No abstract

Cultivar mixtures slow polycyclic epidemics but may also affect the evolution of pathogen populations by diversifying the selection pressures exerted by their plant hosts at field scale. We compared the dynamics of natural populations of the fungal pathogen Zymoseptoria tritici in pure stands and in three binary mixtures of wheat cultivars (one susceptible cultivar and one cultivar carrying the recently broken‐down Stb16q gene) over two annual field epidemics. We combined analyses of population “size” based on disease severity, and of population “composition” based on changes in the frequency of virulence against Stb16q in seedling assays with more than 3000 strains. Disease reductions were observed in mixtures late in the epidemic, at the whole‐canopy scale and on both cultivars, suggesting the existence of a reciprocal protective effect. The three cultivar proportions in the mixtures (0.25, 0.5, and 0.75) modulated the decrease in (a) the size of the pathogen population relative to the two pure stands, (b) the size of the virulent subpopulation, and (c) the frequency of virulence relative to the pure stand of the cultivar carrying Stb16q. Our findings suggest that optimal proportions may differ slightly between the three indicators considered. We argue potential trade‐offs that should be taken into account when deploying a resistance gene in cultivar mixtures: between the dual objectives “efficacy” and “durability,” and between the “size” and “frequency” of the virulent subpopulation. Based on current knowledge, it remains unclear whether virulent subpopulation size or frequency has the largest influence on interepidemic virulence transmission.

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Cultivar mixtures can stabilize yield and reduce pathogen spread in plant populations. A field experiment was performed to determine whether (i) a large difference between the cultivars in the mixture (e.g. plant height or earliness) would have an impact on mixture performance and whether (ii) such differences would modify the classical rules for mixture design. Mixtures were constituted from cultivars with diversity for many traits, including plant height, flowering date, disease resistance and yield potential. The field experiment was conducted in three years testing each year 72 to 90 mixtures of two, four or eight cultivars, and their corresponding pure stands. Disease severity and yield of cultivar mixtures were strongly related to the mean values of the component cultivars in pure stands. Despite the considerable diversity of the mixtures tested, the classic rules (e.g. proportion of susceptible cultivars) already tested in mixtures with similar height and earliness were effective for decreasing disease severity. Agronomic heterogeneity for traits such as plant height, yield potential or earliness of the cultivars in mixtures did not have a negative impact on disease severity and yield relative to pure stands. Increasing the number of cultivars in the mixture from two to eight had no impact on the mean disease severity and yield of the mixtures, but reduced the variability of disease severity and yield in the mixture relative to pure stands. These results suggest that it may be possible to increase within-field wheat diversity by combining more contrasted cultivars in mixtures than was previously thought.

Plant pathogen populations inhabit patchy environments with contrasting, variable thermal conditions. We investigated the diversity of thermal responses in populations sampled over contrasting spatiotemporal scales, to improve our understanding of their dynamics of adaptation to local conditions. Samples of natural populations of the wheat pathogen Zymoseptoria tritici were collected from sites within the Euro‐Mediterranean region subject to a broad range of climatic conditions. We tested for local adaptation, by accounting for the diversity of responses at the individual and population levels on the basis of key thermal performance curve parameters and “thermotype” (groups of individuals with similar thermal responses) composition. The characterization of phenotypic responses and genotypic structure revealed the following: (i) a high degree of individual plasticity and variation in sensitivity to temperature conditions across spatiotemporal scales and populations; and (ii) geographic variation in thermal response among populations, with major alterations due to seasonal patterns over the wheat‐growing season. The seasonal shifts in functional composition suggest that populations are locally structured by selection, contributing to adaptation patterns. Further studies combining selection experiments and modeling are required to determine how functional group selection drives population dynamics and adaptive potential in response to thermal heterogeneity.

Evolution within a given virulence phenotype (pathotype) is driven by changes in aggressiveness: a case study of French wheat leaf rust populations , Peer Community Journal, 3: e39.

 Plant pathogen populations inhabit patchy environments with contrasting, variable thermal conditions. We investigated the diversity of thermal responses in populations sampled over contrasting spatiotemporal scales, to improve our understanding of their dynamics of adaptation to local conditions.  Samples of natural populations of the wheat pathogen Zymoseptoria tritici were collected from sites within the Euro-Mediterranean region subject to a broad range of environmental conditions. We tested for local adaptation, by accounting for the diversity of responses at the individual and population levels on the basis of key thermal performance curve parameters and 'thermotype' (groups of individuals with similar thermal responses) composition. The characterisation of phenotypic responses and genotypic structure revealed: (i) a high degree of individual plasticity and variation in sensitivity to temperature conditions across spatiotemporal scales and populations; (ii) geographic adaptation to local mean temperature conditions, with major alterations due to seasonal patterns over the wheatgrowing season. The seasonal shifts in functional composition suggest that populations are locally structured by selection, contributing to shape adaptation patterns. Further studies combining selection experiments and modelling are required to determine how functional group selection drives population dynamics and adaptive potential in response to thermal heterogeneity.

Plant pathogens are constantly evolving and adapting to their environment, including their host. Virulence alleles emerge, and then increase, and sometimes decrease in frequency within pathogen populations in response to the fluctuating selection pressures imposed by the deployment of resistance genes. In some cases, these strong selection pressures cannot fully explain the evolution observed in pathogen populations. A previous study on the French population of Puccinia triticina, the causal agent of wheat leaf rust, showed that two major pathotypes — groups of isolates with the same combinations of virulences — predominated but then declined over the 2005-2016 period. The relative dynamics of these two pathotypes — 166 317 0 and 106 314 0 — relative to the others present in the population could not be explained solely by the frequency of Lr genes in the landscape. Within these two pathotypes, we identified two main genotypes that emerged in succession. We assessed three components of aggressiveness — infection efficiency, latency period and sporulation capacity — for 44 isolates representative of the four P. triticina pathotype-genotype combinations. We showed, for both pathotypes, that the most recent genotypes were more aggressive than the older ones. Our findings were highly consistent for the various components of aggressiveness for pathotype 166 317 0 grown on Michigan Amber — a 'naive' cultivar never grown in the landscape — or on Apache — a 'neutral' cultivar, with no selection effect on the landscape-pathotype pattern. For pathotype 106 314 0, the most recent genotype was more aggressive on several of the cultivars most frequently grown in the landscape, but not on 'neutral' and 'naive' cultivars, and only in terms of its latency period. We conclude that the quantitative components of aggressiveness can be significant drivers of evolution in pathogen populations. A gain in aggressiveness allowed the maintenance of a declining pathotype, and even further expansion of that pathotype, in the pathogen population, providing evidence that virulence alone is not sufficient, aggressiveness also being required for the adaptation of a pathogen to a changing varietal landscape.