A field evaluation of the effect of rain on wheat rust epidemics

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Seasonal Changes Drive Short-Term Selection for Fitness Traits in the Wheat Pathogen Zymoseptoria tritici

International audienceIn a cross-infection experiment, we investigated how seasonal changes can affect adaptation patterns in a Zymoseptoria tritici population. The fitness of isolates sampled on wheat leaves at the beginning and at the end of a field epidemic was assessed under environmental conditions (temperature and host stage) to which the local pathogen population was successively exposed. Isolates of the final population were more aggressive, and showed greater sporulation intensity under winter conditions and a shorter latency period (earlier sporulation) under spring conditions, than isolates of the initial population. These differences, complemented by lower between-genotype variability in the final population, exhibited an adaptation pattern with three striking features: (i) the pathogen responded synchronously to temperature and host stage conditions; (ii) the adaptation concerned two key fitness traits; (iii) adaptation to one trait (greater sporulation intensity) was expressed under winter conditions while, subsequently, adaptation to the other trait (shorter latency period) was expressed under spring conditions. This can be interpreted as the result of short-term selection, driven by abiotic and biotic factors. This case study cannot yet be generalized but suggests that seasonality may play an important role in shaping the variability of fitness traits. These results further raise the question of possible counterselection during the interepidemic period. While we did not find any trade-off between clonal multiplication on leaves during the epidemic period and clonal spore production on debris, we suggest that final populations could be counterselected by an Allee effect, mitigating the potential impact of seasonal selection on long-term dynamics

Which temperature to simulate foliar epidemics × crop architecture interactions ?

Air temperature measured by weather stations is commonly used in epidemiological models to forecast the effect of temperature on the development of foliar fungal pathogens. However, leaf temperature is the temperature actually perceived by such pathogens. The leaf temperature depends on the leaf energy budget (e.g. air temperature, radiation, wind, transpiration, etc.), which itself strongly depends on the crop architecture (e.g. leaf position, leaf angle, leaf area density). Consequently, differences between air and leaf temperatures vary spatially between canopies with contrasted architectures and between leaves within a given architecture, especially between leaf layers, as well as temporally throughout the course of the epidemic (seasonal variations). We already characterized the effect of leaf temperature on the latent period of the fungus Mycosphaerella graminicola infecting wheat leaves. In this simulation study, we aimed at estimating whether the use of either air or leaf temperature as input data influences the development of the infectious cycle of M. graminicola within contrasted wheat canopy architectures. Various weather conditions were generated using actual weather data. For each leaf layer, leaf temperature was calculated using the one-dimensional Soil-Vegetation-Atmosphere Transfer model CUPID for different canopy architectures. From the thermal performance curves of the latent period established in the aforementioned study, the pathochron, defined as the number of leaves emerging per latent period, was calculated, using either air temperature or leaf temperature as input data. At the leaf scale, the type of temperature used as input data modified the pathochron, which could generate various disease dynamics into the canopy. Our results highlighted the weather conditions for which it is necessary to take into account leaf temperature rather than air temperature to estimate accurately the development of M. graminicola. Our simulation method could be applied to other foliar fungal pathogens. In a further step, we will use future climatic scenarios to explore the impact of climate change on disease dynamics. In a longer term, the integration of these findings to more elaborated epidemiological models is expected to improve their forecasting accuracy

Les populations de Mycosphaerella graminicola s'adaptent-elles aux modifications saisonnières de leur environnement ?

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Biosécurité des cultures et agroterrorisme. Une menace, des questions scientifiques et une opportunité : réactiver un dispositif d'épidémiovigilance

Les 27 et 28 novembre 2007 s’est tenu à Paris un colloque consacré à la biosécurité des cultures et à l’agroterrorisme (European Crop Biosecurity Workshop), organisé par l’INRA dans le cadre d’un projet financé par l’Union européenne (encadré 1). Nous revenons à cette occasion sur la question des risques liés à l’utilisation volontaire d’agents phytopathogènes, abordée dans un précédent article du Courrier de l’environnement de l’INRA (Suffert, 2002

Comparative pathogenicity of sexual and asexual spores of Zymoseptoria tritici (septoria tritici blotch) on wheat leaves.

