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.
Highlights
A continental scale dieback threatens the future existence of common ash (Fraxinus excelsior) and poses a set of cascading impacts upon the biodiversity associated with this keystone tree species in Europe (Pautasso et al, 2013; Mitchell et al, 2014)
Chosen compound leaves from the understory of common ash trees were sampled throughout the growing season in 2011 and 2012 in a stand located 30 km south of Oslo (As municipality, 59°40044′′N, 10°46031′′E,100 m above sea level), exhibiting epidemic levels of ash dieback
The presence of H. fraxineus DNA in all ash leaf tissues was first detected by quantitative PCR (qPCR) on 11 July, after which the pathogen DNA level showed a generally continuous increase (Fig. 2a), which coincided with the vigorous increase of airborne ascospores of H. fraxineus at the stand in July (Fig. 2c)
Summary
A continental scale dieback threatens the future existence of common ash (Fraxinus excelsior) and poses a set of cascading impacts upon the biodiversity associated with this keystone tree species in Europe (Pautasso et al, 2013; Mitchell et al, 2014). Recent studies indicate that the fungus is a leaf endophyte of Manchurian ash (Cleary et al, 2016) and can show some pathogenic potential in its native range (Drenkhan et al, 2016). Since 2001, an intensive spread of the disease has been observed in central, northern, eastern and western Europe (Juodvalkis & Vasiliauskas, 2002; Przybył, 2002; Kowalski & Łukomska, 2005; Lygis et al, 2005; Kowalski, 2006; Timmermann et al, 2011), and only populations at the southern and eastern range margins of common ash currently remain disease-free (McKinney et al, 2014)
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