Browsing by Subject "Fungizid"

Now showing 1 - 3 of 3
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    Analysis of the emerging situation of resistance to succinate dehydrogenase inhibitors in Pyrenophora teres and Zymoseptoria tritici in Europe
    (2018) Rehfus, Alexandra; Vögele, Ralf
    Phytopathogenic fungi such as Pyrenophora teres and Zymoseptoria tritici cause destructive diseases of barley and wheat in all major cereal production areas worldwide. The control of net blotch of barley caused by P. teres and Septoria tritici blotch (STB) of wheat caused by Z. tritici mainly relies on the usage of fungicides. Thereby, three single-site inhibiting fungicide classes, the quinone outside inhibitors (QoIs), the demethylation inhibitors (DMIs) and the succinate dehydrogenase inhibitors (SDHIs) have the highest relevance. The class of SDHIs is the most newly introduced fungicide class and inhibits the fungal succinate dehydrogenase complex (SDH) which is a critical enzyme of the respiratory chain and the tricarboxylic cycle. The upcoming SDHI resistance in European populations of P. teres and Z. tritici was investigated in the present study and resistance mechanisms underlying SDHI resistance were characterised. SDHI resistant isolates of both pathogens were collected in intensive monitoring programmes which covered the major barley and wheat growing areas in Europe. SDHI resistant isolates showed point mutations in the genes SdhB, SdhC and SdhD which cause amino acid alteration in the subunits B, C and D of the SDH complex. First SDHI resistant isolates of both pathogens were detected in 2012 and showed amino acid alteration, histidine to tyrosine at position 277 in SDH B (B-H277Y) in the case of P. teres and a threonine to asparagine exchange at position 79 in SDH-C (C-T79N) in the case of Z. tritici. In P. teres, a significant increase of SDHI resistant isolates from 2012 to 2015 was observed, particularly in countries such as France and Germany. Several target-site mutations leading to amino acid exchanges, namely B-H277Y, C-S73P, C-N75S, C-G79R, C-H134R, C-S135R, D-D124N/E, D-H134R, D-G138V, D-D145G and D-E178K, were identified in those isolates. Sequencing of SdhB, SdhC and SdhD genes of several isolates confirmed that each isolate carried one mutation in the Sdh genes, and not two or more in combination. In vitro and in planta sensitivity tests were performed and revealed that each SDH-variant causes a distinct resistance phenotype towards SDHIs. Commercially available SDHIs were compared and isolates showed cross-resistance towards all SDHIs tested, although some minor differences in the response to different mutations were observed. Most of the SDHI resistant P. teres isolates carried C-G79R substitution which was shown to exhibit one of the strongest effects of all detected alterations. In addition to C-G79R, other substitutions, such as C-N75S and D-D145G, were frequently found in the field. These SDH-variants were shown to confer low to moderate levels of resistance. In contrast to the rapid ‘build-up’ of resistant isolates in the population of P. teres in countries such as France and Germany, the emergence of SDHI resistance in Z. tritici did not evolve as fast as observed in net blotch. Here, only a few resistant isolates have been sampled so far (42 resistent of 3431 investigated isolates, 1.2%). An increase of resistant isolates of Z. tritici was observed mainly in Ireland, the United Kingdom and the Netherlands, however, still at low levels. SDH variants B-N225I, B-T268I/A, C-N86S/A, C-T79N/I, C-W80S, C-H152R and C-V166M were detected in SDHI resistant isolates collected in these and other countries such as France and Germany. Four isolates showed two mutations in the Sdh genes in combination. In vitro and in planta sensitivity measurements demonstrated that C-H152R mutants showed the highest resistance level of all investigated SDH variants collected in the field. C-T79N and C-N86S exchanges which have been detected more frequently in the field than C-H152R, were shown to confer lower levels of resistance compared to C-H152R. Both phytopathogenic species were shown to evolve a range of diverse target-site mutations, which led to different alterations in both pathogen species with exception of C-N75S in P. teres and the homologous variant, C-N86S, in Z. tritici. This can be explained by species-specific variation of the SDH enzyme, a different nature of the pathogens (e.g. host plants and disease geographical spread) as well as a different fungicide use pattern (e.g. mode of action diversity and fungicide application intensity). The absence of a dominant major target-site mutation in the case of SDHI resistance in both pathogens is thought to allow SDHIs as effective control agent against both pathogen species also in the future. Nevertheless, anti-resistance management strategies are highly recommended for the usage of SDHIs. These strategies should not only be based on the use of mixtures and alternations of fungicides but should also implement integrated disease control measurements (e.g. resistant host cultivars).
