By November 2010, ash dieback had been reported in 22 European countries ^[1][2][3,23]. The presence of the fungus Hymenoscyphus fraxineus (anamorph—Chalara fraxinea) was confirmed in all of these countries. At that time, ash dieback was not reported outside of Europe ^[3][28]. Until 2012, this pathogen was not included in the list of the European Union Plant Protection Directive, which allowed the free movement of ash planting material and wood within the European Union ^[4][62].

Currently, the disease has been confirmed in at least 26 countries, and its current south-western limit extends to central France ^[5][26]. In Norway, the disease spreads at a rate of 30 km/year, in Latvia at 40 km/year and in north-eastern Italy at 50–60 km/year ^[6][8]. In 2011, the British Isles’ common ash is highly valued and the fourth most common deciduous tree species ^[4][62] was still considered free of ash dieback ^[1][3]. A year later, however, the pathogen was introduced to England with seedlings from the Netherlands ^[7][63].

Of the over 80 million common ash trees growing in the UK, up to 90% of them are at risk ^[8][9][24,64]. Attempts to burn the infested trees on the islands to control the disease have failed. The case of ash dieback in the UK became a matter of public interest, even making it to government meetings and the BBC media. This forced the government to take urgent remedial action ^[8][24]. One of the solutions implemented in England was to extensively educate the public on how the disease was spread and how to recognise and monitor it, including through mobile phone applications. Educational videos on the symptomatology and aetiology of the disease were made available via social media (You Tube), and a game was organised on Facebook in which participants had to match the DNA sequence of H. fraxineus ^[10][65]. To underline the importance of the problem, a computer program was developed to visualise the hypothetical decline of the ash trees. Numerous studies have also been carried out in the British Isles; the scientists showed that 953 species of organisms are associated with ash tree populations, 44 of which are obligate ^[11][66].

Ash dieback has not been found in southern Italy ^[12][67]. Little is known about the decline of ash trees in eastern and south-eastern Europe (France, Spain) ^[2][23]. In parts of northern Europe, it is already present, e.g., in Ireland ^[13][68], and there are no recent data from Russia (with the exception of Kaliningrad Oblast), Moldova or Bulgaria ^[1][3].

In 2010, Sweden listed the Fraxinus excelsior in the Red Book of Threatened Species ^[14][46], and 60 species of organisms associated with ash were already on this list. If the disease continues to spread and intensify, some of these organisms will probably be on the verge of extinction ^[15][69]. In Sweden, according to data from 2009, up to half of the ash trees were affected by ash disease ^[16][70]. The problem is not only the disease itself, but also the belief by forest owners that removing dying trees en masse will contain the spread of the pathogen ^[7][63]. In Sweden, a nationwide mobile phone monitoring program has been launched to encourage citizens to report the location of healthy ash trees ^[17][9].

In Norway, ash dieback was first reported in 2008 but infection with H. fraxineus probably occurred there at least two years earlier ^[1][18][3,71], and in 2015 the common ash was listed as an endangered species in the Red Book of Plants ^[19][72]. Although low temperatures were initially considered a damaging factor ^[18][71], tree deaths due to infection with H. fraxineus were confirmed in 2009. Since then, Norway has been divided into three zones: Quarantine, Surveillance and Disease-free. As a preventive control measure, a ban on importing ash wood, cuttings and plant parts (also from quarantine zones) into observation zones where the disease is not present has been introduced. Nursery stock for renewal is checked for disease symptoms before being released for sale ^[1][3].

In Denmark, the first symptoms of ash dieback were noticed in 2003, when Danish foresters observed damage to the branches of young ash trees. Soon the problem was reported nationwide, but the cause remained unknown. Only in 2006 was it confirmed that the symptoms were caused by the Chalara fraxinea. The planting of new common ash forests was almost completely stopped, and the economically important species gradually began to disappear from Danish forests ^[20][73]. Efforts are also being made in Denmark to select common ash genotypes that are resistant to ash shoot dieback ^[17][9].

