Abstract
Large-scale ash (Fraxinus spp.) dieback caused by the fungus Hymenoscyphus fraxineus has been a major concern throughout Europe for more than two decades. Most of the related research has been focused on Fraxinus excelsior L., and there is still little information on fungal involvement in the dieback of Fraxinus angustifolia Vahl, especially in roots and stem bases, which play an important role in decline progress and tree stability. The objectives of this study were to identify fungi present in visually healthy and symptomatic wood tissues in basal parts of narrow-leaved ash trees in different decline phases, in order to determine the possible role of these fungi and their importance in the dieback process. The stem bases and roots of 90 trees in three different health categories, determined based on crown defoliation, were sampled in natural stands affected by ash dieback. Isolated fungal cultures were identified based on the rDNA ITS (Internal transcribed spacer) region and their association with tree health status was analyzed. In total, 68 different fungal taxa were confirmed, including Hymenoscyphus fraxineus and Armillaria spp., which were mainly present in roots, although overall in lower frequencies than on common ash in other studies. Most frequently isolated fungal taxa, which encompassed 51% of all obtained isolates, were Trichoderma spp., Ilyonectria robusta, Fusarium solani, Cladosporium cladosporioides, and Diaporthe cotoneastri. Their associations with tree health categories and presence in both symptomatic and visually healthy wood indicate that they act as opportunistic pathogens and early colonizers of weakened ash tissues. Research also revealed that, although the extent of crown defoliation and presence of root and stem necroses were associated, basal symptoms occurred on a number of trees with healthy looking crowns, meaning they can develop independently.
Highlights
Narrow-leaved ash (Fraxinus angustifolia Vahl) is a species of great ecological and economical value in lowland and riparian forests of central Europe, the Pannonian Basin, and the Balkans [1], as well as in Croatia where it grows in single-species or mixed stands as the second most abundant floodplain tree species after pedunculate oak (Quercus robur L.), with growth stock of 17,619,000 m3 (3.19% of total stock), occupying 72,690 ha (3.06% of total forest area) [2]
Out of 68 different fungal taxa found in this research in roots and stems of narrow-leaved ash, 20 were found on declining common ash in similar studies, including already confirmed and well-known ash pathogens Hymenoscyphus fraxineus and Armillaria spp., and Coprinellus micaceus, Diaporthe cotoneastri, Ilyonectria robusta, Metapochonia bulbillosa (Pochonia bulbillosa), Neonectria punicea, Psathyrella candolleana, Pseudeurotium bakeri, Trametes versicolor, and Xylaria polymorpha in roots and stem bases [25,30,31,53]; Cladosporium cladosporioides, Clonostachys rosea (Bionectria ochroleuca), Nemania serpens, Paraphaeosphaeria neglecta, and Penicillium glabrum in shoots or branches [11,12,13,14,23,54,55]; and Coprinellus disseminatus, Epicoccum nigrum, Fusarium lateritium, and Fusarium solani in both basal parts and crowns [11,12,13,14,15,19,25,30,31,53,54,55,56]
This study revealed 68 different fungal taxa present in roots and stem bases of narrow-leaved ash trees in different phases of decline, including previously reported and well-known pathogens, Hymenoscyphus fraxineus and Armillaria spp
Summary
Narrow-leaved ash (Fraxinus angustifolia Vahl) is a species of great ecological and economical value in lowland and riparian forests of central Europe, the Pannonian Basin, and the Balkans [1], as well as in Croatia where it grows in single-species or mixed stands as the second most abundant floodplain tree species after pedunculate oak (Quercus robur L.), with growth stock of 17,619,000 m3 (3.19% of total stock), occupying 72,690 ha (3.06% of total forest area) [2]. In spite of possible explanations, the absence of H. fraxineus in parts of analyzed declining narrow-leaved ash trees in Croatia raised the question of the role of other fungi in the dieback process, as this theory had already been introduced in similar research conducted mainly on common ash, where various known pathogenic and opportunistic fungi were found in symptomatic tissues. Some of these species (Alternaria alternata (Fr.) Keissl., Cytospora pruinosa (Fr.) Sacc., Diaporthe eres Nitschke, Diplodia mutila (Fr.) Mont., Epicoccum nigrum Link, Fusarium avenaceum (Fr.) Sacc., F. lateritium Nees, F. oxysporum
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