Managing Ramularia collo-cygni through varietal resistance, seed health and forecasting

Abstract

Ramularia leaf spot is a fungal disease of barley caused by Ramularia collo-cygni. The disease has become widespread in the north of Europe but it also occurs further south and has become endemic in France. An assessment of seed from harvested grain showed the causal fungus to be present throughout the UK. Levels on seed can vary by season and also by region with the highest levels detected in the north and west of the UK and the lowest levels in the east of England. Winter and spring barley varieties showed different levels of Ramularia leaf spot and on the basis of these observations a varietal resistance rating can be made. No varieties exhibited total resistance. The causal fungus developed inside plants of both resistant and susceptible varieties. This was detected using Polymerase Chain Reaction (PCR) diagnostics which can detect R. collo cygni DNA before symptoms are seen. This phase of the disease (asymptomatic) occurs both in resistant and susceptible varieties and it shows DNA levels of the fungus develop initially in the lower leaves and then in the upper leaves, but symptom expression is most extensive on the upper leaves.

Field trials using seed stocks with different levels of contamination with R. collo-cygni show that the level of seed infection can have a part to play in the final level of symptoms seen. This was greatest in the spring barley variety NFC-Tipple where a 50% reduction in visible symptoms was observed by using seed with less than 1pg R. collo-cygni DNA. It was not possible to determine the impact of sowing seed where R. collo-cygni was absent, since no seed stock was completely free from infection. It is recommended however that seed levels are kept as low as possible in regions where the disease is not endemic (1pg DNA England).

Site had the greatest impact on the disease epidemic with disease severity varying by 78% depending upon the site. The main differentiation between high and low disease pressure sites was leaf wetness in early June. A correlation between leaf wetness events in early June with subsequent symptom expression in July was determined on the basis of this research and this corroborates with results taken from previous trials since 2005. In early June, spring barley crops were at Growth Stages (GS) 25-32 and had yet to reach the optimum timing for fungicide control (at GS49). This new information can therefore be used as a risk forecast to warn growers about the potential risk of disease in time to take preventative action. It provides a big step forward in forecasting the risk of Ramularia leaf spot when used alongside other factors including variety and seed contamination. It will enable action to be taken to protect crops most at risk with fungicide before symptoms appear.

Extending the spring barley risk forecast to other UK and European regions and extending the risk forecast for winter barley, will be the focus priority of future research. Since seed contamination had a lower impact on symptom expression compared to site, using seed contaminated with R. collo cygni is likely to have a low impact on the varietal resistance score, but it is recommended that seed stocks used for Recommended List trials have as low a level of seed contamination as is possible. Understanding the impact of seed treatments to eradicate disease from seed is the focus of new research.