Browsing by Subject "Septoria tritici"

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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).
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.
Mapping of quantitative-trait loci (QTL) for adult-plant resistance to Septoria tritici in five wheat populations (Triticum aestivum L.)
(2010) Risser, Peter; Miedaner, Thomas
Septoria tritici blotch (STB), caused by Septoria tritici (teleomorph Mycosphaerella graminicola), is one of the most important diseases in wheat varieties worldwide, responsible for severe damage of the leaves causing yield losses between 30 and 40 %. Control of STB includes crop rotation, soil tillage, fungicide application, and cultivation of resistant varieties. Profit-making wheat growers are forced to apply narrow crop rotations under reduced tillage. Some fungicides including widely-used strobilurins are no longer effective due to mutations in the highly variable pathogen population of S. tritici. Therefore, resistance breeding using genetic mapping to identify quantitative-trait loci (QTL) associated with STB resistance provides a promising strategy for controlling the disease. The main goal of this study was to detect chromosomal regions for quantitative adult-plant resistance of winter wheat to STB. Besides this, we analyzed the genetic diversity of 24 European varieties after inoculation with four different isolates of S. tritici. Multienvironmental field trials inoculated with S. tritici were applied to test isolates and varieties and to phenotype mapping populations. In detail, the objectives were to (1) compare natural infection and inoculation, (2) evaluate genotypic variation of adult-plant resistance to STB in European varieties, (3) analyze genotype x environment (G x E) interaction, (4) evaluate and analyze phenotypic data including STB severity, heading date (HED), and plant height (PLH) of five mapping populations, (5) construct genetic linkage maps of these populations using AFLP, DArT, and SSR markers, (6) determine number, positions, and genetic effects of QTL for evaluated traits, and (7) reveal QTL regions for multiple-disease resistance within mapping populations using QTL meta-analysis. In all trials, inoculation with one to four preselected isolates was performed and STB severity was visually scored plotwise as percentage coverage of flag leaves with lesions bearing pycnidia. 24 winter wheat varieties were chosen with maximal differentiation in resistance to STB and evaluated across three years including nine environments. Five mapping populations, Florett/Biscay, Tuareg/Biscay, History/Rubens, Arina/Forno, and Solitär/Bussard, each comprising a cross of a resistant and a susceptible variety, with population sizes ranging from 81 to 316, were phenotyped across four to six environments. In parallel, 221 to 491 polymorphic genetic markers were assigned to linkage groups covering 1,314 to 3,305 cM of the genome. Based on these linkage maps, the number, positions, and genetic effects of QTL could be determined by composite interval mapping. Furthermore, raw data of different experiments evaluated for resistance to two other pathogens, Fusarium head blight and Stagnospora glume blotch, were used to reveal multiple-disease resistance QTL within Arina/Forno and History/Rubens populations by the software package PLABMQTL. Results of inoculated field trials coincided with not inoculated trials showing natural infection (r = 0.84 to 0.99, P < 0.01), thus inoculation method was accurate to evaluate STB severity in the field. Genotypic variation between 24 varieties ranged from 8 % (Solitär) to 63 % (Rubens) flag leaf area infected. In the analysis of variance, genotypic variance had highest impact followed by G x E interaction (P < 0.01). Therefore, environmental stability of varieties should be a major breeding goal. The varieties Solitär, History, and Florett were most stable, as revealed by a regression approach. In contrast, disease symptoms of Biscay ranged from 19 to 72 % within the three experimental years. Phenotypic data revealed significant (P < 0.01) genotypic differentiation for STB, HED, and PLH within all five mapping populations and between the parents. Entry-mean heritabilities (h²) ranged from 0.69 to 0.87 for STB, the only exception was Tuareg/Biscay (h² = 0.38). For HED (h² = 0.78 to 0.93) and PLH (h² = 0.92 to 0.98) heritabilities were high. All correlations between STB and HED (r = -0.18 to -0.33) as well as between STB and PLH (r = -0.13 to -0.45) were negative and moderate. The exception was History/Rubens which is segregating at the Rht-D1 locus showing considerably higher correlation between STB and PLH (r = -0.55, P < 0.01). The five mapping populations showed a wide and continuous distribution of mean STB severity averaged across three to six environments in field trials at adult-plant stage. In QTL analysis, one to nine, zero to nine, and four to eleven QTL were detected for STB, HED, and PLH, respectively, across five wheat populations using composite interval mapping. One to two major QTL for resistance to STB were detected consistently across environments in each population (QStb.lsa_fb-3B, QStb.lsa_fb-6D, QStb.lsa_tb-4B, QStb.lsa_tb-6B, QStb.lsa_hr-4D, QStb.lsa_hr-5B.1, QStb.lsa_af-3B, QStb.lsa_bs-7A) explaining more than 10 % of normalized adjusted phenotypic variance. Altogether, resistance QTL explained 14 to 55 % of adjusted phenotypic variance. Both parents contributed resistant alleles. Major QTL, however, were all from the resistant parent. QTL meta-analysis revealed each of four loci for multiple-disease resistance located on chromosomes 3B, 4B, 5B, and 6D in Arina/Forno, and on chromosomes 2B, 4D, 5B, and 7B in History/Rubens. The most effective meta QTL was on chromosome 4D in History/Rubens closely linked to Rht-D1. The resistance allele from History reduced disease severity by 9.8 % for STB and 6.3 % for FHB, thus explaining 47 % and 60 % of partial phenotypic variance. In general, European wheat varieties showed a wide range of genotypic variation for STB resistance useful for breeding. Although the influence of environment and G x E interaction was high, some resistant varieties which were stable across multiple environments were found (Solitär, History, Florett). Genomic regions associated with STB resistance were mapped across 13 out of 21 wheat chromosomes. Together with the continuous distribution of five segregating populations for flag leaf infection, it can be concluded that the adult-plant resistance to S. tritici was inherited quantitatively depending on several loci explaining part of phenotypic variance. QTL meta-analysis across three severe pathogens, including Fusarium head blight, Stagnospora glume blotch, and STB, within two populations revealed eight loci for multiple-disease resistance with closely linked markers applicable in resistance breeding. Combining detected major QTL as well as meta QTL in present breeding material by applying marker-assisted selection seems a promising approach to the breeding of varieties with improved resistance to Septoria tritici blotch, Fusarium head blight, and Stagnospora glume blotch.