Zymoseptoria tritici ascospores and pycnidiospores are considered the main forms of primary and secondary inoculum, respectively, in septoria tritici blotch epidemics. The pathogenicity of the two types of spores of the same genotypic origin were compared through a two-stage inoculation procedure in controlled conditions. Adult wheat leaves were inoculated with ascospores collected from field sources, yielding 119 lesions; pycnidiospores collected from 12 lesions resulting from these ascospore infections were then used for inoculation. Lesion development was assessed for 5 weeks; latent period, lesion size, and pycnidium density were estimated for different isolates. The latent period was calculated as the maximum likely time elapsed between inoculation and either the appearance of the majority of the sporulating lesions (leaf scale) or the appearance of the first pycnidia (lesion scale). The latent period was significantly longer ( c. 60 degree-days, i.e. 3-4 days) after infection with ascospores than with pycnidiospores. No difference was established for lesion size and density of pycnidia. A comparison with other ascomycete fungi suggested that the difference in latent period might be related to the volume of spores and their ability to cause infection. Fungal growth before the appearance of lesions may be slower after inoculation with an ascospore than with a pycnidiospore. The mean latent period during the very beginning of epidemics, when first lesions are mainly caused by ascospores, may be longer than during spring, when secondary infections are caused by pycnidiospores. Disease models would be improved if these differences were considered

How large and diverse are field populations of fungal plant pathogens? The case of Zymoseptoria tritici

Pathogen populations differ in the amount of genetic diversity they contain. Populations carrying higher genetic diversity are thought to have a greater evolutionary potential than populations carrying less diversity. We used published studies to estimate the range of values associated with two critical components of genetic diversity, the number of unique pathogen genotypes and the number of spores produced during an epidemic, for the septoria tritici blotch pathogen Zymoseptoria tritici. We found that wheat fields experiencing typical levels of infection are likely to carry between 3.1 and 14.0 million pathogen genotypes per hectare and produce at least 2.1-9.9 trillion pycnidiospores per hectare. Given the experimentally derived mutation rate of 3 x 10(-10) substitutions per site per cell division, we estimate that between 27 and 126 million pathogen spores carrying adaptive mutations to counteract fungicides and resistant cultivars will be produced per hectare during a growing season. This suggests that most of the adaptive mutations that have been observed in Z. tritici populations can emerge through local selection from standing genetic variation that already exists within each field. The consequences of these findings for disease management strategies are discussed.ISSN:1752-4571ISSN:1752-456

Une épidémie, combien ça coûte ?

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How large and diverse are field populations of fungal plant pathogens? The case of Zymoseptoria tritici

Pathogen populations differ in the amount of genetic diversity they contain. Populations carrying higher genetic diversity are thought to have a greater evolutionary potential than populations carrying less diversity. We used published studies to estimate the range of values associated with two critical components of genetic diversity, the number of unique pathogen genotypes and the number of spores produced during an epidemic, for the septoria tritici blotch pathogen Zymoseptoria tritici. We found that wheat fields experiencing typical levels of infection are likely to carry between 3.1 and 14.0 million pathogen genotypes per hectare and produce at least 2.1 to 9.9 trillion pycnidiospores per hectare. Given the experimentally derived mutation rate of 3 x 10-10 substitutions per site per cell division, we estimate that between 27 and 126 million pathogen spores carrying adaptive mutations to counteract fungicides and resistant cultivars will be produced per hectare during a growing season. This suggests that most of the adaptive mutations that have been observed in Z. tritici populations can emerge through local selection from standing genetic variation that already exists within each field. The consequences of these findings for disease management strategies are discussed