Winter barley (Hordeum vulgare L.) is the third most cultivated crop after corn and wheat in Austria but one of the most challenging for disease control. The foliar pathogen Ramularia collo-cygni B. Sutton and J.M. Waller, causing Ramularia leaf spots (RLS), is one of the most important diseases in barley. In the recent years, control has only been achieved using fungicide mixtures including the multi-site inhibitor chlorothalonil, however this compound is totally banned in the EU. The objective of this study was to assess fungicide dose-rates and spray mixtures for RLS control. Furthermore, a field monitoring within the main barley growing areas of Austria was carried out, to analyse the current resistance situation to DMI and SDHI fungicides, which are still the backbone in RLS control. The results indicate that only the mixture with chlorothalonil achieved a good RLS control. Prothioconazole or benzovindiflupyr (alone or additively) decrease the severity of RLS but increase the local frequency of Cyp51 and sdhC mutations, especially the high dose rates. Based on a low Cyp51 mutation frequency of 16% in untreated control this frequency increased over 3.8 times following an application with 300 g ha−1 prothioconazole. The cumulative-sdhC mutations were even more increased after an application with benzovindiflupyr. This study showed that Ramularia collo-cygni is present in 91% of barley fields presented in this field survey. Widespread use of chlorothalonil fungicide maintained a low to moderate mutation frequency (Cyp51-I325T, Cyp51-I328L, sdhC-H146R and sdhC-H153R) in Austrian barley regions with no increase between 2017 and 2019.
Ramularia collo-cygni B. Sutton and J.M. Waller is a major disease in Austrian barley-growing regions. To date, fungicide application is the most effective method to manage the disease; however, fungicide resistance to demethylation and succinate dehydrogenase inhibitors has developed over the last few years. In the growing seasons 2016/2017 and 2017/2018, field trials were carried out to analyze the efficiency of fungicide strategies based on different fungicide classes. Disease development, growth parameters and monitoring of CYP51 and sdh mutations were determined. Fungicide treatments resulted in higher disease control, green leaf area and grain yield. In Austrian R. collo-cygni field populations, the frequency of the mutations CYP51-I325T and CYP51-I328L was low to moderate. Frequency of mutations sdhC-H146R and sdhC-H153R was low. Frequencies of CYP51-I325T and -I328L were similar and increased following DMI application. Frequency of sdhC-H146R was higher compared to sdhC-H153R. The SDHI benzovindiflupyr showed a higher selection rate for sdh mutations compared to bixafen. These sdh mutations were not selected if chlorothalonil was used as mixing partner, leading to a stable composition of sdh resistance alleles over the last two years. Chlorothalonil was proven to be an effective tool for anti-resistance strategies. Currently, SDHIs and DMIs are the backbone of Ramularia leaf spot control in Austria; however, the level of resistance is likely to increase in absence of suitable anti-resistance strategies and following the ban of chlorothalonil.
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