Fungal diversity and seasonal succession in ash leaves infected by the invasive ascomycete Hymenoscyphus fraxineus

Abstract

Summary High biodiversity is regarded as a barrier against biological invasions. We hypothesized that the invasion success of the pathogenic ascomycete Hymenoscyphus fraxineus threatening common ash in Europe relates to differences in dispersal and colonization success between the invader and the diverse native competitors.Ash leaf mycobiome was monitored by high‐throughput sequencing of the fungal internal transcribed spacer region (ITS) and quantitative PCR profiling of H. fraxineus DNA.Initiation of ascospore production by H. fraxineus after overwintering was followed by pathogen accumulation in asymptomatic leaves. The induction of necrotic leaf lesions coincided with escalation of H. fraxineus DNA levels and changes in proportion of biotrophs, followed by an increase of ubiquitous endophytes with pathogenic potential. H. fraxineus uses high propagule pressure to establish in leaves as quiescent thalli that switch to pathogenic mode once these thalli reach a certain threshold – the massive feedback from the saprophytic phase enables this fungus to challenge host defenses and the resident competitors in mid‐season when their density in host tissues is still low. Despite the general correspondence between the ITS‐1 and ITS‐2 datasets, marker biases were observed, which suggests that multiple barcodes provide better overall representation of mycobiomes.

The use of ascospores of the dieback fungus Hymenoscyphus fraxineus for infection assays reveals a significant period of biotrophic interaction in penetrated ash cells

“…() and Cross et al . () may simply reflect more extensive biotrophic development. Alternatively there may be a period of stasis on the leaf surface, perhaps after formation of an anchoring appressorial structure or within the initially penetrated cell.…”

“…Cleary et al (2013) recorded a latent period between exposure to ascospores and lesion appearance on leaves, estimated to be c. 2 weeks. A recent study using high-throughput sequencing and quantitative PCR identified pathogen accumulation in symptomless leaf tissue under forest conditions and suggested the establishment of quiescent thalli of the pathogen before lesion appearance, but no correlative microscopical studies were carried out (Cross et al, 2017). Screening accessions for sources of resistance to dieback has relied on mycelial inocula and wound inoculation (Kowalski et al, 2015;Lobo et al, 2015;Gross & Sieber, 2016;Schwanda & Kirisits, 2016).…”

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

“…This could be due to a better ''ecological match'' between pathogen and new host (Desprez-Loustau et al 2016), and because only few evolutionary changes are needed for the pathogen in order to adapt to the new host (Schulze-Lefert and Panstruga 2011). Other determining factors are the presence of pre-evolved defense mechanisms in the tree (Ennos 2015) and the ability of the new host to support the buildup of high infection pressure that is obviously crucial for the pathogenicity and invasiveness of H. fraxineus (Cross et al 2016). Taking the long generation time of trees into consideration, adaptation of trees to newly introduced and quickly evolving pathogens is likely to rely on pre-existing genetic variation rather than the buildup of new mutations (Barrett and Schluter 2008).…”

Direct evidence of Hymenoscyphus fraxineus infection pathway through the petiole‐shoot junction

“…Another factor that may affect spread through the petiole is competition from endophytic fungi that may be present in the petioles (Cross et al, 2016;Scholtysik, Unterseher, Otto, & Wirth, 2013). Another factor that may affect spread through the petiole is competition from endophytic fungi that may be present in the petioles (Cross et al, 2016;Scholtysik, Unterseher, Otto, & Wirth, 2013).…”

“…Another factor that may affect spread through the petiole is competition from endophytic fungi that may be present in the petioles (Cross et al, 2016;Scholtysik, Unterseher, Otto, & Wirth, 2013). However, their protective effect in vivo remains unclear, and H. fraxineus appears to hold an advantage due to its early and immense sporulation pressure (Cross et al, 2016). However, their protective effect in vivo remains unclear, and H. fraxineus appears to hold an advantage due to its early and immense sporulation pressure (Cross et al, 2016).…”