Fungicide Resistance Evolving in Ramularia collo-cygni Population in Estonia.

Riinu Kiiker,Marite Juurik,Andres Mäe

doi:10.3390/microorganisms9071514

Abstract

Ramularia leaf spot caused by the fungus Ramularia collo-cygni, has recently become widespread in Estonian barley fields. Currently, disease control in barley fields relies on SDHI and DMI fungicides, which might be threatened by R. collo-cygni isolates that are well-adapted to fungicide pressure. In a two-year study, 353 R. collo-cygni isolates were collected from spring barley fields in Estonia. A total of 153 R. collo-cygni isolates were examined for sensitivity to azoles (DMIs; prothioconazole-desthio, epoxiconazole, mefentrifluconazole) and succinate dehydrogenase inhibitors (SDHIs; boscalid, fluxapyroxad). Epoxiconazole was the least effective and a new fungicide mefentrifluconazole was the most effective DMI. Among SDHIs, fluxapyroxad was more effective than boscalid. Also, single R. collo-cygni isolates with high resistance to tested fungicides occurred, which could affect fungicide control of the pathogen. The entire collection of R. collo-cygni was analysed for mutations in fungicide target proteins. Six mutations were identified in CYP51 gene, the most dominant being I381T, I384T, and S459C. Also, numerous point mutations in the SdhC gene were present. The mutation G143A in strobilurin target protein CytB dominates in over 80% of the R. collo-cygni population, confirming the low efficacy of strobilurin fungicides in barley disease control.

Highlights

Barley (Hordeum vulgare L.) is the fourth most relevant cereal crop, and is grown in diverse environments worldwide [1]
To study the fungicide sensitivity of the Estonian R. collo-cygni population, a total of isolates were collected from spring barley plants in the ripening stage before harvest
The R. collo-cygni population was tested for active ingredients of demethylation inhibitors (DMIs) and succinate dehydrogenase inhibitors (SDHIs) class fungicides, and relevant mutations in fungicide target genes (CYP51, SdhC, and cytochrome bb (CytB)) were identified

Summary

Introduction

Barley (Hordeum vulgare L.) is the fourth most relevant cereal crop, and is grown in diverse environments worldwide [1]. Ramularia leaf spot (RLS) caused by Ramularia collo-cygni is a late-season disease that has emerged as a major problem in barley production in Europe, South America, and New Zealand over the last 20 years [3,4,5]. The pathogen infects barley seeds, where it is adapted to survive, and during plant development it colonises the emerging leaf layers in the absence of external inoculum [9]. RLS symptoms typically develop after ear emergence, symptoms can be observed earlier in the season in especially conducive environmental conditions [5,13]. Expression of diverse R. collo-cygni virulence factors at the time of leaf infection is likely to promote RLS disease symptoms [14]

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