Written by Francesco Benvenuti, University of Edinburgh
Hymenoscyphus pseudoalbidus Q. (anamorph Chalara fraxinea K.) is an ascomycete fungal pathogen which, since the early 90’s, has caused a serious ash tree disease in Europe known as “ash dieback”[1,2,3]. Early episodes were reported in Poland since 1992, affecting forest and urban trees as well as saplings in nurseries. Since then, it rapidly spread across European countries causing extensive death of various ash species such as Fraxinus excelsior and Fraxinus angustifolia [4-7]. Scandinavia for instance, reported over 90% of national ash trees diseased .
Since February 2012 its presence was also reported in the UK, unleashing a rapid governmental and scientific community response to design appropriate responsive management strategies for the disease mitigation[9,10]. As ash is the third most abundant UK native broadleaf tree species, such an outbreak poses a serious threat to the national forestry sector, domestic wood supply and aesthetic natural heritage.
By assembling evidence from recent literature, this report will firstly carry out a review of the current state of knowledge about H. pseudoalbidus ecology. Secondly, management action already taken by the UK government will be discussed. Finally, management recommendations are put forward to aid currently unaffected tree nursery managers and forest land-owners to take future decisions with awareness of their long-term implications on the future of national forests.
H. pseudoalbidus ecology
Although ash dieback was initially associated to the fungus asexual stage C. fraxinea , the fungal pathogen sexual form, H. pseudoalbidus, was most recently recognised as the actual strain causing the disease. H. pseudoalbidus is an Ascomycete fungus which infects the living tissue of ash leaves, rapidly spreading across the tree canopy as well as through the branches to reach the main tree stem[1,9,10].
Although the infection symptoms are initially limited to the widespread and rapid death of canopy leaves, the disease also aggressively infects shoots, bark and the tree stem, ultimately leading to evident stem abrasions (i.e. necrotic lesions, cankers and sapwood discolouration) and to the tree’s death[5,9,10]. Namely, the fungus grows within the tree living stem and, by blocking its xylem vessels, it ultimately girdles the trees and starves it of water. Kirisits and Cech have observed that once infected leaves are shed, ascospores are produced and wind disperses from fruiting bodies occurring on their decomposing surface. Queloz et al. also noted that such phenomena predominantly occurs over the summer months and is promoted in humid settings (June–September). Finally, most recent observations indicate that spores are also produced on tree shoots the year subsequent to contagion and that they remain infectious only for a few days after being dispersed.
Ash species vulnerability
Ash species are members of the Oleaceae family and most of these belonging to the genus Fraxinus seem to be vulnerable to the pathogen’s attack[1,9]. However, although no treatment is yet available, some trees are believed to show genomic resistance. Namely, no presence of the pathogen on F. ornus has yet been reported and some Asian tree species have shown some degree of resistance.
In the UK, F. excelsior (i.e. Common Ash) has been one of the most susceptible and severely affected species, often resulting in rapid tree death, particularly of young saplings[7,9]. In contrast, larger and older trees are less rapidly affected by the pathogen and, subsequent to the infections, lower branches remain vigorous and new shoots are produced for a few years. As F. excelsior is the second most abundant planting and the third most common broadleaved species in the UK(Figure 1), ash dieback poses a great economic and environmental threat to the country.
It was originally believed that the pathogen entered the UK as a result of saplings’ import from infected nurseries in continental Europe.However, infected trees were also reported in established UK woodland without any evident link to recent sapling plantation,. Consequently, it has been suggested that the pathogen may have arrived by natural means. Namely, wind dispersal of airborne ascospores is believed to be the most likely dispersal route[1,2], with evidence suggesting a possible dispersal rate of 30 km per year. Alternatively, the fungal spores might have being dispersed by vectors such as birds or other animals, or on human clothes, vehicles, forestry machinery or imported wooden products[9,10,11].
Secondary agents of ash dieback
Tree age, local environmental characteristics, climatic conditions as well as the co-occurrence of secondary fungal organisms such as Armillaria sp. fungi or other pathogens drawing on the tree resources, are also important factors in determining the extent of H. pseudoalbidus impact on ash[8,9]. Particularly, highly infected woodlands have been often associated to widespread attacks of Armillaria honey fungus (Armillaria mellea P.) at the tree root level which often contributes as a secondary agent to the tree death.
NB Part of In Class Assessment for Edinburgh University’s 4th Year Ecological Science Field Course, November 2012
1) Queloz V., Grünig, C., Berndt, R., Kowalski, T., Sieber, T.N., Holdenrieder, O.(2011). Cryptic speciation in Hymenoscyphus albidus. For. Pathol. 41: 13-142.
2) Kowalski, T.,Holdenrieder, O. (2008). A new fungal disease of ash in Europe. Schweiz. Z. Forstwes 159, 45–50.
3) Kowalski, T., Holdenrieder, O.(2009). Pathogenicity of Chalara fraxinea. Forest Pathology 39, 1–7.
4) Kirisits, T., Matlakova, M., Mottinger-Kroupa, S., Cech, T.L., Halmschlager, E. (2009). The current situation of ash dieback caused by Chalara fraxinea in Austria. In: Faculty of Forestry Journal, ISSN: 102-7085, Serial: A, Special Issue: pp. 97–119.
5) Schumacher, J., Kehr, R., Leonhard, S. (2010). Mycological and histological investigations of Fraxinus excelsior nursery saplings naturally infected by Chalara fraxinea. For. Pathol. 40: 419-429.
6) Schumacher, J. (2011). The general situation regarding ash dieback in Germany and investigations concerning the invasion and distribution strategies of Chalara fraxinea in woody tissue. EPPO Bull. 40: 7-10.
7) Ogris, N., Hauptman, T., Jurc, D., Floreancig, V., Marsich, F., Montecchio, L. (2010) First report of Chalara fraxinea on common ash in Italy. Plant Disease 94(1), p 13
8) Lygis, V., Vasiliauskas, R., Larson, K.-H., Stenlid, J. (2005). Wood-inhabiting fungi in stems of Fraxinus excelsior in declining ash stands of northern Lithuania, with particular reference to Armillaria cepistipes. Scandinavian Journal of Forest Research 20, 337-346.
9) Forest Research (2012). Rapid assessment of the need for a detailed Pest Risk Analysis for Chalara fraxinea [Online] Available at: http://goo.gl/mFH5j – [Accessed on 18-11-2012]
10) Timmermann, V., Børja, I., Hietaka, A.M., Kirisits, T., Solheim,. H,. (2011). Ash dieback: pathogen spread and diurnal patterns of ascospore dispersal, with special emphasis on Norway. EPPO Bulletin, 41: 14-20. doi: 10.1111/j.165-2338.2010.02429.x
11) Husson, C., Caël, O., Grandjean, J.-P., Nageleisen, L.-M., Marçais, B., (2012). Occurrence of Hymenoscyphus pseudoalbidus on infected ash logs. Plant Pathology 61: 889-895.