In Latvia, common ash is the most common deciduous tree species in the forests ^[21][74]. In 2003, forests were designated under the EUFORGEN program to protect the gene pool of the common ash. In 2007, however, a massive dieback of the trees was observed, which is why these stands could no longer fulfill the task of conserving genetic resources. Until the introduction of the disease to Latvia, common ash was the most common deciduous tree species (readily cultivated among the hardwoods), and now it only occupies about 0.5% of the total forest area and is no longer the dominant species in most stands ^[22][75]. In Latvia, the area of common ash stands decreased by 40.6% ^[6][8]. In the case of stands of younger age classes, the area decreased by more than four times, disrupting the age structure of the population to such an extent that the proportion of younger age classes fell from 43% (at the beginning of the 21st century) to 15% in 2015. Currently, the intensity of ash dieback in Latvia has decreased ^[21][74], and observations show that it is progressing at a rate of about 40 km per year ^[6][8].

In Lithuania, ash dieback was first detected in 1996 in forests in the northern part of the country. Soon the decline spread to the whole country. From 2001 to 2012, the area of common ash decreased from 2.7 to 1.7%. Virtually all remaining Lithuanian common ash stands have been decimated, and their health is still deteriorating ^[23][7].

The national forest monitoring network recorded the beginning of the mass drying of ash trees in Belarus in 2003. At that time, 6.8% of trees died at the permanent registration points, and in 2004, 12.2% died. More than 80% of ash forests are classified as plantations with impaired stability, and only 12% are recognised as biologically sustainable ^[24][49]. According to the forest management data, almost all recorded ash stands in the “National park “Białowieża Forest” are affected by root rot and stem pests, which leads to the disappearance of ash stands as a formation of Białowieża forests.

In Germany, ash dieback disease symptoms were already observed in Brandenburg in 2002 ^[25][76]. The investigations started in Bavaria in 2008, resulting in numerous confirmations of the occurrence of H. fraxineus. Common and narrow-leaved ash Fraxinus angustifolia Vahl. trees of all ages were found to be affected, both in forest nurseries and in forests growing in different locations, as well as in urban plantations ^[26][77].

Ash dieback in Slovenia was first observed in 2006 in the north-eastern part of the country. In 2007 and 2008, typical disease symptoms appeared throughout the country. In 2009, due to the spread of the fungus H. fraxineus and the problems in obtaining healthy seedlings in Slovenian nurseries, it was decided to replace common ash with other tree species such as sycamore maple or poplar ^[27][78].

In Austria, the phenomenon of ash dieback was first confirmed in plantations in 2005 ^[28][14], although ash trees with characteristic disease symptoms had already been found in 1997 ^[29][79]. By 2007, the disease was very widespread, and the observed symptoms of ash dieback were consistent with those in other European countries ^[28][14].

In Ukraine, the disease has been observed since 2010, but the health of ash trees there had already deteriorated significantly four years earlier, especially those growing on the outskirts of the forest. However, the disease spreads quite slowly, especially in the eastern part of Ukraine ^[2][23]. In 2014–2015, the mortality of common ash trees was observed in all age groups. Hot and dry summers prevailing in the south and south-east of Ukraine were likely not conducive to the development of the disease, in contrast to the cooler climate of the western and northern parts of the country ^[30][80].

In the Czech Republic, the weakening of common ash trees has been observed since the mid-1990s, but the presence of the pathogen was confirmed only in 2007 ^[31][81]. Natural forests, commercial plantations, forest nurseries, riparian vegetation, avenues and urban plantations were all affected. Trees of all ages died, but the most severe losses were suffered by young trees. Young stands up to 50 years old were significantly more affected by ash shoot dieback than mature stands ^[32][60].

In Hungary, H. fraxineus was first detected in the western part of the country in 2008, although the pathogen was probably already present in the area three years earlier. Originally, the damage to the trees was attributed to low temperatures. This pathogen was also found on the narrow-leaved ash. In 2008–2009, the cause of tree death was already present throughout Hungary and occurred in both younger and older stands, but caused more frequent damage to younger forest stands (two- to ten-year-olds) ^[33][16].

