Preferential spread of the pathogenic fungus Rhizoctonia solani through structured soil

Most studies on soil fungi have been carried out with little explicit characterisation of soil structure within which fungi spread and biotic interactions occur. In this paper we use a combination of epidemiological (colonisation efficiency) and soil bio-physical (thin sectioning) techniques to investigate the role of macropores in soil on the spread of a fungal colony. Macropores, in the form of gaps orientated in various directions, were artificially introduced in replicated samples of sand and a sandy loam. The pathogenic fungus Rhizoctonia solani AG4 was introduced on the surface (encountering a gap whilst the colony expands over the surface) or within soil (encountering a gap whilst the colony spreads through the bulk soil). Depending on the orientation, location and width, gaps were demonstrated to act as preferential pathways (increasing the colonisation efficiency of R. solani), or as a partial barrier (reducing the colonisation efficiency). Within bulk soil, R. solani preferentially followed larger pores, enabling the fungus to by-pass more densely areas. Study of soil thin sections revealed that hyphal densities were greater in gaps than in the surrounding bulk soil. We use the results to discuss how macropore structure in soil can either enhance or reduce the parasitic spread and saprotrophic invasion of soil by fungi.