Soil physics, fungal epidemiology and the spread of Rhizoctonia solani

The role of soil structure on colonization efficiency of the pathogen Rhizoctonia solani is analysed by linking experimental techniques that control and quantify soil structure with epidemiological concepts that deal with the fungal dynamics of colonization. The dynamics of colonization efficiency are quantified for the fungal colony spreading through replicated samples of a nonsterile sandy loam with different physical properties. The effect of bulk-density and aggregate-size on the air-filled pore volume is assessed by examination of thin soil sections and by quantification of pore-size distributions estimated from water-retention characteristics. An increase in bulk-density and a reduction in aggregate size increased the fraction of micropores in the air-filled pore volume at the expense of larger pores. These changes resulted in smaller, more slowly expanding fungal colonies and significantly reduced the volume of soil from which a nutrient source can become colonized. It is concluded that colonization efficiency is a convenient way to summarize the complicated dynamics of mycelial growth through soil. We use the results to discuss the effect of soil structure on the saprotrophic and parasitic invasion of soil by fungi.