Root Endophytes Pathogens

In addition to the facultative necrotrophic abilities of fairy ring fungi, other agarics, e.g. P. semilanceata, are able to colonize healthy cortical tissues of grasses but without clear evidence of any deleterious symptoms in the host (Keay and Brown, 1990). Similarly colonization of grass roots has been observed under field or microcosm conditions by species such as Melanoleuca grammopodia and Con-ocybe dunensis (McKay, 1968). In both there was some evidence of host specificity, with P. semilanceata exhibiting a preference for Agrostis tenuis and Poa annua over L. perenne, and infection rates by basidiomycete (clamped) hyphae being much higher for Ammophila arenaria than other sand dune grasses. Thanatephorus cucumeris (anamorph Rhizoctonia solani) is commonly isolated from grassland and arable soil (Garrett, 1951; Warcup and Talbot, 1962) and is a capable cellulolytic saprotroph. It is also an economically important necrotrophic pathogen in grassland, causing various diseases (e.g. 'brown patch', root rot and aerial blight) in turfgrasses and other grassland plants (Couch, 1995). However, T. cucumeris and related species in the Ceratobasidiaceae are detected in healthy roots (Jump-ponen and Johnson, 2005), and are also able to form mycorrhizal symbioses with orchids, and Carex spp. (Haselwandter and Read, 1982; Roberts, 1999).

Presence of basidiomycetes is occasionally revealed by culture-based examination of healthy roots from grasslands but at low frequency (Warcup, 1959; Wilberforce et al., 2003). However, use of fungal-specific PCR primers has recently shown a great diversity of basidiomycetes in healthy root tissues. Wilberforce (2003) found that basidiomycetes comprised 15% of clones from an oligotrophic temperate grassland in the UK, while Jumpponen and Johnson (2005) found ca. 30% of clones in a library derived from tallgrass prairie roots to be basidiomycete in origin. However, like many aspects of root biology, decomposition of these organs is poorly understood and further work is required to elucidate the function of many of these endophytes. The recent discovery, by Harrington and

Mitchell (2002), of ectomycorrhiza-like structures formed by Cortinarius cinna-momeus on the roots of Carex flacca and C. pilulifera in calcareous grassland, consistent with earlier observation of the association of Tricholoma melaleucum with Carex glauca (Wilkins and Patrick, 1939), illustrates that mycorrhizal associations involving agarics and non-woody hosts may be more common in temperate habitats than previously thought. Distinctive assemblages of ectomycorrhizal fungi do occur with shrubs in grasslands (e.g. Helianthemum nummularium), but association of agarics with non-woody hosts is usually restricted to Arctic-alpine habitats (Gardes and Dahlberg, 1996). Thus, the assignment of mycorrhizal status can be problematic especially in the absence of evidence of distinctive morphological structures.

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