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 field experiments in southern New South Wales where the soil-borne pathogen Rhizoctonia solani had been implicated as a cause of slower seedling growth in direct-drilled soil compared with cultivated soil. Experiments 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 cores from both sites at a very early stage. The growth reductions were completely removed by fumigation, confirming previous reports that biological factors rather than physical factors were responsible for reduced growth. In cores from the Galong site, Rhizoctonia symptoms were severe at the 5-leaf stage (8 on a scale of 0-10) and the reduction in leaf growth at earlier stages coincided with the appearance of Rhizoctonia symptoms on the roots. Seedling growth was not reduced until the severity 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 fumigated controls with no further improvement when the depth of disturbance was increased to 10 cm, suggesting the pathogens responsible were confined to the 0-5 cm layer and were not completely eliminated by disturbance. Pre-sowing disturbance had no effect on shoot growth in fumigated soil. In cores from the Harden site, Rhizoctonia symptoms were moderate at the 5-leaf stage; however, reductions in leaf and root growth at earlier stages occurred 7 days before any symptoms of Rhizoctonia or other pathogens were visible on the roots. The results suggest that other inhibitory organisms are present at the Harden site and cause significant reductions in seedling growth in the absence of visible disease symptoms. Pythium spp, pathogenic to wheat are known to be present at the site but their role has not been confirmed. Further studies are required to identify the inhibitory organisms and to determine the mechanisms by which they reduce early seedling growth.