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Septoria tritici blotch (STB), caused by Mycosphaerella graminicola, is the most prevalent disease of wheat worldwide. Primary inoculum and the early stages of STB epidemics are still not fully understood and deserve attention for improving management strategies. The inoculum build-up and overseasoning involves various fungal structures (ascospores, pycnidiospores, mycelium) and plant material (wheat seeds, stubble and debris; wheat volunteers; other grasses). Their respective importance is assessed in this review. Among the mechanisms involved in the early stages of epidemics and in the year-to-year disease transmission, infection by ascospores wind-dispersed from either distant or local infected wheat debris is the most significant. Nevertheless, infection by pycnidiospores splash-dispersed either from neighbouring wheat debris or from senescent basal leaves has also been inferred from indirect evidence. Mycosphaerella graminicola has rarely been isolated from seeds so that infected seed, although suspected as a source of primary inoculum for a long time, is considered as an epidemiologically anecdotal source. Mycosphaerella graminicola can infect a few grasses other than wheat but the function of these grasses as alternative hosts in natural conditions remains unclear. Additionally, wheat volunteers are suspected to be sources of STB inoculum for new crops. This body of evidence is summarized in a spatio-temporal representation of a STB epidemic aimed at highlighting the nature, sources and release of inoculum in the early stages of the epidemic.

Septoria tritici blotch, caused by Mycosphaerella graminicola, is a major foliar disease of wheat. The quantitative traits of pathogenicity are not comprehensively described in this pathosystem. The objective of this study was to identify and quantify the most relevant variables to describe traits of aggressiveness. Four wheat cultivars were inoculated in a greenhouse with four isolates. Inoculation was performed on a limited surface of the two uppermost leaves of adult plants. The dynamics of chlorotic, necrotic and sporulating areas were assessed twice a week. Pycnidia were counted at the same time. A Gompertz model was fitted to the resulting curves. Parameter combinations with easily interpreted biological relevance were examined further as descriptors of aggressiveness. Within each category of descriptor, those which were the most pairwise correlated and which explained the largest part of the variance were retained: incubation and latent period, development rate of sporulating area, maximal sporulating area, pycnidial density, and sporulation capacity. Correlations between these variables were discussed, assuming they reflect biological relationships between the corresponding aggressiveness quantitative traits. It is suggested that the selected variables, providing a good measure of M. graminicola fitness, can be used to estimate quantitative resistance of wheat to septoria tritici blotch, to characterize differences among isolates within a pathogen population, and to study quantitative adaptation of the pathogen to its host and to its environment.

A first comprehensive inventory of alien fungi and fungal-like organisms (in Stramenopila) recorded in France since 1800 was established, comprising 227 species, with 64.7% plant pathogens, 29.5% saprotrophic species, 3.5% ectomycorrhizal fungi, 1.3% animal parasites and 0.9% mycopathogenic fungi. Using this and a previously built European dataset, correlates of invasion success in fungi (sensu lato) were investigated, especially for pathogenic species occurring in wild environments (mostly forest tree pathogens). Several common features were demonstrated at the two spatial scales. Some taxonomic/phylogenetic orders were shown to be over-represented in alien fungi and Stramenopila pseudo-fungi, e.g. Peronosporales and to have faster spread, e.g. Erysiphales. Residence time and economic variables, especially imports, were important explaining variables of the levels of invasion. The influence of climatic factors was also suggested.

SummaryThe thermal performance curve is an ecological concept relating the phenotype of organisms and temperature. It requires characterization of the leaf temperature for foliar fungal pathogens. Epidemiologists, however, use air temperature to assess the impacts of temperature on such pathogens. Leaf temperature can differ greatly from air temperature, either in controlled or field conditions. This leads to a misunderstanding of such impacts.Experiments were carried out in controlled conditions on adult wheat plants to characterize the response of Mycosphaerella graminicola to a wide range of leaf temperatures. Three fungal isolates were used. Lesion development was assessed twice a week, whereas the temperature of each leaf was monitored continuously.Leaf temperature had an impact on disease dynamics. The latent period of M. graminicola was related to leaf temperature by a quadratic relationship. The establishment of thermal performance curves demonstrated differences among isolates as well as among leaf layers.For the first time, the thermal performance curve of a foliar fungal pathogen has been established using leaf temperature. The experimental setup we propose is applicable, and efficient, for other foliar fungal pathogens. Results have shown the necessity of such an approach, when studying the acclimatization of foliar fungal pathogens.

The contribution of wheat debris to the early stages of septoria leaf blotch epidemics was assessed in a 3-year field experiment. First lesions were detected very early (December) in the case of an early sowing (mid-October), showing that the first contamination could occur as soon as the seedlings emerge. The tested debris management options (chopped debris, removal of debris followed by tillage, or tillage in absence of debris) had a strong effect, although transient, on the epidemic dynamic: the more debris present on the soil surface, the more severe initial disease was. The magnitude of differences between treatments differed substantially between years. The relative production of pycnidiospores and ascospores was measured on the chopped debris. Peaks in pycnidiospore and ascospore production coincided in October-November. Both types of spores can be involved as primary inoculum in north-west European conditions. The local amount of pycnidiospores available on debris in the field, estimated per square metre, was 1000-fold the local ascospore production. Moreover, inoculum production was quantified on debris exposed to different environmental conditions. Autumnal conditions, characterized by moderate temperature with alternating wet and dry periods, were favourable for the production of both pycnidiospores and ascospores, as shown by the high inoculum production on debris exposed to field or outdoor conditions. By late autumn, the canopy became the most important source of pycnidiospores, and this period, characterized by the decreasing role of debris as a local source of inoculum compared to distant potential sources, can be considered as the end of the early epidemic stages.

cIn 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.T hermal variation, including seasonal fluctuations and also climate change, appears to be one of the main drivers of the predicted change in plant disease distributions (1-3). The adaptive response of plant pathogens to thermal variations may involve the movement of pathogen populations, as well as phenotypic plasticity and selection, so that full characterization requires extensive measurements of fitness traits under various environmental conditions. How the seasonality of disease development (e.g., the duration of the epidemic and interepidemic periods, through modifications of the biotic and abiotic environment) will be modified by climate change is a question increasingly addressed in the literature (4-6). In contrast, how seasonal climate variation within a year currently affects the dynamics of pathogen populations has received less attention. However, this knowledge would be very useful for characterizing the potential effects of warming on future population dynamics. Indeed, the effects of seasonal variation are measurable at this time and offer very interesting prospects for experimentation. Investigation of such current effects of seasonality is particularly interesting because they might induce shortterm selection in the fitness traits of pathogen populations that...

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