Fraxinus

The invasive pathogen, ash dieback fungus Hymenoscyphus fraxineus, is spreading rapidly across Europe. It shows high levels of outcrossing and limited population structure, even at the epidemic front. The anamorphic (asexual) form produces prolific conidia, thought to function solely as spermatia (male gametes), facilitating gene flow between sympatric strains. Here, we show that conidia are capable of germination on ash leaves and in vitro, and can infect seedlings via leaves or soil. In leaves, germlings form structures resembling fruiting bodies. Additionally, H. fraxineus colonises ash debris and grows in soil in the absence of ash tissues. We propose an amended life-cycle in which wind-dispersed, insect-vectored or water-spread conidia infect ash and may sporulate in planta, as well as in forest debris. This amplifies inoculum levels of different strains in ash stands. In combination with their function as spermatia, conidia thus act to maximise gene flow between sympatric strains, including those originally present at low inoculum. Such mixing increases evolutionary potential, as well as enhancing the likelihood of gene introgression from closely-related strains or assimilation of further genetic diversity from parental Asian populations. This scenario increases the adaptability of H. fraxineus to new climates and, indeed, onto new host species. The ascomycete fungus Hymenoscyphus fraxineus 1 is the causal agent of ash dieback 2. This disease is currently destroying ash trees across Europe 3–5. First observed in Poland in 1992 3 , it has now spread to more than 25 European countries 6. This pathogen is aggressive 7 , causing leaf necrosis, premature leaf drop, shoot wilting and crown dieback, as well as necrotic lesions on petioles, stems and root collars 7. Mortality is high in infected seedlings , whilst older trees develop chronic infections which are often eventually fatal (e.g. ref. 8). The loss of trees has a significant ecological and economic impact 9 , as ash is an important tree of mature woodland and hedgerows, as well as yielding commercial timber 10,11. Hymenoscyphus fraxineus is pleiomorphic; the anamorph (previously Chalara fraxinea 12) produces prolific asexual spores 13,14. However, these conidia have not been thought to have a role in the spread of the fungus, being described as " sticky " 3,15 and unlikely to become airborne. They are instead assumed to act as spermatia 13,16 during ascospore formation by the teleomorph (previously H. pseudoalbidus 17). This hypothesis is supported by the discovery that ascospores derived from individual apothecia show levels of genetic polymorphism indicative of multiple mating partners 13,14 and by the observation that the anamorph sporulates at the edge of the pseudoscle-rotial plate from which the apothecia emerge 18. Evidence suggests that sexual reproduction is of key importance the spread of H. fraxineus, a heterothallic fungus 13. Population genetic studies have found high intra-population variability, along with little inter-population variability 19 and a lack of population genetic structure 14,20–24 across the entire continent of Europe. These data indicate high gene flow between populations, consistent with a high rate of outcrossing 14. Indeed, while clear evidence of a founder effect can be seen when comparing the genotypes of the invasive pathogen populations in Europe with the much more variable genotypes displayed by native H. fraxineus populations in Asia, no such founder effect has been detected in the study of newly-discovered populations at the epidemic front 19. The closely-related species, Hymenoscyphus albidus, a non-pathogenic saprophyte on ash debris 25,26 , is being displaced