLING GROWTH OF DIRECT-DRILLED WHEAT - STUDIES WITH INTACT CORES

Wheat seedlings were grown in intact cores of soil removed from 2 fiel d experiments in southern New South Wales where the soil-borne pathoge n Rhizoctonia solani had been implicated as a cause of slower seedling growth in direct-drilled soil compared with cultivated soil. Experime nts were conducted with intact cores in controlled environments using fumigation to assess the role of Rhizoctonia in the growth responses. Seedling growth was lower in direct-drilled cores than cultivated core s from both sites at a very early stage. The growth reductions were co mpletely removed by fumigation, confirming previous reports that biolo gical factors rather than physical factors were responsible for reduce d growth. In cores from the Galong site, Rhizoctonia symptoms were sev ere at the 5-leaf stage (8 on a scale of 0-10) and the reduction in le af growth at earlier stages coincided with the appearance of Rhizocton ia symptoms on the roots. Seedling growth was not reduced until the se verity was >5. This corresponded with a root-cutting experiment where shoot growth was unaffected until 3 of the 5 seminal roots of healthy seedlings were removed. Pre-sowing soil disturbance to a depth of 5 cm in unfumigated soil increased shoot growth from 47 to 84% of the fumi gated controls with no further improvement when the depth of disturban ce was increased to 10 cm, suggesting the pathogens responsible were c onfined to tile 0-5 cm layer and were not completely eliminated by dis turbance. Pre-sowing disturbance had no effect on shoot growth in fumi gated soil. In cores from the Harden site, Rhizoctonia symptoms were m oderate at the 5-leaf stage; however, reductions in leaf and root grow th at earlier stages occurred 7 days before any symptoms of Rhizoctoni a or other pathogens were visible on the roots. The results suggest th at other inhibitory organisms are present at the Harden site and cause significant reductions in seedling growth in the absence of visible d isease symptoms. Pythium spp, pathogenic to wheat are known to be pres ent at the site but their role has not been confirmed. Further studies are required to identify the inhibitory organisms and to determine th e mechanisms by which they reduce early seedling growth.