Zymoseptoria tritici causes leaf spot, also called Septoria, which is the most prevalent fungal disease in wheat in Northern Europe. Septoria can cause great damage in wheat fields and have a negative effect on the farmer's yield. Control of the disease is highly dependent on fungicides within the group of azoles. The use of the fungicides has increased steadily since their introduction around the 1980s, and azoles have been the most widely used fungicides for the past 20 years. In two independent projects, researchers from, among others, the Department of Agroecology at Aarhus University investigated how fungicide resistance evolves in samples from Denmark and Sweden conducted at AU in Flakkebjerg since 2012 and samples from Estonia, Latvia, Lithuania, and Finland, to investigate the evolvement of resistance in these countries. The studies in the Baltic countries help to create a picture of how resistance to azoles has travelled in Europe.

“By examining samples in vitro (in the laboratory) and with DNA-based methods, we have uncovered the mechanisms that cause resistance. This has allowed us to discover how the resistance moves in a very specific geographical pattern. The two projects support each other very well, and the results show the same development,” says postdoc Thies Marten Heick from the Department of Agroecology at Aarhus University.

From west to east

According to the researchers, the resistance is travelling from Ireland in the west and slowly towards east. One of the aims of the project, which is based in the Baltic countries, was precisely to confirm this travel pattern.

"Mostly the resistance starts in Ireland or the UK, because the weather conditions there are completely optimal for the fungus. The climate there is very humid and favourable for Septoria, and also the Irish farmers use more fungicide than the Danish. In a previous project we have had experiments in many different countries, and we discovered the gradient from west to east (Read more about the project here). That result was confirmed by our new measurements in the Baltic countries, where resistance has gradually begun to show itself, while we have seen a greater degree of resistance further west,” explains Thies Marten Heick.

The researchers have primarily focused on azoles, which is active substance in the fungicides mainly used in Denmark.

"Experiments in the Baltic countries and Finland show that so far there has been a very good effect of azoles. But now the resistance is also slowly starting to show itself here too,” says Thies Marten Heick.

Reference measurements from 2014 for the four countries have been compared with new measurements for 2018, and the researchers could see a change in both in-vitro measurements and an increased incidence of specific mutations that cause resistance. This fits in with the previous studies, and helps to establish tthe gradient of resistance travelling from west to east.

Molecular changes cause resistance

In the past many years the Department of Agroecology has examined samples from Denmark and Sweden for resistance to fungicides.

“After testing for resistance to fungicides in relation to Septoria for years, we now know about the mechanisms that change genetically in the fungus so that it becomes resistant to the azoles. In our second project, we examined our resistance tests from Denmark and Sweden, to see what actually happens at a molecular level. This may help us avoid resistance in relation to the new fungicides that are gradually appearing on the market,” explains Thies Marten Heick.

The purpose of the Danish-Swedish project is to find out how the resistance arises, so that it is possible to strengthen the principles of anti-resistance management, so that both new and old remedies against Septoria can have an extended effective life. 

“Even though we have problems with resistance, new products appear from time to time, but the risk that resistance may also built up in relation to new agents is something we have to take into account. We simply need to think in terms of anti-resistance strategies from the very beginning, so we can ensure that a resistance to new fungicides is not quickly built up. We hope that with these research projects we can shed some light on how important the anti-resistance strategies are, so that we can continue to fight Septoria in wheat in the future,” says Thies Marten Heick.

Behind the research