Symptomless infection by Rhynchosporium commune in relation to control of barley leaf blotch

Rhynchosporium leaf blotch (caused by Rhynchosporium commune) is the most economically important disease of barley in the UK, but epidemics can be difficult to control with fungicides and the basis of cultivar resistance is not well understood. Data were collected from three seasons of experiments using two susceptible and two resistant cultivars of both winter and spring barley. Amounts of R. commune DNA were quantified from samples taken at several growth stages (GS) throughout the growing season and visual symptoms were assessed. There was widespread symptomless infection by R. commune in winter barley crops, with extensive colonisation and sporulation on apparently healthy leaves. During a growing season, the pathogen could spread from seed sown (in autumn) through successive leaf layers to seed harvested (the following summer) without causing visible symptoms. There was generally far more symptomless R. commune in winter barley than spring barley (sown in spring). Fungicide sprays decreased the amount of symptomless infection but the relationship to their effects on yield was not clear. Leaves of resistant cultivars generally contained less symptomless R. commune than those of susceptible cultivars