Role of inoculum sources in Rhynchosporium population dynamics and epidemics on barley

Rhynchosporium leaf blotch of barley, caused by the fungus Rhynchosporium secalis, is of increasing importance in world agriculture. It is the most serious disease of winter and
spring barley in the UK, causing substantial losses nationally, despite expenditure of £50M per year on fungicides. The disease is difficult to control with fungicides and severe epidemics may appear suddenly. The sources of inoculum responsible for starting such
epidemics are not well understood.
• This project aimed to clarify the origin and early dynamics of epidemics using molecular techniques (quantitative PCR) that can detect and quantify the DNA of the pathogen in
barley plants before symptoms occur. The same techniques can also detect genetic characteristics of the fungus, such as mating type, virulence, and genes responsible for
resistance to fungicides. Each season, epidemics were monitored on both winter (Octobersown) and spring (March-sown) barley on samples from current crops from sites in England and Scotland. Work was also done on historical spring barley samples archived at Rothamsted over the last 150 years. Thus short-term and long-term changes in the
pathogen population were studied.
• Seed-borne inoculum was identified as a significant source for early infection of barley crops, with substantial amounts of R. secalis DNA found in seedlings of crops grown from
infected seed. However, there was little evidence that severity of seed infection influenced amounts of pathogen DNA in leaves, disease severity (leaves) or yield loss later in the cropping season.
• Whilst small amounts of airborne R. secalis inoculum were collected in different seasons at different sites, this did not provide evidence that airborne inoculum played an important role in the development of epidemics.
• The discovery that R. secalis can colonise barley crops extensively throughout the cropping season (from seed to seed) in the absence of visual symptoms has completely changed the understanding of the disease by the industry (with implications for use of fungicides, breeding programmes and the HGCA Recommended List of barley cultivars).
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• Substantial early symptomless infection was identified in winter barley crops but epidemic severity late in the season was largely dependent on the amount of spring rainfall, which encouraged secondary disease spread by splash dispersal of pathogen spores. Therefore the PCR quantification of early R. secalis infection could not be accurately used to predict epidemic severity late in the season.
• Use of quantitative PCR provided new insights into the operation of host resistance against R. secalis, especially in work with a barley mapping population. However, the early season PCR assessments of cultivar resistance could not accurately be used to predict resistance ratings based on late season disease assessments.
• Markers associated with new sources of resistance to R. secalis were identified in a barley mapping population and methods to screen material for resistance were improved.
• Work with samples from the 150-year barley archive at Rothamsted provided unique insights into the long-term dynamics of R. secalis on barley crops. Temporal patterns in the amounts of R. secalis DNA over the past 40 years were found to be consistent with those recorded in the Defra/HGCA barley disease surveys since the 1970s. Analysis over the entire 150-year period, using quantitative species-specific PCR, confirmed the increase in severity of rhynchosporium epidemics observed in national surveys. Of the factors investigated, the most likely explanation was the change in height of barley cultivars through the introduction of new short-strawed cultivars. This work also provided information about the long-term dynamics of other barley pathogens, such as Pyrenophora.
• Knowledge from this project is being combined with new information from related projects
being funded by BBSRC LINK, Defra, HGCA and RERAD (at ADAS, and in Scotland, SAC
and SCRI) to develop guidelines for crop husbandry and agronomic practices to reduce
Rhynchosporium secalis population size and genetic variation to achieve sustainable control of rhynchosporium disease of barley