Infection strategy of Ramularia collo-cygni and development of Ramularia leaf spot on barley and alternative graminaceous hosts

Ramularia leaf spot (RLS) is a newly important disease of barley across temperate regions worldwide. Despite this recent change in importance, the infection biology of the causal agent Ramularia collo-cygni (Rcc) remains poorly understood. Confocal microscopy of the infection process of two transgenic Rcc isolates, expressing either GFP or DsRed reporter markers, was combined with light microscopy during field infection to track the progression of Rcc in planta. Infection of stomata, including the development of a previously unreported stomatopodium structure, results in symptomless development and intercellular colonization of the mesophyll tissue. Transition to necrotrophy is associated with breakdown of host chloroplasts and the formation of aggregates of conidiophores. In addition to barley, Rcc forms a compatible interaction with winter wheat and a number of perennial grass species. An incompatible reaction was observed with two dicotyledonous species. These results provide further insights into the host interactions of this fungus and suggest that RLS could be a potential threat to other agriculturally important crops.