In general, the common ash was considered to be more susceptible to the disease ^[4][62] than the narrow-leaved ash ^[3][26][28,77], but new findings (for example, from Croatia), show that narrow-leaved ash is highly susceptible to the ash dieback, and the least susceptible is manna ash (F. ornus L.) ^[34][82]. Symptoms of the disease were also found in black ash (F. nigra Marsh.), green ash (F. pennsylvanica Marsh.) and white ash (F. americana Marsh.) ^[3][28]. Manchurian ash (F. mandshurica Rupr.), along with F. chinensis Rox., are included in the group of natural hosts of the fungus H. fraxineus, although the disease does not usually lead to the death of these trees ^[35][83], and the pathogen has even been found in healthy specimens of F. mandshurica growing in far eastern Russia ^[34][82]. H. fraxineus has also been found in healthy Manchurian ash trees in China and Japan. Artificial inoculations of Manchurian ash seedlings with the fungus showed the possibility of tissue infection, but the damage observed was minor and did not lead to plant death ^[34][82].

Following Koch’s postulates, H. fraxineus was re-isolated from infected tissue with mild disease symptoms ^[34][82]; it is worth emphasizing again that the infected tissue in question is the one from Manchurian ash. This is why, in Europe, the fungus is considered to be an alien that has become an invasive species causing a mass mortality of trees. For this reason, in Europe, it is the East Asian and North American ash trees that tolerate the disease the most and show only mild symptoms. Perhaps it was the diseased, asymptomatic Manchurian ash trees transported to Estonia that brought the disease to Europe ^[34][82], although Manchurian ash had been cultivated there for a long time and other ash species were not diseased ^[35][83]. Also in Sweden, common ash trees planted next to infected Asian ash trees showed no signs of dieback ^[34][82].

In Estonia, the phenomenon of tree death was found in the species Fraxinus sogdiana Bunge, and it was the first case where the disease was confirmed in ash trees from Central Asia ^[35][83]. Furthermore, Drenkhan et al. ^[35][83] noted that there are twenty ash species naturally occurring in Eurasia, reaching as far as Estonia, and that this could be a potential route of spread for the pathogen.

H. fraxineus attacks Asian species of ash but does not kill them because it has co-evolved with them. Trees that co-evolved have natural resistance mechanisms, whereas European ash species are therefore sensitive to infection by this fungus. In addition, the climate in Central Asia may not favor the development of the pathogen as much as in Europe.

It is not yet certain whether the current spread of the pathogen is limited to the eastern range of the host trees or also to the western range. It is doubtful that the pathogen accompanied its host during the transcontinental spread of ash from East Asia to Europe and survived only in northeastern Poland in a latent form until 1992, when the phenomenon of mass ash shoot death was first described in Europe ^[35][83]. H. fraxineus was probably brought to Central Asia together with the Manchurian ash.

In 2006, ash shoot dieback affected almost 11,000 ha in Poland ^[36][5] and was already reported in eastern Poland in the forest districts of Czerwony Dwór and Borki in the early 1990s ^[1][3]. In 2006, however, the discovery of Prof. Tadeusz Kowalski, who first described the fungus Chalara fraxinea, pointed to the new species as the cause of ash dieback, which was later confirmed by other researchers ^[28][37][14,30]. In 2008, based on genetic studies and in vitro analyses of colony characteristics, the fungus was found to form fruiting bodies on petioles of fallen leaves from the previous year. The fungus was described as Hymenoscyphus albidus (Robergere ex Desm.) W. Philips ^[38][84]. However, H. albidus has been known in Europe since 1851 and has not shown pathogenicity to common ash. This leads to speculation about mutations and the emergence of new pathogenic strains of the fungus, or the introduction of a closely related, morphologically similar species.

Confirmation of the introduction of a new related species was conducted by a team from the Swiss Federal Institute of Technology Zurich (ETH) in collaboration with the Department of Forest Phytopathology in Cracow ^[3][28]. Investigations showed that two phylogenetic subgroups were distinguishable within known isolates of H. albidus ^[39][85]. The pathogenic subgroup was represented by the species Hymenoscyphus pseudoalbidus (whose anamorph is Chalara fraxinea, described in 2006). It was assumed that H. pseudoalbidus displaced the non-pathogenic fungus H. albidus from the environment ^[3][28].

However, studies in the southeast of the Czech Republic showed the presence of inoculum in the air of both species, indicating their coexistence in the environment, and that H. albidus is still found at individual sites in Norway, Belgium and France ^[31][81]. Attempts to inoculate plants with the fungus H. albidus showed no pathogenicity on common ash or green ash ^[40][86].

