Several target-site mutations in the genes of subunits SDH-B, SDH-C and SDH-D with different impact on SDHI fungicides were detected. The pattern of mutations varied from year to year and between different regions. Strict resistance management strategies are recommended to maintain SDHIs as effective tools for net blotch control, especially in areas with low frequencies of resistant isolates. © 2016 Society of Chemical Industry.

Zymoseptoria tritici is the causal agent of septoria tritici blotch (STB), a foliar wheat disease important worldwide. Succinate dehydrogenase inhibitors (SDHIs) have been used in cereals for effective control of STB for several years, but resistance towards SDHIs has been reported in several phytopathogenic fungi. Resistance mechanisms are target-site mutations in the genes coding for subunits B, C and D of the succinate dehydrogenase (SDH) enzyme. Previous monitoring data in Europe indicated the presence of single isolates of Z. tritici with reduced SDHI sensitivity. These isolates carried mutations leading to amino acid exchanges: C-T79N, C-W80S in 2012; C-N86S in 2013; B-N225T and C-T79N in 2014; and C-V166M, B-T268I, C-N86S, C-T79N and C-H152R in 2015. The current study provides results from microtitre and greenhouse experiments to give an insight into the impact of different mutations in field isolates on various SDHIs. In microtitre tests, the highest EC 50 values for all tested SDHIs were obtained with mutants carrying C-H152R. Curative greenhouse tests with various SDHIs confirmed the findings of microtitre tests that isolates with C-H152R are, in general, controlled with lower efficacy than isolates carrying B-T268I, C-T79N and C-N86S. SDHI-resistant isolates of Z. tritici found in the field were shown to have cross-resistance towards all SDHIs tested. So far, SDHI-resistant isolates of Z. tritici have been found in low frequencies in Europe. Therefore, FRAC recommendations for resistance management in cereals, including a limited number of applications, alternation and combination with other MOAs, should be followed to prolong SDHI field efficacy.

Zymoseptoria tritici causes septoria tritici blotch (STB), one of the most devastating diseases of wheat worldwide. C-14 demethylation inhibitors (DMIs) belong to the most relevant fungicides in the control of STB. Intensive and longlasting exposure to DMIs has led to an adaptation of Z. tritici towards these fungicides. The most important mechanism leading to reduced DMI sensitivity is based on the accumulation of mutations in the CYP51 gene. Different attempts have been made to describe CYP51 haplotypes in the past. However, due to the ongoing evolution of CYP51, a new nomenclature has become necessary. This study has developed such a new nomenclature and used it to adequately describe 33 different CYP51 haplotypes found in a collection of 331 isolates across Europe in 2016. Nine of these haplotypes were found to represent 85% of all isolates, and have a heterogeneous distribution across Europe. Haplotypes carrying the substitution S524T, which is associated with a decreased sensitivity of Z. tritici to all DMIs, were only found at frequencies of around 5%. These haplotypes were mostly identified in Ireland and the UK and at lower frequencies in Germany, the Netherlands, Poland and France. In vitro studies using epoxiconazole and prothioconazole-desthio revealed similar trends of the nine most frequent haplotypes with respect to their sensitivity towards these compounds. The data here confirm the ongoing evolution of CYP51 in the European population of Z. tritici and helps to establish a new, easy-to-apply nomenclature to support future descriptions of CYP51 haplotype development and evolution.

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