Frederic Suffert

Interactions between Head Blight Pathogens: Consequences for Disease Development and Toxin Production in Wheat Spikes

ABSTRACT Head blight (HB) is one of the most damaging diseases on wheat, inducing significant yield losses and toxin accumulation in grains. Fungal pathogens responsible for HB include the genus Microdochium, with two species, and the toxin producer genus Fusarium, with several species. Field studies and surveys show that two or more species can coexist within a same field and coinfect the same plant or the same spike. In the current study, we investigated how the concomitant presence of F. graminearum and another of the HB complex species influences the spike colonization and the toxin production by the fungi. To study these interactions, 17 well-characterized isolates representing five species were inoculated alone or in pairs on wheat spikes in greenhouse and field experiments. The fungal DNA in the grains was estimated by quantitative PCR and toxin contents (deoxynivalenol and nivalenol) by ultraperformance liquid chromatography-UV detection-tandem mass spectrometry. The responses of the different isolates to the presence of a competitor were variable and isolate specific more than species specific. The development of the most aggressive isolates was either unchanged or a slightly increased, while the development of the less aggressive isolates was reduced. The main outcome of the study was that no trend of increased toxin production was observed in coinoculations compared to single inoculations. On the contrary, the amount of toxin produced was often lower than expected in coinoculations. We thus conclude against the hypothesis that the co-occurrence of several HB-causing species in the same field might aggravate the risk linked to fusarium toxins in wheat production.

Development of a rapid multiplex SSR genotyping method to study populations of the fungal plant pathogen Zymoseptoria tritici

BackgroundZymoseptoria tritici is a hemibiotrophic ascomycete fungus causing leaf blotch of wheat that often decreases yield severely. Populations of the fungus are known to be highly diverse and poorly differentiated from each other. However, a genotyping tool is needed to address further questions in large collections of isolates, regarding regional population structure, adaptation to anthropogenic selective pressures, and dynamics of the recently discovered accessory chromosomes. This procedure is limited by costly and time-consuming simplex PCR genotyping. Recent development of genomic approaches and of larger sets of SSRs enabled the optimization of microsatellite multiplexing.FindingsWe report here a reliable protocol to amplify 24 SSRs organized in three multiplex panels, and covering all Z. tritici chromosomes. We also propose an automatic allele assignment procedure, which allows scoring alleles in a repeatable manner across studies and laboratories. All together, these tools enabled us to characterize local and worldwide populations and to calculate diversity indexes consistent with results reported in the literature.ConclusionThis easy-to-use, accurate, repeatable, economical, and faster technical strategy can provide useful genetic information for evolutionary inferences concerning Z. tritici populations. Moreover, it will facilitate the comparison of studies from different scientific groups.

Localized septoria leaf blotch lesions in winter wheat flag leaf do not accelerate apical senescence during the necrotrophic stage.

Mycosphaerella graminicola, the most damaging disease of wheat in Northern Europe, induces yield or grain quality losses usually related to green area losses. This work aims at clarifying whether induced senescence is due to an acceleration of apical senescence and thus to a modification of the leaf nitrogen remobilization rate. The effect on apical leaf senescence of a restricted diseased leaf area was investigated. The experiment involved four winter wheat cultivars varying in their susceptibility to four M. graminicola isolates. Flag leaves were inoculated at a dose representative of field epidemics. As soon as symptoms appeared, dynamics of apical senescence (Sapi) and around the inoculation zone (Ssep) were measured by repeatedly taking digital photos until senescent areas merged. Ssep trends were fit to a logistic function whose parameters depended on cultivar×isolate interactions. Disease severities ranged from 0 to 24% total leaf area. Apical senescence followed an exponential pattern in control and inoculated leaves. Sapi was twice as great in inoculated shoots as in controls, even when no disease occurred. The relative rate of Sapi depended on the cultivar, but no isolate effect was detected despite wide variations in Ssep extent. Paired comparisons between inoculated and control leaves showed that the relative rate of Sapi was not increased by inoculation. It is concluded, that over a wide range of Septoria disease, apical senescence was not accelerated in inoculated leaves as compared to control. Results suggest that the disease did not modify the regulation of leaf N remobilization rate.

Epidemiological trade-off between intra- and interannual scales in the evolution of aggressiveness in a local plant pathogen population