Until 2011, the two names Chalara fraxinea and Hymenoscyphus pseudoalbidus were used interchangeably. However, according to the decision of the International Botanical Congress of 2011, dimorphic fungi should have only one name. Genetic studies have shown that the causal agent of ash dieback is closer to the genus Hymenoscyphus than to Chalara. Since the term “fraxinea” was widely used in the literature, it was decided to combine it with the taxonomically correct “Hymenoscyphus” ^[41][33]. Currently, the accepted name of the fungus is Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz, Hosoya, comb. Nov.; basionym: Chalara fraxinea T. Kowalski; synonym: Hymenoscyphus pseudoalbidus Queloz et al. The term ash cup was adopted as the Polish name because the fungus in the teleomorphic stage produces characteristic “cups” (Figure 1) with a diameter of 1.5 to 3.0 (7.0) mm ^[42][87].

Before Baral and Zhao ^[43][34] revealed the difference in the structure at the base of asci in the two fungal species, it was believed that a distinction between H. albidus and H. fraxineus based on morphological characteristics was not possible ^[44][88]. Recent studies indicate that H. fraxineus is most likely an invasive species ^[1][3], as it was found to be genetically and morphologically identical to Lambertella albida, which has been known in Japan since 1993 ^[15][44][69,88]. This name is often used in the literature as a synonym for the species H. fraxineus ^[34][82]. The speculation that the pathogen was introduced was supported by the finding of greater genetic diversity of this fungus in Japan than in Europe, and the lack of natural resistance of common ash trees in interactions with the pathogen ^[15][69]. Modeling of the spread of the pathogen showed that the climate in south-western Europe is too warm (it was not found in Spain). The assumption is that the disease will continue to spread northwards ^[45][89].

Scientists in Asia have reported the presence of H. fraxineus in China; it has also been found in South Korea ^[44][88], and it is thought to occur naturally in Central and East Asia ^[35][83], as a saprotroph ^[34][82]. In this area, there are fungal species belonging to the same genus, but their pathogenicity has not been reported so far; however, should they be accidentally introduced into Europe, they could pose a new threats to European forests ^[46][90]. The sexual reproduction of the pathogen ensures genetic diversity of the pathogen, which allows it to adapt to changing environmental conditions and successfully compete with other fungi ^[15][69], which may be how trees that initially appear to be resistant eventually become diseased ^[5][26]. Samples of plant material from a herbarium in Switzerland show that the pathogen has been introduced to Europe several times in the past, but has only recently become successfully established ^[1][3].

In light of the current scientific knowledge, it is recognised that the fungus H. fraxineus is present throughout Poland ^[42][87] and forms apothecia on foliage from the previous year (including under laboratory conditions).

Despite evidence that urea and carbendazim are effective in limiting the growth of the pathogen in in vitro experiments (in cultures in Petri dishes), there are still no effective methods of protection against this pathogen ^[8][33][16,24]. The use of urea may prove practical (e.g., for the protection of individual trees), as it is relatively cheap and non-toxic to the environment, and also promotes the development of numerous organisms hostile to the fungus ^[47][91].

The species H. fraxineus spreads in the air by ascospores that reach maturation at night, under high humidity ^[1][3], and are released in the early morning from April to October, in central Europe, with the greatest intensity in August ^[31][48][81,92]. Ascospores are produced in apothecial cups on the fallen leaves of dead seedlings ^[48][92], and their production is favoured by high humidity ^[2][31][23,81]. Under such conditions, the spores can remain airborne even when fruiting bodies are no longer present. A similar process of spore dispersal also occurs for H. albidus ^[31][81]. Studies have also shown that the species H. fraxineus prefers cooler conditions; its optimal growth on solid media is between 20 °C to 22 °C, and hyphal growth stops above 28 °C ^[45][89]. The fungus penetrates the host through epidermal cells ^[49][93] and can develop in the living part of the bark, phloem or xylem ^[50][19], although no preference has been for specific tissue types ^[42][49][87,93]. Tissue necrosis occurs at the infection sites ^[2][15][23,69], while the pathogen develops in various tissue types and in different directions ^[42][49][87,93].

In nurseries, the incubation period of the fungus in host plants lasts about 10–14 days and the first tissue necroses appear after two weeks, while clear disease symptoms may only appear after several months ^[4][62].

