Abstract
The fungal genus Rhynchosporium (causative agent of leaf blotch) contains several host-specialised species, including R. commune (colonising barley and brome-grass), R. agropyri (couch-grass), R. secalis (rye and triticale) and the more distantly related R. orthosporum (cocksfoot). This study used molecular fingerprinting, multilocus DNA sequence data, conidial morphology, host range tests and scanning electron microscopy to investigate the relationship between Rhynchosporium species on ryegrasses, both economically important forage grasses and common wild grasses in many cereal growing areas, and other plant species. Two different types of Rhynchosporium were found on ryegrasses in the UK. Firstly, there were isolates of R. commune that were pathogenic to both barley and Italian ryegrass. Secondly, there were isolates of a new species, here named R. lolii, that were pathogenic only to ryegrass species. R. lolii was most closely related to R. orthosporum, but exhibited clear molecular, morphological and host range differences. The species was estimated to have diverged from R. orthosporum ca. 5735 years before the present. The colonisation strategy of all of the different Rhynchosporium species involved extensive hyphal growth in the sub-cuticular regions of the leaves. Finally, new species-specific PCR diagnostic tests were developed that could distinguish between these five closely related Rhynchosporium species.
Highlights
Leaf blotch caused by the fungal pathogen Rhynchosporium commune is an economically important disease of barley (Hordeum vulgare) crops throughout the world [1] with yield losses of 1–10% common [2]
DNA Fingerprint Analysis Both RAPD-PCR (Fig. 1A) and repetitive-sequence-based PCR (rep-PCR) (Fig. 1B) methodologies could discriminate between isolates of R. commune, R. agropyri, R. secalis and R. orthosporum obtained from proximate geographical origins
Two Rhynchosporium isolates from Italian ryegrass (2lm11 and 5lm11) clustered within the main R. commune species group according to both DNA fingerprinting and sequence data
Summary
Leaf blotch (scald) caused by the fungal pathogen Rhynchosporium commune is an economically important disease of barley (Hordeum vulgare) crops throughout the world [1] with yield losses of 1–10% common [2]. United Kingdom (UK) survey data indicates that barley crops grown in 2005 had 0.6% area with rhynchosporium lesions on leaf two at growth stage 75 (medium milk development stage; [3]). This equates to an estimated UK national yield loss of £10.8 million per annum (at a price of £225/tonne) despite fungicide treatment [4]. Data from phylogenetic analyses using the R. commune avirulence gene NIP1 and flanking regions [7] suggested that modern populations of R. commune originated in northern Europe approximately 2500–5000 years ago, when the pathogen switched from a wild grass species onto cultivated barley shortly after barley was introduced there, and that it subsequently spread into other barley-growing areas of the world
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