Spore dispersal patterns of the ascomycete fungus Ramularia collo-cygni and their influence on disease epidemics

Ramularia leaf spot is a major economic disease of barley caused by the dothidiomycete fungus Ramularia collo-cygni. The fungus has a complex life cycle which includes extensive late season spore release events and a seed-borne phase. Predicting disease epidemics during the growing season remains a difficult challenge. To better understand the interaction between spore movement and disease epidemics, spore samplers were set up in Germany (2 sites over 4 years), Poland (7 sites over 2 years) and the UK (2 sites over 9 years), where the disease has been observed. Spore concentrations were determined using a real-time PCR assay, and meteorological data were obtained from co-located automatic stations. Spore release events were seen to peak in June on mainland Europe and July in the UK. The pattern of spore release was broadly similar across countries with earlier peaks in mainland Europe. A relationship was observed in the UK between July spore levels and disease in following winter barley crops. Rainfall and temperature were proposed as significant drivers of spore release in these months. The major environmental parameter associated with spore release across the two UK sites was crop surface wetness, although some site-specific interactions were noted for rainfall and wind movement. Regression analysis of spore patterns and disease epidemics indicates a relationship between spore levels 75 and 105 days pre-harvest and final disease levels in UK winter barley crops. This relationship was not observed in spring barley. The implications on risk forecasts are discussed.