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The anamorph fungus Ramularia collo‐cygni Sutton and Waller, the cause of a leaf spot disease on barley, produces biologically active rubellins on infected leaves, the main component of which is the anthraquinone derivate rubellin B. In severely infected barley leaves originating from Germany and Austria we found approximately 2–6 μg rubellin B per cm2 leaf area. Rubellin B isolated for the first time from mycelium and culture filtrate of R. collo‐cygni acts as reference substance. As already shown for rubellin D, the isomeric compound rubellin B induces symptoms in barley leaves only in the light. The light‐dependency of symptom expression can be explained by the photodynamic formation of reactive oxygen species by rubellin B and D which was demonstrated in a model system.