Expansion of Ash Dieback towards the scattered Fraxinus excelsior range of the Italian peninsula

Abstract

Hymenoscyphus fraxineus, causal agent of Ash Dieback, has posed a threat to Fraxinus excelsior (common ash) in Europe since the 1990s. In south-western Europe, optimal climatic conditions for H. fraxineus become scattered and host density decreases, reducing disease spread rates. To date, the Ash Dieback agent has not been reported from southern and most of central Italy, where native F. excelsior is present as small fragmented populations. This study examines the expansion of Ash Dieback into central Italy, and it considers the consequences of further local spread with regards to the loss of F. excelsior genetic resource. Symptomatic F. excelsior were sampled from sixteen sites in northern and central Italy during 2020. Specimens were analyzed with a culturomics and a quantitative PCR approach. A bibliographic search of F. excelsior floristic reports was conducted for the creation of a detailed range map. The combined use of both techniques confirmed the presence of H. fraxineus in all the sites of central Italy where host plants were symptomatic. These new records represent the southern limit of the current known distribution of this pathogen in Italy, and together with Montenegro, in Europe. The characterization of the F. excelsior scattered range suggests that further spread of Ash Dieback across southern Italy is a realistic scenario. This presents a threat not just to the southern European proveniences of F. excelsior, but to the species as a whole, should Ash Dieback lead to the loss of warm climate adapted genetic material, which may become increasingly valuable under climate change.

Botryosphaeriaceae Species Associated with Stem Canker, Shoot Blight and Dieback of Fraxinus ornus in Italy

“…Studies dealing with Fraxinus dieback in Europe, including the Mediterranean areas, have mainly focused on assessing the pathogenic role of the ascomycete fungus H. fraxineus [4][5][6][7]. Only recently, this and a few other investigations have started taking into consideration the possible role of fungi other than H. fraxineus, among which the Botryosphaeriaceae, in the onset of ash decline/dieback [10,31,32].…”

“…Due to the poor quality of its wood, F. ornus always received little attention from humans, and this is why no serious threat had been documented for this tree species until the reporting in Italy of the ash dieback disease caused by the ascomycete fungus Hymenoscyphus fraxineus [4]. H. fraxineus occurrence has been documented in some Italian regions since 2009, but although the fungus is able to infect all the ash species growing there, i.e., common ash (Fraxinus excelsior), flowering ash (Fraxinus ornus) and narrow-leaved ash (Fraxinus angustifolia), serious damage has been reported only on F. excelsior [5][6][7].…”

Features of the Territorial Distribution, Composition and Structure of Phytocenoses with the Participation of Fraxinus excelsior, Their Resource Qualities, Ecological and Economic Importance (Southeastern Part of the East European Plain)

“…The expediency of protecting populations of this species can be traced in different parts of the range of the species under consideration [ 82 , 83 , 84 , 85 ]. The protection of the existing forests with the participation of Fraxinus excelsior L. in the south-eastern part of the East European Plain is a strategic ecological and economic task.…”

Pathogenic fungi and oomycetes causing dieback on Fraxinus species in the Mediterranean climate change hotspot region

“…All these variable symptoms represent a complex syndrome that substantially differ in its etiology and pattern from the simple pathosystem model ash dieback-Hymenoscyphus fraxineus. Regarding this latter fungus, it has being expanding since 2009 in various Italian regions starting from the North-Eastern Alps to some areas in the center of the country along the Apennines (Ogris et al, 2010;Luchi et al, 2016;Migliorini et al, 2022). The disease involves all the three above-named ash species, with particular incidence on common ash (Fraxinus excelsior) (Panconesi et al, 2014;Rigling et al, 2018).…”

“…The disease involves all the three above-named ash species, with particular incidence on common ash (Fraxinus excelsior) (Panconesi et al, 2014;Rigling et al, 2018). This helotiaceous fungus prefers the cold and humid valleys of the mountain areas of North Italy and North-Central Apennines, its current southern range being some scattered sites in mountain areas with Mediterranean climatic conditions, characterized by cold and snowy winters and cool summers with absence of drought (Migliorini et al, 2022). However, it is unlikely that it will succeed in expanding southward, being limited by the unfavorable conditions of the Mediterranean climate, characterized by mild-dry winters and hot summers with prolonged droughts even in mountainous areas.…”