Ramularia leaf spot: a newly important threat to barley production

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 (Fig. 1a) is caused by the dothideomycete fungus Ramularia collo-cygni (Fig. 1b, c). The fungus produces phytotoxins called rubellins (Heiser et al., 2003) in the plant which cause foliar necrosis and reduce photosynthetic area (Fig. 1d). 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 (Havis et al., 2015). 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 (Kaczmarek et al., 2017). Typical RLS lesions can be observed on some nonbarley 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. collocygni has been detected in archive samples and the disease has been known in Europe since the 19th Century (Havis et al., 2015). Recent research developments have begun to improve our understanding of RLS with the long-term aim of developing novel strategies to combat this disease.