Control of ramularia leaf spot in a changing climate

Abstract

The LINK project on Control of Ramularia Leaf Spot in a Changing Climate (CORACLE) made substantial advances in understanding and controlling this disease of barley. Ramularia leaf spot, caused by the fungus Ramularia collo-cygni, has become important in temperate regions throughout the world. It has been important in Scotland and Ireland since 1998 and has become widespread in England since 2009. While it is most significant on spring barley in the UK, it is increasingly found on winter barley.

CORACLE showed that Ramularia has a negative effect on barley yields and must be controlled. Fungal spores were shown to disperse mainly at the end of the growing season, in July and August, following prolonged periods of leaf are wetness. It also showed that Ramularia epidemics can be predicted from leaf surface wetness during the period of stem extension, normally in May or June for spring barley. Using these insights into the epidemiology of Ramularia, CORACLE improved a disease forecasting system to help farmers apply appropriate fungicide sprays.

In research on disease control, CORACLE showed that seed treatment can reduce levels of R. collo-cygni in seed and thus reduce epidemics in susceptible varieties. The most effective treatment for established Ramularia remains a late-season spray of a foliar fungicide but the R. collo-cygni fungus is genetically diverse and has evolved resistance to several important groups of fungicides. It is regarded as a medium-to-high risk for further development of fungicide resistance. This means that, in future, control will increasingly need to focus on choosing a resistant variety and high standards of seed hygiene.

CORACLE found good genetic resistance to Ramularia in diverse varieties of winter and spring barley. An important discovery was that the mlo gene, which controls powdery mildew in many spring barley varieties, substantially increases susceptibility to Ramularia. The strength of the effect of mlo depends on environmental conditions. However it is possible to select for improved Ramularia-resistance in mlo varieties. This is reflected in the current Recommended List of Spring Barley, which includes several varieties, which combine excellent resistance to mildew with improved resistance to Ramularia.
A critical stage in the life-cycle of the R. collo-cygni fungus is the transition from growing harmlessly within the barley plant to becoming an aggressive parasite. Research in CORACLE showed that physical stress plays a central role in this transition and that plants which are better able to tolerate physical stress are generally less susceptible to Ramularia. A key goal for barley breeding is to produce varieties which combine Ramularia resistance, resistance to other diseases and tolerance of stress.