The efficiency of plant resistance to fungal pathogen populations is expected to decrease over time, due to their evolution with an increase in the frequency of virulent or highly aggressive strains. This dynamics may differ depending on the scale investigated (annual or pluriannual), particularly for annual crop pathogens with both sexual and asexual reproduction cycles. We assessed this time‐scale effect, by comparing aggressiveness changes in a local Zymoseptoria tritici population over an 8‐month cropping season and a 6‐year period of wheat monoculture. We collected two pairs of subpopulations to represent the annual and pluriannual scales: from leaf lesions at the beginning and end of a single annual epidemic and from crop debris at the beginning and end of a 6‐year period. We assessed two aggressiveness traits—latent period and lesion size—on sympatric and allopatric host varieties. A trend toward decreased latent period concomitant with a significant loss of variability was established during the course of the annual epidemic, but not over the 6‐year period. Furthermore, a significant cultivar effect (sympatric vs. allopatric) on the average aggressiveness of the isolates revealed host adaptation, arguing that the observed patterns could result from selection. We thus provide an experimental body of evidence of an epidemiological trade‐off between the intra‐ and interannual scales in the evolution of aggressiveness in a local plant pathogen population. More aggressive isolates were collected from upper leaves, on which disease severity is usually lower than on the lower part of the plants left in the field as crop debris after harvest. We suggest that these isolates play little role in sexual reproduction, due to an Allee effect (difficulty finding mates at low pathogen densities), particularly as the upper parts of the plant are removed from the field, explaining the lack of transmission of increases in aggressiveness between epidemics.

Mutual Exclusion between Fungal Species of the Fusarium Head Blight Complex in a Wheat Spike

ABSTRACT Fusarium head blight (FHB) is one of the most damaging diseases of wheat. FHB is caused by a species complex that includes two genera of Ascomycetes: Microdochium and Fusarium. Fusarium graminearum, Fusarium culmorum, Fusarium poae, and Microdochium nivale are among the most common FHB species in Europe and were chosen for these experiments. Field studies and surveys show that two or more species often coexist within the same field or grain sample. In this study, we investigated the competitiveness of isolates of different species against isolates of F. graminearum at the scale of a single spike. By performing point inoculations of a single floret, we ensured that each species was able to establish independent infections and competed for spike colonization only. The fungal colonization was assessed in each spike by quantitative PCR. After establishing that the spike colonization was mainly downwards, we compared the relative colonization of each species in coinoculations. Classical analysis of variance suggested a competitive interaction but remained partly inconclusive because of a large between-spike variance. Further data exploration revealed a clear exclusion of one of the competing species and the complete absence of coexistence at the spike level.

Trade-off between intra- and interannual scales in the evolution of aggressiveness in a local plant pathogen population

This preprint has been reviewed and recommended by Peer Community In Evolutionary Biology (http://dx.doi.org/10.24072/pci.evolbiol.100039). The efficiency of plant resistance to fungal pathogen populations is expected to decrease over time, due to its evolution with an increase in the frequency of virulent or highly aggressive strains. This dynamics may differ depending on the scale investigated (annual or pluriannual), particularly for annual crop pathogens with both sexual and asexual reproduction cycles. We assessed this time-scale effect, by comparing aggressiveness changes in a local Zymoseptoria tritici population over an eight-month cropping season and a six-year period of wheat monoculture. We collected two pairs of subpopulations to represent the annual and pluriannual scales: from leaf lesions at the beginning and end of a single annual epidemic, and from crop debris at the beginning and end of a six-year period. We assessed two aggressiveness traits – latent period and lesion size – on sympatric and allopatric host varieties. A trend toward decreased latent period concomitant with a significant loss of variability was established during the course of the annual epidemic, but not over the six-year period. Furthermore, a significant cultivar effect (sympatric vs. allopatric) on the average aggressiveness of the isolates revealed host adaptation, arguing that the observed patterns could result from selection. We thus provide an experimental body of evidence of an epidemiological trade-off between the intra- and inter-annual scales in the evolution of aggressiveness in a local plant pathogen population. More aggressive isolates were collected from upper leaves, on which disease severity is usually lower than on the lower part of the plants left in the field as crop debris after harvest. We suggest that these isolates play little role in sexual reproduction, due to an Allee effect (difficulty finding mates at low pathogen densities), particularly as the upper parts of the plant are removed from the field, explaining the lack of transmission of increases in aggressiveness between epidemics.

Stability in the genetic structure of a Zymoseptoria tritici population from epidemic to interepidemic stages at a small spatial scale

Subpopulations of the wheat pathogen Zymoseptoria tritici (26 sample groups composed of 794 pure strains) were collected in two nearby wheat fields, during both epidemic and interepidemic periods of three successive years (2009–2013). The alternative presence of wheat debris allowed taking into account different forms (ascospores vs. pycnidiospores) of inoculum and, thus, its putative origin (local vs. distant). We used a molecular epidemiology approach, based on population genetic indices derived from SSR marker analysis. The aim was to describe putative changes in the neutral genetic structure and diversity of these subpopulations over the time-course of several epidemics, in two close fields, i.e. at a spatiotemporal scale consistent with epidemiological observations, and to determine the possible structuring factors. Genetic structure was stable over time (within and between years) and between fields. All subpopulations displayed very high levels of gene and genotypic diversity. The low levels of linkage disequilibrium and the low clonal fraction, and the similar frequencies of the two mating types in most subpopulations were consistent with the regular occurrence of sexual reproduction in the two fields over the epidemic and interepidemic stages. In conclusion, at this fine spatiotemporal scale, we found that the period, the nature of inoculum and its putative origin had little effect on the short term evolution of the local population, with population size and diversity apparently large enough to prevent genetic drift, and with a major contribution of migration between and within plots to the stabilization of genetic diversity.