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Chalara fraxinea: yet another threat to European forests

The symptoms of ash dieback ("desiccation of ash") were observed for the first time in Poland and Estonia between 1992 and 1996, this phenomenon has since spread rapidly, targeting 23 European countries in just 20 years (Timmermann et al. 2011). In the affected areas are observed the same symptoms: foliar browning and wilting, characterized by premature abscission, apical dieback of the jets with the issue of epicormic branches and necrosis diamond-shaped intersection of the branches on the stem (Fig.1-2) (Kowalski & Holdenrieder 2008). To be affected are either young or mature plants, regardless of environment (forest, nurseries and urban areas), even if only in young plants (below 10 years of age) decay leads to a quick death (Schumacher, Wulf, & Leonhard, 2007).

Difficult to classify. Initially, European researchers have thought to undermine the species of the genus Fraxinus had been a sharing of causes: climate change, pollution or the sudden aggressiveness of a pathogen due to changes in environmental conditions. This belief has had credit until 2006 when Kowalski described, as an agent of deterioration of the ash, a new pathogen, the ascomycete fungus Chalara fraxinea (Kowalski 2006). Initially Chalara fraxinea seemed to be the form of anamorfa Hymenoschiphus albidus, known in Europe since 1851 as a non-pathogenic saprophyte of the rachis of the leaves of ash in litter (Kowalski & Holdenrieder 2009). Queloz et al. (2011) have recognized, however, the presence of two morphologically very similar taxa: H. albidus and H. pseudoalbidus sp. November, suggesting a different origin of the latter, it was compounded with the fact that the pattern of geographical spread of the disease did think of an alien organism. Recent studies have confirmed this hypothesis by demonstrating the genetic affinity between H. pseudoalbidus and a parasite of little importance to a Japanese species of ash, known as Lambertella albida. (Zaho et al. 2012).

So we are probably facing yet another case of introduced parasite responsible for a full-scale invasion of the new host environment, just think that in Poland 80% of populations are affected by ash ash dieback (Lindgren 2008) or in Lithuania between '95 and 2011, the populations of F. excelsior increased from 50800 to 36300 hectares (Pliura, Lygis, Suchockas, & Bartkevicius, 2011).

European coverage and risk fitasintario. Since 2009, H. pseudoalbidus is also present in the north of Italy (Ogris et al., 2010), Friuli Venezia Giulia, Trentino and Veneto, although for now the damage seems limited (Floreancing, 2009). Among the last countries affected there are also Belgium (2009), Holland (2010) and the United Kingdom (2012), where C. fraxinea was found for the first time in the nursery, of material likely German import (Forest Research, 2012), the damage caused by the pathogen led the Anglo-Saxons to impose specific rules which allow the importation of ash only from areas declared officially free.

In order to combat the spread of the pathogen the EPPO (European and Mediterranean Plant Protection Organization) has added to its Chalara fraxinea Alert List, in order to alert the NPPOs (National Plant Protection Organizations) in the regions of the potential EPPO pest risk associated with this pathogen.

Risks and control. This pathogen can become a danger to our ash trees in both forest and ornamental contexts, especially in the regions of northern Italy and in the Apennines, where the climate is cool enough, a key condition for its spread. The species most susceptible are the ash and ash leaved, while the manna ash is more resistente.La spread of this new disease may be limited if all operators in the sector, from nursery to the technicians of the green, to pay particular dieback, also if minimal, visible on the soles of ash and it will prompt Phytopathological regional information services.

In the great picture - Classic symptoms of C. fraxinea: a) and b) of the sprig discolorations, c) necrosis diamond shaped d) dark spots in cross section and sprig) wilting of the jets apical d) of the spine foliar necrosis.

Many ash trees affected by this fungus