Genetic analysis reveals efficient sexual spore dispersal at a fine spatial scale in Armillaria ostoyae, the causal agent of root-rot disease in conifers

Armillaria ostoyae (sometimes named Armillaria solidipes) is a fungal species causing root diseases in numerous coniferous forests of the northern hemisphere. The importance of sexual spores for the establishment of new disease centres remains unclear, particularly in the large maritime pine plantations of southwestern France. An analysis of the genetic diversity of a local fungal population distributed over 500 ha in this French forest showed genetic recombination between genotypes to be frequent, consistent with regular sexual reproduction within the population. The estimated spatial genetic structure displayed a significant pattern of isolation by distance, consistent with the dispersal of sexual spores mostly at the spatial scale studied. Using these genetic data, we inferred an effective density of reproductive individuals of 0.1-0.3 individuals/ha, and a second moment of parent-progeny dispersal distance of 130-800 m, compatible with the main models of fungal spore dispersal. These results contrast with those obtained for studies of A. ostoyae over larger spatial scales, suggesting that inferences about mean spore dispersal may be best performed at fine spatial scales (i.e. a few kilometres) for most fungal species

Multiple introductions of divergent genetic lineages in an invasive fungal pathogen, Cryphonectria parasitica, in France

The occurrence of multiple introductions may be a crucial factor in the successful establishment of invasive species, but few studies focus on the introduction of fungal pathogens, despite their significant effect on invaded habitats. Although Cryphonectria parasitica, the chestnut blight fungus introduced in North America and Europe from Asia during the 20th century, caused dramatic changes in its new range, the history of its introduction is not well retraced in Europe. Using 10 microsatellite loci, we investigated the genetic diversity of 583 isolates in France, where several introductions have been hypothesized. Our analyses showed that the seven most frequent multilocus genotypes belonged to three genetic lineages, which had a different and geographically limited distribution. These results suggest that different introduction events occurred in France. Genetic recombination was low among these lineages, despite the presence of the two mating types in each chestnut stand analysed. The spatial distribution of lineages suggests that the history of introductions in France associated with the slow expansion of the disease has contributed to the low observed rate of recombination among the divergent lineages. However, we discuss the possibility that environmental conditions or viral interactions could locally reduce recombination among genotypes

Resistance, tolerance and competence for a root pathogen in six woody species

The outcome of host-pathogen interactions depends on host defence and pathogen infection strategies. On the host side, two strategies, resistance and tolerance, have long been identified. They differ in their underlying mechanisms and their effects on pathogens. However, the dynamics of disease transmission also rely on host competence. This trait, defined as the ability of the host to allow pathogen transmission, is rarely studied for pathogen-supporting plants. We assessed the competence for Phytophthora cinnamomi of seedlings of five Quercus and one Castanea species and studied how this trait is correlated with resistance and tolerance. P. cinnamomi is responsible for the chestnut ink disease, alongside with Phytophthora x cambivora, which has a much less generalist behaviour. Each seedling was inoculated in minirhizotrons by root-mycelium contact and phenotyped for its resistance, tolerance and competence. The amount of necrosis (lesion length), the effect of infection on plant performance (root growth) and the sporangia number were measured. Moreover, we developed a droplet digital (dd) PCR assay to quantify the pathogen within host tissues near the inoculation point. This measure was used as a proxy of pathogen load. We were thus able to relate individual host competence to the pathogen content measured in the same root fragment. No significant correlation between the phenotyped traits was found at the individual level. However, at the species level, the least competent species (Q. robur, Q. petraea) proved to be the most resistant. These results may have important implications for management of chestnut and oak ecosystems threatened by ink disease

Foliar fungal communities strongly differ between habitat patches in a landscape mosaic

ABSTRACT Background: Dispersal events between habitat patches in a landscape mosaic can structure ecological communities and influence the functioning of agrosystems. Here we investigated whether short-distance dispersal events between vineyard and forest patches shape foliar fungal communities. We hypothesized that these communities homogenize between habitats over the course of the growing season, particularly along habitat edges, because of aerial dispersal of spores. Methods: We monitored the richness and composition of foliar and airborne fungal communities over the season, along transects perpendicular to edges between vineyard and forest patches, using Illumina sequencing of the Internal Transcribed Spacer 2 (ITS2) region. Results: In contrast to our expectation, foliar fungal communities in vineyards and forest patches increasingly differentiate over the growing season, even along habitat edges. Moreover, the richness of foliar fungal communities in grapevine drastically decreased over the growing season, in contrast to that of forest trees. The composition of airborne communities did not differ between habitats. The composition of oak foliar fungal communities change between forest edge and centre. Discussion: These results suggest that dispersal events between habitat patches are not major drivers of foliar fungal communities at the landscape scale. Selective pressures exerted in each habitat by the host plant, the microclimate and the agricultural practices play a greater role, and might account for the differentiation of foliar fugal communities between habitats

