Friend or foe? Biological and ecological traits of the European ash dieback pathogen Hymenoscyphus fraxineus in its native environment

Abstract

Hymenoscyphus fraxineus, an introduced ascomycete fungus and primary causal agent of European ash dieback, was investigated on Fraxinus mandshurica trees in its native range in Primorye region of Far East Russia. This evidence is the first report of H. fraxineus on healthy, asymptomatic F. mandshurica trees. High-throughput sequencing revealed 49 distinct fungal taxa associated with leaves of F. mandshurica, 12 of which were identified to species level. Phyllosphere fungal assemblages were similar among sites despite being largely geographically distant. Many organisms comprising the foliar fungal community on F. mandshurica in Far East Russia have similarity to those reported inhabiting F. excelsior in Europe based on previous studies. However, Mycosphaerella sp., the most dominant species in this study and detected in nearly all samples, was associated only with F. mandshurica. Genetic diversity of H. fraxineus was significantly higher in the Far East Russian population than in Europe. In contrast to its aggressive behaviour on Fraxinus excelsior in Europe, H. fraxineus appears to be a benign associate of indigenous F. mandshurica that initially induces quiescent and asymptomatic infections in healthy trees prior to active host colonization normally associated with modification of host tissue during senescence.

The susceptibility of Asian, European and North American Fraxinus species to the ash dieback pathogen Hymenoscyphus fraxineus reflects their phylogenetic history

“…A recent study confirmed the presence of H. fraxineus in asymptomatic leaves of F. mandshurica from Eastern Russia (Cleary et al 2016). In another recent study, significantly higher levels of H. fraxineus DNA were July 1st, 3 July 15th, 4 July 29th 2014.…”

“…In Europe the disease was first reported in Poland in the early 1990s and Eastern Poland and/ or the South Baltic has been suggested as epicenter for the disease (Drenkhan et al 2014;McKinney et al 2014). Current evidence indicates that H. fraxineus originates from Asia, where it is associated with local Fraxinus species, Fraxinus mandshurica and F. chinensis (Cleary et al 2016;Gross et al 2014a;Zhao et al 2013;Zheng and Zhuang 2014). Observations from a botanical garden in Estonia (Drenkhan and Hanso 2010) and from forest conditions in Japan (Zhao et al 2013) and far East Russia suggest that F. mandshurica has lower susceptibility to shoot infection by H. fraxineus than F. excelsior, although lesions may develop after direct stem inoculation [on F. mandshurica var.…”

“…Regarding potential pathways of introduction of H. fraxineus to North America arrival via airborne ascospores seems less plausible than introduction via import of infected but asymptomatic plants, seeds (Cleary et al 2013a) or wood (Husson et al 2012). Given the higher genetic variation (Cleary et al 2016;Gross et al 2014b) and presumably also higher virulence of East Asian strains (Gross and Sieber 2016), introductions from East Asia would be especially critical (Landolt et al 2016). Our results highly support measures taken to prevent unintended introduction of H. fraxineus via ash products to North America, a scenario that would likely result in species decline in one of the two main centers of diversity of genus Fraxinus.…”

Effects of Host Variability on the Spread of Invasive Forest Diseases

“…Although most plant species are susceptible to infection and damage by one or more pathogens, some species or individuals may exhibit different degrees of susceptibility [50]. For example, within the genus Fraxinus, there is large variation between species in susceptibility to the ash dieback pathogen Hymenoscyphus fraxineus [51,52] (see Section 3.1 Case studies), and large genotypic effects in susceptibility among individuals of European ash (Fraxinus excelsior L.) [53]. Similarly, R. lauricola affects several members of the laurel family (Lauraceae) in the USA though lethal damage is most prominent on only a few native hosts including red bay (Persea borbonia (L.) Spreng.…”

Control of Hymenoscyphus fraxineus, the causal agent of ash dieback, using composting

“…The fungus produces large numbers of these spores which are aerially dispersed during summer. The apothecia are produced largely on the blackened pseudosclerotial rachises from the previous year in the leaf litter (Cleary et al, 2016). The apothecia which release the ascospores are produced seasonally and have been observed in the field between June and October in Europe, occurring mainly on fallen ash rachises in the fallen litter layer from the previous year, but also occasionally on dead ash shoots.…”