Scald or Rhynchosporium, caused by the fungus Rhynchosporium commune, is difficult to control with fungicides and severe epidemics may appear suddenly. Its epidemiology is not well understood as it is based on disease symptoms rather than the presence of the pathogen. Quantitative PCR enables detection and quantification of pathogen DNA in barley plants in both presymptomatic phases of infection and where they remain asymptomatic throughout their life cycle. Seed-borne inoculum was identified as a significant source for early infection of barley crops, with substantial amounts of R. commune DNA found in crops from infected seed but severity of seed infection correlated poorly with amounts of pathogen DNA (leaves), disease severity (leaves) and yield loss later in the cropping season. R. commune can colonise barley crops extensively throughout the cropping season (from seed to seed) in the absence of visual symptoms which has implications for the use of fungicides, breeding programmes and national variety recommended lists. The genetic basis of several different components of resistance to R. Commune in barley was investigated in a mapping population derived from a winter x spring barley cross. Relative expression of symptoms quantified using the residual values from a linear regression of amount of R. Commune DNA against visual plot disease score and was generally highly correlated. A QTL on chromosome 7H was identified as having a significant effect on the expression of visual disease symptoms relative to overall amount of R. commune colonisation.