Cryphonectria parasitica vegetative compatibility type analysis of populations in south-western France and northern Spain

A comprehensive study of the population biology of Cryphonectria parasitica, the causal agent of chestnut blight, is required to understand the spread of the epidemic in Europe and its natural regulation by the Cryphonectria hypovirus-1 (CHV-1). With this objective in mind, the diversity in vegetative compatibility (vc) types and mating types of C. parasitica populations was assessed in 43 chestnut sites in western France and northern Spain. Isolates were sampled in three regions along the Pyrénées mountains (Hautes Pyrénées, Pyrénées Atlantiques and Navarra) and in two regions north of the Pyrénées (Landes and Dordogne). There were 61 vc types observed in 682 isolates sampled. Ninety-five isolates (14%) could not be assigned to any one of the previously known European vc types. The finding of 47 incompatible vc types confirmed that the genetics of vegetative compatibility could not be totally accounted for by six diallelic vic genes. The two idiomorphs of the mating type gene were detected in all regions, indicating that sexual reproduction could occur within and between vc types. In all regions except Dordogne, C. parasitica populations were dominated either by the vc type EU-66 or EU-72. Neither vc type has been reported elsewhere in Europe, which suggests that two different introductions of C. parasitica have occurred in the study area. In Dordogne, populations were dominated by EU-33 and EU-2, and to a lesser extent EU-66 and EU-72. The low diversity in vc types for most of the C. parasitica populations provides good opportunities for natural regulation in forest coppices and for biocontrol with CHV-1 in orchards

Effects of variable root damage caused by Phytophthora cinnamomi on water relations of chestnut saplings

International audienc

Spatial variability of phyllosphere fungal assemblages: genetic distance predominates over geographic distance in a European beech stand (Fagus syluatica)

Despite being major drivers of the dynamics and diversity of plant populations and communities, the spatial variability of phyllosphere fungal assemblages has been seldom explored. We used CE-SSCP and 454 pyrosequencing to quantify the spatial variability of European beech phyllosphere fungal assemblages with a hierarchical sampling design including four different spatial scales (tree, branch, group of leaves and individual leaf). Fungal assemblages were highly diverse, with high proportions of generalist and cosmopolitan fungi. The major part of the variability was at the smallest spatial scale, between individual leaves. Within a single tree canopy, dissimilarity between fungal assemblages generally increased with distance between leaves. This pattern may be driven by within-canopy gradients of leaf traits and microclimate. At the stand scale, dissimilarity between fungal assemblages was correlated with the genetic distance rather than the geographic distance between trees, consistent with the findings of community genetics studies. Our results were constant over a small simulated sequencing depth, providing opportunities for the design of large-scale studies addressing the relationship between the genetic variation of trees and the variation of associated phyllosphere fungal assemblages

Distribution de Phytophthora Spp. dans les chênaies et chataigneraies françaises

National audienc

The composition of phyllosphere fungal assemblages of European beech (Fagus sylvatica) varies significantly along an elevation gradient

Little is known about the potential effect of climate warming on phyllosphere fungi, despite their important impact on the dynamics and diversity of plant communities. The structure of phyllosphere fungal assemblages along elevation gradients may provide information about this potential effect, because elevation gradients correspond to temperature gradients over short geographic distances. We thus investigated variations in the composition of fungal assemblages inhabiting the phyllosphere of European beech (Fagus sylvatica) at four sites over a gradient of 1000 m of elevation in the French Pyrenees Mountains, by using tag-encoded 454 pyrosequencing. Our results show that the composition of fungal assemblages varied significantly between elevation sites, in terms of both the relative abundance and the presenceabsence of species, and that the variations in assemblage composition were well correlated with variations in the average temperatures. Our results therefore suggest that climate warming might alter both the incidence and the abundance of phyllosphere fungal species, including potential pathogens. For example, Mycosphaerella punctiformis, a causal agent of leaf spots, showed decreasing abundance with elevation and might therefore shift to higher elevations in response to warming