Abstract

Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is the most important foliar pathogen of sugar beet worldwide. Extensive reliance on fungicides to manage CLS has resulted in the evolution of fungicide resistance in C. beticola worldwide, including populations in the Czech Republic. One important class of fungicides used to manage CLS is the sterol demethylation inhibitors (DMI). The aim of our study was to assess DMI resistance in C. beticola from the Czech Republic and elucidate the molecular basis of DMI resistance in this population. A total of 50 isolates were collected in 2018 and 2019 from the major sugar beet growing regions of the Czech Republic and assessed for in vitro sensitivity to the DMI fungicides propiconazole, prochloraz, and epoxiconazole. These analyses identified three strains that exhibited 50% effective concentration (EC50) values > 1.0 μg mL–1 against respective fungicides, which were therefore considered resistant. In contrast, strains that exhibited lowest EC50 values were considered sensitive. To explore the molecular basis of resistance in these three strains, the cytochrome P450-dependent sterol 14α-demethylase (Cyp51) gene was sequenced. Sequence analysis identified a Y464S mutation in all three resistant strains. To assess whether Cyp51 gene expression may play a role in DMI resistance, selected strains were grown in vitro with and without fungicide treatment. These analyses indicated that Cyp51 gene expression was significantly induced after fungicide treatment. Thus, we conclude that Y464S point mutation along with induced Cyp51 gene overexpression is likely responsible for resistance against DMI fungicides in C. beticola from the Czech Republic.

Highlights

  • IntroductionCercospora leaf spot (CLS), caused by Cercospora beticola, is one of the most damaging foliar diseases in Beta vulgaris L

  • We identified a point mutation in all three-resistant C. beticola strains correlating with reduced sensitivity towards demethylation inhibitors (DMI) fungicides

  • This mutation was found in highly conserved domains of the target cytochrome P450-dependent sterol 14α-demethylase (Cyp51) gene, in C. beticola and in other plant pathogenic fungi displaying reduced DMI sensitivity

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Summary

Introduction

Cercospora leaf spot (CLS), caused by Cercospora beticola, is one of the most damaging foliar diseases in Beta vulgaris L. (sugar beet) [1,2] If left unmanaged, this disease can cause losses of up to 50% of the yield and sugar content [3,4,5]. Symptoms of CLS are necrotic leaf spots of around 2–3 mm that eventually coalesce in the later stages of the disease development resulting in large necrotic regions on the leaf surface, reducing the photosynthetic activity of affected leaves, which effects the yield [2]. While host resistance is an important means of managing CLS, fungicides are critical tools to manage this disease in most sugar beet growing regions of the worldwide [6,7]

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