Preferential spread of the pathogenic fungus Rhizoctonia solani through structured soil W. Otten a, * , K. Harris b , I.M. Young c , K. Ritz b,d , C.A. Gilligan a a Epidemiology and Modelling Group, Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, UK b Plant Soil Interface Programme, Scottish Crop Research Institute (SCRI), Dundee DD2 5DA, UK c Scottish Informatics, Mathematics, Biology and Statistics (SIMBIOS) Centre, University of Abertay Dundee, Bell Street, Dundee DD1 1HG, UK d National Soil Resources Institute, Cranfield University, Silsoe, Bedfordshire MK45 4DT, UK Received 24 March 2003; received in revised form 29 August 2003; accepted 10 September 2003 Abstract 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. q 2003 Elsevier Ltd. All rights reserved. Keywords: Rhizoctonia solani; Colonisation; Soil thin section; Soil structure; Epidemiology; Fungal spread; Hyphal density 1. Introduction Infection of roots or plants by many economically important plant pathogens including Rhizoctonia solani, Pythium spp., or Gaeumannomyces graminis depends on the ability of fungal hyphae to spread several millimetres or centimetres through soil. A number of experimental studies in the absence of a host have shown that soil physical conditions including soil water potential, aggregate size, bulk-density and air-filled pore volume have a substantial effect on the variability, extent and rate of mycelial spread of fungal pathogens such as R. solani (Gill et al., 2000; Glenn and Sivasithamparam, 1990; Otten and Gilligan, 1998; Otten et al., 1999), G. graminis (Glenn et al., 1987; Grose et al., 1984), or Fusarium oxysporum (Toyota et al., 1996b). In particular it is known that the geometry of the air-filled pore volume constrains fungal spread (Glenn and Sivasitham- param, 1990; Otten et al., 1999) and that R. solani spreads faster and further along surfaces than through soil (Otten and Gilligan, 1998). This raises such questions as: to what extent does structured soil provide a framework of surfaces formed by root channels, aggregate surfaces and cracks, and what are the consequences for fungal spread and biotic interactions? These questions are particularly critical for soil under conservation tillage management, in which relatively persistent flow paths develop (Edwards et al., 1992). Novel techniques have greatly advanced the study of spread of pathogenic and saprotrophic fungi through soil (Harris et al., 2002; Neate and Benger, 1995; Otten et al., 2001, 2003). Laboratory based systems involving controlled conditions have permitted quantitative analysis of mycelial exploration and nutrient acquisition (Bolton and Boddy, 1993; Donnelly and Boddy, 1997; Otten et al., 2001; Ritz 0038-0717/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2003.09.006 Soil Biology & Biochemistry 36 (2004) 203–210 www.elsevier.com/locate/soilbio * Corresponding author. Tel.: þ 44-1223-330229; fax: þ 44-1223- 333953. E-mail address: wo200@cus.cam.ac.uk (W. Otten).