Trials highlight the effect of old and new azoles on wheat

Leaf spot disease, otherwise known as Septoria, is caused by the fungus Zymoseptoria tritici. Photo: Maccheek
Leaf spot disease, otherwise known as Septoria, is caused by the fungus Zymoseptoria tritici. Photo: Maccheek

The disease septoria causes huge wheat yield losses across Europe. An analysis of 55 field trials across 10 European countries uncovered the effect of different azoles on the disease.

Septoria is the most prevalent fungal disease in wheat in western Europe, particularly in more humid growing seasons when it causes great damage to wheat fields. Wheat varieties grown in Europe are only partially resistant, and so to reduce infestation and loss, spraying with fungicides is common.

“The septoria fungus produces large amounts of infectious material on stubble residues in the autumn, which spread from field to field over greater distances,” said senior researcher, Lise Nistrup Jørgensen from the Department of Agroecology – Crop Health, at Aarhus University in Denmark.

Azoles as fungicide in wheat

Farmers are increasingly experiencing problems with resistance due to the intensive use of azoles to combat septoria. This resistance is seen in relation to the “old” azoles, which have been on the market for decades.

Azoles are a class of 5-membered heterocyclic compounds containing a nitrogen atom and at least 1 other non-carbon atom. These substances act on many different types of fungi and have been used in agriculture to control fungal diseases on leaves in wheat for more than 40 years. Azoles are still considered an important group and are part of many blend products and control strategies despite the growing problems of resistance.

Varying effects of azoles on wheat across Europe

In a European network, Eurowheat, field trials led by Aarhus University were carried out across 10 European countries (Scotland, England, Poland, France, Germany, Ireland, Belgium, Latvia, Lithuania, and Hungary) with the aim of creating an updated overview of the effect of azoles on relevant diseases in wheat. In these trials, both septoria and rust diseases have been included.

“We performed a total of 55 field trials in the period 2015-2018, where we, among other things, tested the 4 most common azoles: epoxiconazole, prothioconazole, tebuconazole and metconazole. In the experiments, we found that the effect of the azoles generally varied greatly across Europe. We also saw that the effect was reduced over the trial period,” said Jørgensen.

Some “unacceptable effects” of azoles on wheat

“On average, the azoles protect approximately 60% against septoria, but it varies a lot – not just between the different regions but also from year to year. In about 30% of our field trials the azoles could control less than 50% of the septoria, and in some trials from England, France and Germany, some field trials even showed disease reductions of less than 30%, which must be said to be unacceptable effects.”

Jørgensen added that although there are areas where the impact is challenged, “we can also conclude that the azoles are still beneficial in the fight against septoria in most parts of Europe, not least as a mixing partner, which helps reduce the development of resistance to other groups of fungicides.”

New azole has a greater effect

In 2 of the growing seasons, a new azole – mefentrifluconazole – introduced in 2020, was also included in the trial and showed a significantly higher effect than the 4 ‘old’ azoles. This, according to the researchers, confirms that there is no clear cross-resistance between the azoles, which means that the new azole is not inhibited by the mutations found in the septoria population, which inhibit the effect of the ‘old’ azoles.

“The new azole…is currently of great importance for the control of septoria in Europe,” Jørgensen stated.

Fungicides – controversial and problematic

The university notes that the use of fungicides is “controversial and problematic” in relation to the public’s desire for a lower consumption of pesticides. Furthermore, further restrictions are expected on the number of approved azoles legal in the EU.

Even with the increased resistance in several places in Europe, the new and ‘old’ azoles are useful against Septoria, notes Jørgensen, warning, however, that we cannot “sit back and relax”. “We need to focus more on increasing the prevalence of septora-resistant varieties, which reduces the need for spraying. It is also important that we have built-in strategies against fungicide resistance that can optimise the way we utilise the fungicides, including fewer sprays, spraying according to needs assessment, spraying with mixtures of active substance groups, and spraying with low and adapted dosages. These elements will help ensure that we can continue to grow wheat in Europe in the future without major losses from septoria,” concluded Jørgensen.

Berkhout
Natalie Berkhout Freelance journalist
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