Ramularia leaf spot, caused by the fungus Ramularia collo-cygni, has recently become one of the most important barley diseases. Although it was first described in 1893, it has been causing the most problems since the 1980s. The disease is widespread throughout Europe and some other countries around the world. It causes necrotic and chlorotic spots on barley, which are often confused with symptoms of abiotic stress and infection with other fungal pathogens. The spots reduce the green area of the leaves, resulting in yield loss. The disease may also be reflected in reduced seed size. The fungus also produces host nonspecific phytotoxins (rubellins), that are activated when exposed to sunlight, causing fatty acid peroxidation and the formation of reactive oxygen species that destroy cell membranes. As a result, the tissue dies and signs of disease appear. The fungus has a long latent growth phase, usually until flowering of barley, and then under the influence of various factors, passes from the endophytic to the necrotrophic state. The cause of this transition is not yet well understood. Control of Ramularia leaf spot is mainly based on the use of fungicides, as there are no resistant barley varieties on the market yet.
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