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Ramularia Leaf Spot: A Newly Important Threat to Barley Production

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Ramularia leaf spot (RLS) has emerged as a serious threat to barley production across temperate regions of Europe, South America and New Zealand. The disease, typified by characteristic reddish brown, rectangular lesions visible on both sides of the leaf and often ringed by a chlorotic halo is caused by the dothideomycete fungus Ramularia collo-cygni. The fungus produces phytotoxins called rubellins in the plant which cause foliar necrosis and reduce photosynthetic area. With this reduction in green leaf area, RLS can affect yields by as much as 70%, but losses are more usually in the region of 5–10%. Importantly, RLS can also increase the proportion of small grains (screenings) by as much as 4% consequently lowering the quality of the grain. The losses attributed to RLS are intriguing as disease symptoms are usually only visible late in the growing season after the barley crop has flowered. Although RLS is primarily a disease of barley R. Collo-cygni is also able to infect other cereal crops such as wheat and oats as well as some grasses including weed species. Typical RLS lesions can be observed on some non-barley host species but infection is often asymptomatic. The importance of other non-barley host species in the epidemiology of RLS and the risk this disease poses to other cereal hosts is currently unknown. Why RLS has emerged as an important disease of barley crops is something of a mystery as R. Collo-cygni has been detected in archive samples and the disease has been known in Europe since the 19th Century. Recent research developments have begun to improve our understanding of RLS with the long-term aim of developing novel strategies to combat this disease. As RLS becomes more readily recognised as a major threat to barley production it is clear that improving our understanding of the biology of this organism is essential to devising sustainable management strategies against this disease. There are still many questions surrounding the pathology of this disease that remain unanswered. Defining the key changes in both fungal and barley gene expression that correspond to the transition from benign endophyte to aggressive necrotroph will provide important insights into the mediation of this switch. Furthermore, determining how abiotic stress influences the expression of RLS and how varietal interactions with the environment can affect this disease is important to help inform how barley genetics can be used to control RLS. The release of the first draft R. Collo-cygni genome sequence will help advance research into genetic control of fungal biology and may provide novel targets for chemical control of the disease. By answering these questions and gaining a better understanding of R. Collo-cygni biology it will be possible to devise improved control measures for this newly important late season disease.
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Keywords: ENDOPHYTE; FUNGICIDE INSENSITIVITY; MLO; NECROTROPH; RAMULARIA COLLO-CYGNI

Document Type: Research Article

Publication date: 01 April 2017

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