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    Characterisation of the sensitivity of Zymoseptoria tritici to demethylation inhibitors in Europe
    (2021) Huf, Anna; Vögele, Ralf
    The fungal pathogen Zymoseptoria tritici (formerly Septoria tritici) causes Septoria tritici blotch (STB), one of the most yield reducing diseases of wheat worldwide. In addition to cultural control measures and the cultivation of wheat varieties with a level of disease resistance, STB control relies heavily on the application of foliar fungicides with different modes of action. The demethylation inhibitors (DMIs) have been one of the most widely applied fungicides for many decades and belong to one of the most important fungicide modes of action in STB management. DMIs inhibit the sterol 14α-demethylase, an essential enzyme in the ergosterol biosynthesis pathway, encoded by the CYP51 gene of fungi. Widespread and intensive use of the DMIs over time has led to a continuous negative shift in the sensitivity of Z. tritici towards DMIs that have been used for a long time. This shift in sensitivity is mainly driven by the accumulation of mutations in the CYP51 gene resulting in the selection of various CYP51 haplotypes. More recently, CYP51 overexpression and an increased efflux activity, based on the overexpression of the MFS1 transporter, have been shown to be additional mechanisms affecting DMI sensitivity of Z. tritici. Inserts in the CYP51 promotor (CYP51p) and MFS1 promotor (MFS1p) were observed to be responsible for CYP51 and MFS1 overexpression. The prevalence and contribution of different DMI resistance mechanisms to a reduced DMI sensitivity of Z. tritici were investigated in isolates from across Europe in 2016 and 2017. The CYP51 gene of all isolates was sequenced and the CYP51p and MFS1p was investigated for inserts in order to determine the character of the CYP51 haplotypes as well as to identify CYP51 overexpression or if an increased efflux activity was occurring in these isolates. Overall, it was shown that the occurrence of CYP51 haplotypes was still the most frequent and important mechanism conferring a reduction in sensitivity to DMIs by Z. tritici in Europe. Nevertheless, an increase in the frequency of isolates exerting CYP51 overexpression and those exhibiting increased efflux activity was observed compared to earlier studies. Glasshouse data demonstrated that DMIs can still contribute to disease control, and in some cases give full control, of STB even if isolates expressed CYP51 overexpression and/or an increased efflux in addition to also carrying moderately or highly adapted CYP51 haplotypes. However, in order to prevent the further increase and spread of further adapted CYP51 haplotypes plus additional resistance mechanisms in the Z. tritici population across Europe, anti-resistance-management strategies should be a high priority in the use of DMIs. In addition, especially integrated disease management strategies, such as the appropriate choice of cultivars, should be applied in order to keep STB disease pressure low and consequently reduce the number of fungicide applications. Moreover, resistance-management strategies may exploit the limited cross-resistance between different DMIs, for example, by the use of mixtures or alternation of different DMI fungicides. However, control strategies should also incorporate the use of fungicides with different MOAs. The aim of all these strategies is to reduce selection of adapted Z. tritici isolates and consequently to prolong the efficacy of DMIs in STB management.
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    In vivo und molekularbiologische Untersuchungen zur Sensitivität von Phakopsora pachyrhizi gegenüber Demethylierungs-Inhibitoren und Qo-Inhibitoren
    (2013) Schmitz, Helena Katharina; Vögele, Ralf
    Soybeans are one of the most important crops worldwide. Since 1980, soybean production attained increasing distinction in Brazil. Following the leading producer USA, Brazil counted as the second biggest soybean producer in 2010. A number of threats are involed reducing soybean yield, rating the Asian Soybean Rust, Phakopsora pachyhrizi, as one of the worst pathogens since its invasion in 2001. Until this date the American Soybean Rust, Phakopsora meibomiae, was known in Brazil only, which is of minor importance. Not only did P. pachyrhizi reduce soybean yield in Asia, but also in Brazil considerable additional costs were caused by yield reduction and disease management. Control is mainly based on fungicide treatment, demethylation inhibitors (DMI) and quinone outside inhibitors (QoI) being the most important and effective classes used. Both fungicide groups are frequently applied in combinations to ensure prolonged effects. While efficiency of QoIs remained unchanged, protection by DMIs was significantly narrowed. The primary objective of the recent study was to survey the sensitivity of P. pachyrhizi isolates towards fungicides and to analyse the genetical background of a conceivable adaption. Indeed, an adaption towards DMIs could be observed, while efficiency of QoIs was stable. Due to the P. pachyrhizi genetical structure of the cytochrome b (cyt b) gene, which corresponds to the QoI fungicide target protein, resistance towards QoIs based on the most important mechanism known from other pathogens is rather unlikely. The major resistance mechanism of phytopathogenic fungi against QoIs is an alteration of the cyt b-sequence, in particular point mutations F129L, G137R and G143A, whereas G143A results in highest resistance factors. An intron after codon 143 of the cyt b-gene prevents the development of G143A-mutation. In contrast, genetical analyses of the cyp51-gene, which corresponds to the target protein of DMI fungicides, revealed that adaption is based on different resistance mechanisms which have an additive or synergistic impact. In P. pachyrhizi, point mutations within the cyp51-gene and a modified expression of cyp51 were involved. An altered expression of cyp51 was due to a selective expression of a mutated cyp51-allele and due to up-regulation of cyp51. Six point mutations (F120L, Y131H, Y131F, K142R, I145F and I475T) which appeared in defined combinations, except for one mutation which was found as a single character, correspond to a reduced sensitivity. Additionaly, in some of the isolates cyp51 was up-regulated three- to thenfold compared to the reference strain, leading to decreased efficiency of DMIs. Indications were found, assuming that the copy number of the cyp51-gene in P. pachyrhizi is responsible for the observed alterations in the expression. Other resistance mechanisms than described in the recent study (such as expression of efflux transporters) may additionally play a role now or in future. Resistance towards DMIs evolved by P. pachyrhizi was caused by nesting mechanisms which appeared in quite a short time period due to enormous selection pressure by extensive DMI-use in large areas and the clonal, dicaryotic live cycle with short generation times. Thus, applications of fungicides with different modes of actions are recommended for management of P. pachyrhizi to prevent further resistance development and -distribution.