Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands 6907, Western Australia

Background and objectives

Rhizoctonia bare patch disease caused by Rhizoctonia solani Kuhn (anastomosis group 8) is the major disease problem in the no-till farming system in southern Australia. This disease is more severe in sandy soils in Australia but in the USA, Rhizoctonia root rot is also a problem in loamy and heavy soils. De Beer [1]reported that in South Australia there were eight times as many patches in light soil compared with a heavier soil nearby. The aim of this glasshouse study was to investigate the effect of physical properties of different soil types on root growth of wheat plants and the severity of disease.

Material and methods
Bulk soil samples were collected from three sites with different textures (sand, loam and clay). Soils were air dried and sieved through a 2-mm sieve and analysed for their physical and chemical properties. Twelve pots (8.7 ;cm diameter and 40 ;cm deep) were filled with each soil type for the two inoculation treatments (inoculated and control) with four replications and rest of the pots were sacrificed to measure root growth during the experiment. Twenty-one days after applying the treatments, five wheat seeds were sown into each pot. In a second experiment, nine pots, each made up of sixteen pieces of 8.7 ;cm diameter and 2.5 ;cm tall PVC pipe stuck together with PVC tape were used. These pots, each filled with one of the three soils, were used to measure soil moisture levels and root penetration resistance at various depths during the experiment. Pots were watered with 40 ;ml of water every second day to keep the soils near field capacity. Plants were harvested 24 ;days after sowing and root and shoot growth measured.

Results and conclusion
The reduction in root and shoot biomass production following inoculation with AG-8 was more in sand than in loam or clay soils. Dry root weight of wheat in sand, loam and clay soil inoculated with AG-8 was 19%, 46% and 66%, respectively, less than in uninoculated controls. There was better moisture retention in loam and clay soils compared with sand in the upper 4-5 ;cm. The root penetration resistance was greater in loam and clay than sand. Root growth in the uninoculated soil columns was faster in sand than in the loam and clay soils.

Experiments are now underway to determine whether soil growth of the pathogen under these conditions are responsible for the responses observed.

1. De Beer JF, 1965. PhD Thesis. Study on the ecology of Rhizoctonia solani Kuhn. University of Adelaide, SA, Australia.