Chalara fraxinea (Kowalski)
Significance: Chalara fraxinea was recently identified as the causal agent of the intensive dieback of European ash (Fraxinus excelsior) occurring throughout Northern Europe (Kowalski, 2006). Symptoms begin at the top of the tree with small necrotic spots on stems and branches. These spots enlarge, resulting in wilting and dieback of branches, death of the top of the crown, and eventually death of the trees (Dobrowolska et al., 2008; EPPO, 2008; Halmschlager and Kirisits, 2008; Kowalski, 2006). Ash dieback affects trees of all ages, but is particularly severe on young saplings (Halmschlager and Kirisits, 2008). Symptoms of infection are often similar to those seen with infestation by the emerald ash borer (Agrilus planipennis) that is currently attacking North American ash trees. By 2002, approximately 60 percent of all ash stands in Lithuania had been killed by ash dieback (Vasaitis and Lygis, 2008). Eighty percent of ash stands in Poland are known to be affected (Lingren, 2008).
In 2006, the newly described fungus was causing intensive dieback of European ash in Poland (http://www.pestalert.org/viewNewsAlert.cfm?naid=26), and now C. fraxinea has spread throughout northern Europe (see distribution list). The teleomorph (sexual stage of the fungus) has recently been shown to be Hymenoscyphus albidus (Helotiaceae : Helotiales), a saprophyte apparently native and widespread in Europe (Kowalski and Holdenrieder, 2009b).
Issues of Concern: It is not known if Chalara fraxinea is more widely distributed in parts of Europe (or elsewhere) or if it poses a threat to other ash species, such as our 16 native North American species. Additional information is also needed regarding the host and distribution information for the teleomorph, H. albidus. While it has been concluded that Chalara fraxinea is the primary causal agent of ash dieback, its exact role is not clear, as it is commonly found in declining ash trees that are also colonized by other potentially pathogenic fungi (Kowalski and Holdenrieder, 2009a). The reasons behind the recent emergence of ash dieback from the widespread teleomorph are not clear, though it may be attributed to changes in environmental conditions or the behavior of the pathogen (Kowalski and Holdenrieder, 2009b). Currently, there are no effective control measures for Chalara fraxinea.
Pathways: The teleomorph, Hymenoscyphus albidus, is wind-transmitted and is likely to be more important for dispersal than the sticky conidia of C. fraxinea (Kowalski and Holdenrieder, 2009b). Chalara fraxinea may be able to disperse aerially, in soil, water, plants for planting, seeds, or wood (EPPO, 2008; Kile, 1993). Insect vectors are also important in moving various Chalara species (Nag Raj and Kendrick, 1993; Webber and Brasier, 2001), though none have been identified for C. fraxinea.
Hosts: European ash (Fraxinus excelsior) is the only known host for Chalara fraxinea. Single observations have also occurred on Fraxinus angustifolia subsp. danubialis(narrow-leaved ash) and Fraxinus excelsior ‘Pendula’ (weeping ash) (Kirisits et al., 2008). Susceptibility of other species to Chalara fraxinea is unknown.
Distribution: Austria; Denmark; Czech Republic; Finland; Germany; Hungary; Lithuania; Norway; Poland; Slovenia; Sweden (EPPO, 2008; Jankovsky et al., 2008; Ogris et al., 2009; Thomsen, 2008). Based on symptoms observed in European ash, Chalara fraxinea is also suspected to occur in Estonia, Latvia, and Switzerland (EPPO, 2008).
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