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Resistance against triazoles is once again on the rise

Resistance to azole fungicides is still increasing in the most important fungi infecting crops.

2018.03.27 | Janne Hansen

In all the approved triazole compounds there has been a tendency towards a decreasing effect against the fungus septoria in the period from 2011 to 2017. Photo: Lise Nistrup

Fungal diseases that affect crops is a serious problem in Denmark and globally. In Denmark the fungus septoria tritici blotch is the one that causes the greatest losses in wheat, and triazoles are among the measures used against it. The problem is that the fungi develop resistance to the fungicides.   

In order to follow developments, researchers from the Department of Agroecology at Aarhus University have studied septoria’s resistance to triazoles on a continual basis. The results show that resistance to triazoles continues to increase.   

This is valid for, among others, Rubric (epoxiconazol) and Proline (prothioconazol). The reduced effects in the field of the pure azoles have also resulted in reduced yield increases.    

Mixtures more efficient that single compounds 

Combining products gives better field results. The triazol mixtures Armure (difenoconazol + propiconazol) and Prosaro (prothioconazol + tebuconazol) had better effects than applications of just one triazole. 

 Figure 1: In all the tested triazole compounds there was a tendency towards a decreasing effect against fungal diseases - septoria in the period 2011 to 2017. The best effect is presently achieved by mixing azoles. 

Some of the increased effect of azole mixtures is related to the fact that the active substances have different impacts on the individual fungus mutations. Difenoconazol and tebuconazol typically have a good effect on some mutations, while prothioconazol and epoxiconazol affect others. The best effect is therefore achieved by using mixture products like Armure or Prosaro/Proline Xpert. 

Field trials have also shown clearly that protection against septoria is improved when the group of fungicides called SDHI is part of the solution.  

Folicur (tebuconazol) has, in contrast to previous years, shown a slightly better effect. This improvement can be explained by that fact that tebuconazol – just like difenoconazole in Armure – has a relatively good effect on some of the new fungal mutations (D134G, V136A), which increasingly dominate the Danish septoria population. 

Differences from place to place

As in previous years, there were differences in the effects between the different locations. At AU Flakkebjerg by Slagelse on the island of Zealand, prothioconazole (Proline) had a slightly poorer effect than epoxiconazole (Rubric), while the opposite was the case in the trials near Horsens. 

Corresponding differences have been seen between the effects of triazoles on the European level. In some countries metconazol works best, in other countries tebuconazol is best, while in countries with less intensive protection against septoria prothioconazole and epoxiconazole  are still the best products. 

Emergence of new mutations

A range of leaf samples from problem fields in 2017 was specifically investigated for mutation frequency. The samples came from fields where the farmers – despite extensive efforts – experienced insufficient effects.   

The samples from these fields had a complex composition of new mutations. There was a marked increase in the number of CYP51 mutations, and new mutations emerged continuously. These observations have been backed up by laboratory studies. The latest sensitivity tests have thus shown a significant shift towards gradually decreasing fungus sensitivity, as shown in figure 2. 

Figure 2: Changes in the sensitivity of the septoria isolates from 2013 to 2017. The more the curves shift towards the right, the less sensitive the tested isolates from Denmark are.  

The prospect is that only a few new compounds for protecting against septoria will be approved in the years to come. Contrary to expectations, Elatus Era will not be on the market in 2018, and GF-3307 (Inatreq + prothioconazol) and Revysol (triazole), in which GF-3307 only contains a new active agent, is expected to be on the market at the earliest in 2020. 

You can also read “Four azoles’ profile in the control of Septoria, yellow rust and brown rust in

wheat across Europe”.

For more information please contact:  

Senior researcher Lise Nistrup Jørgensen, Department of Agroecology, email: lisen.jorgensen@agro.au.dk, telephone: +45 8715 8234, mobile: +45 2228 3352

Sustainable Pest Management is one of the research areas in which the Department of Agroecology is particularly strong and from which results are delivered in line with national and global societal challenges and goals.

Agro, DCA