Temporal Changes in Sensitivity of Zymoseptoria tritici Field Populations to Different Fungicidal Modes of Action

Tim Birr,Mario Hasler,Joseph-Alexander Verreet,Holger Klink

doi:10.3390/agriculture11030269

Abstract

Septoria tritici blotch (STB; Zymoseptoria tritici), one of the most important foliar diseases in wheat, is mainly controlled by the intensive use of fungicides during crop growth. Unfortunately, Z. tritici field populations have developed various extents of resistance to different groups of fungicides. Due to the complete resistance to quinone outside inhibitors (QoIs), fungicidal control of STB relies mainly on demethylation inhibitors (DMIs) and succinate dehydrogenase inhibitors (SDHIs) as well as multi-site inhibitors. In this study, temporal changes in the sensitivity of Z. tritici to selected DMIs (tebuconazole, propiconazole, prothioconazole, prochloraz), SDHIs (boscalid, bixafen), and multi-site inhibitors (chlorothalonil, folpet) were determined in microtiter assays using Z. tritici field populations isolated in 1999, 2009, 2014, and 2020 in a high-disease-pressure and high-fungicide-input area in Northern Germany. For the four tested DMI fungicides, a significant shift towards decreasing sensitivity of Z. tritici field populations was observed between 1999 and 2009, whereby concentrations inhibiting fungal growth by 50% (EC50) increased differentially between the four DMIs. Since 2009, EC50 values of tebuconazole, propiconazole, and prochloraz remain stable, whereas for prothioconazole a slightly increased sensitivity shift was found. A shift in sensitivity of Z. tritici was also determined for both tested SDHI fungicides. In contrast to DMIs, EC50 values of boscalid and bixafen increased continuously between 1999 and 2020, but the increasing EC50 values were much smaller compared to those of the four tested DMIs. No changes in sensitivity of Z. tritici were observed for the multi-site inhibitors chlorothalonil and folpet over the last 21 years. The sensitivity adaptation of Z. tritici to both groups of single-site inhibitors (DMIs, SDHIs) mainly used for STB control represents a major challenge for future wheat cultivation.

Highlights

Septoria tritici blotch (STB), caused by Zymoseptoria tritici, is one of the most important foliar wheat diseases worldwide [1]
Averaged over all tested fungicides and years of Z. tritici isolation, analysis of variances (ANOVA) results showed that EC50 values were significantly affected by the interaction of fungicide and year of Z. tritici isolation (p < 0.0001; Table 1)
We investigated potential changes in fungicide sensitivity of Z. tritici towards the remaining three groups available for STB control over the last 21 years in a high-disease-pressure and high-fungicide-input area in Northern Germany

Summary

Introduction

Septoria tritici blotch (STB), caused by Zymoseptoria tritici (formally Mycosphaerella graminicola), is one of the most important foliar wheat diseases worldwide [1]. In Europe, especially in regions with humid-temperate conditions such as Germany, Northern France, Ireland, or the United Kingdom, STB is currently regarded as the primary yield-reducing disease in wheat production almost every year, causing significant yield losses of up to 50% [2,3,4]. The loss of sensitivity to QoIs is associated with a singlepoint mutation in the target gene, the mitochondrial cytochrome b, replacing alanine for glycine at position 143 [11]. This mutation confers full resistance and dominates in current Z. tritici populations in Europe since the mid-2000s (e.g., United Kingdom, Germany, France) [12,13,14,15,16]. QoIs no longer provide reliable field control of STB in most European countries

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Conclusion