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Impact d’associations variétales de blé tendre sur la dynamique annuelle d’adaptation d’une population de Zymoseptoria tritici à un gène de résistance qualitative récemment contourné

Abstract : Improving the efficacy and the durability of resistance genes against plant pathogens is a major challenge for farming systems. Increasing the cultivated genetic diversity within a field is a lever to achieve this goal. We need to understand how such deployment of the resistance affects pathogen populations during the whole cropping season including the post-epidemic period. To address this question, first I investigated the impact of wheat cultivar mixtures on the adaptation dynamics of a population of the fungal pathogen Zymoseptoria tritici to the recently broken down qualitative resistant gene (Stb16q). I assessed how two wheat cultivars, carrying Stb16q or not and cultivated in mixtures in different proportions (0.25, 0.5, 0.75), modified the pathogen population over two annual field epidemics compared to pure stands. Three key moments were considered in the whole pathogen life cycle: the epidemic period, which involves successive asexual multiplication cycles driven by rain-splash dispersal of pycnidiospores between growing plants of both cultivars (two dates: early and late epidemic stages), and the inter-epidemic period, which involves sexual reproduction in wheat residues releasing ascospores. The frequency of virulent strains in each cultivar and mixture (population composition) was assessed by phenotyping in seedling assays under greenhouse conditions. Population size was assessed by measuring disease severity (epidemic period) or estimating the number of ascospores discharged from residues (inter-epidemic period). At the end of the epidemic period, severity reductions were observed in mixtures at the whole canopy scale and on both cultivars, highlighting a reciprocal “protective” effect. Mixtures decreased the frequency of virulent strains relative to the pure stand of the cultivar carrying Stb16q at the end of the epidemic period and at the inter-epidemic period, showing that sexual reproduction did not counter the “benefic ial” impact of mixtures. Even so, the frequency of virulent strains on this cultivar slightly decreased after sexual reproduction for some mixture proportions, highlighting the capacity of cultivar mixtures to actively maintain avirulent strains in the offspring population through specific epidemiological mechanisms. Secondly, I investigated in semi-controlled conditions different combinations of sexual crosses that may have occurred in field conditions between virulent and/or avirulent strains, in both cultivars. The offspring were quantified, and the parenthood of recombinant strains from each type of cross was checked by genotyping. I established that avirulent × virulent crosses, and even avirulent × avirulent, can occur on the cultivar carrying Stb16q. This result demonstrates that the expression of symptoms on living plant tissues are not essential for a Z. tritici strain to subsequently reproduce sexually. Finally, I suggested a conceptual modeling framework to further invest igate the impact of the initial frequency of virulent strains and the proportion of cultivars on the mixture efficacy at the field and annual scales. To conclude, I showed that, besides disease reductions, cultivar mixtures may be used to restrict the overall propagation of a virulence. Sexual reproduction can make these mixture effects persist over the subsequent epidemic and avirulent strains can take part in it, allowing to “dilute” the virulence within recombinant populations. For these reasons, cultivar mixtures, when deployed at larger spatio-temporal scales, could be an efficient strategy to improve the durability – (i.e. maintain, even slightly re-increase, the efficacy –) of a broken down qualitative resistance gene.
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Submitted on : Wednesday, June 29, 2022 - 2:09:13 PM
Last modification on : Friday, August 5, 2022 - 2:41:12 PM

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  • HAL Id : tel-03708743, version 1

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Carolina Orellana Torrejon. Impact d’associations variétales de blé tendre sur la dynamique annuelle d’adaptation d’une population de Zymoseptoria tritici à un gène de résistance qualitative récemment contourné. Agronomie. Université Paris-Saclay, 2022. Français. ⟨NNT : 2022UPASB031⟩. ⟨tel-03708743⟩

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