The disease symptoms are usually observed under natural conditions in summer or autumn (in the year of infection or spring of the following year) ^[4][62]. Hymenoscyphus fraxineus is a primary pathogen because the fungus is isolated from infected tissues only at the beginning of the infection development ^[51][94], and then there is the fungus retreat place for secondary pathogens, especially species of Phomopsis and Fusarium, which quickly colonise the dead tissues previously killed by H. fraxineus ^[15][42][69,87]. This situation limits the possibility of isolating the Chalara conidia produced in phialides of the anamorphs known as C. fraxinea ^[42][87].

Although conidia can also develop on fallen tree trunks, their growth on seedlings is unlikely ^[1][42][3,87]. However, later in vitro studies showed that conidia can germinate on ash leaves and infect seedlings via leaves or roots in the soil. Seedlings inoculated with conidia also show necrosis and wilting of leaves and even death ^[52][95].

Despite the low germination frequency of the conidia, they could play an important role in the rapid spread of the pathogen. H. fraxineus can grow in plant debris on the ground ^[52][95]. An important factor limiting the spread of the pathogen H. fraxineus is the low resistance of the ascospores to UV radiation and their low nutrient supply; thus, long distance natural invasion from areas of natural occurrence (e.g., by ascending air currents and further into the stratosphere) seems unlikely ^[15][69].

Tree dieback moves faster along valleys and is consistent with bird migration routes ^[6][8]. Therefore, there is a high probability that birds using infected leaves to build their nests may carry pathogen spores on their feathers over long distances (Oszako, unpublished).

An obstacle to the spread of the fungus may be a thick layer of dry leaves of other species (e.g., beech) and a poorly developed undergrowth. Such conditions lead to a lack of development of H. fraxineus apothecia on leaves fallen onto such substrates ^[42][87].

Scientists disagree on whether diseased trees (with symptoms of dying) and dead trees are reservoirs for inoculum. According to Kowalski ^[42][87], trees with symptoms of dieback, with local necrosis or withered branches, do not produce spores that could threaten neighboring healthy trees. Therefore, they do not compose a reservoir of infectious material. On the other hand, the primary inoculum is from the leaves of the previous year, where H. fraxineus conidia are formed in autumn and winter ^[53][96], while ascospores are formed on the fallen leaves in the following summer. Under natural conditions, the fungus can survive in the petioles for up to five years after leaf fall ^[54][97].

Infected trees show some symptoms resembling frost damage from a distance ^[7][63], such as loss of foliage, leaf discolouration and necrosis, reduced fruiting, etc. ^[55][98]. A characteristic symptom and one of the earliest is frequent discolouration near the central vein of leaves. The growth of hyphae through plant tissue leads to the death of successive shoots, and this can even cover entire tree crowns. The most important symptom is the wilting of leaves (as hyphae develop inside vessels and parenchyma of whorl rays, and interfere with the transport of water and assimilates), and the subsequent tissue necrosis is visible on the surface of the shoot bark ^[42][48][87,92]. This leads to the death of parts of the shoots located above the infection site (Figure 2).

Diseased ash trees tend to drop their leaves by early autumn and many trees are deprived of photosynthetic tissues by the end of September ^[27][42][78,87]. In younger trees, cancerous changes can be observed on the trunks, rarely in connection with sap secretions from diseased tissues (Figure 3).

The wood in the affected areas is characterised by tissue discolourations ^[42][87]. Kowalski ^[42][87] also lists the symptoms of the death of the underground parts, such as the rotting of small roots and their blackening, unhealed necrosis (tissue discolouration starting from the infection sites), often near the root collars ^[42][87]. These symptoms are similar to the damage caused by pathogens of the genus Phytophthora. H. fraxineus infects the roots only sporadically and only of seedlings, which show a grey-brown discolouration of the underground parts of the root collars ^[8][42][24,87] with sporadic fruiting bodies ^[56][99]. In heavily infected plants, the pathogen has also been isolated from the roots ^[49][93], although it was previously believed that the pathogen (then recognised as H. pseudoalbidus) did not infect roots ^[26][77]. This was confirmed by later studies, which showed that the fungus not only infects seedling roots but also moves within xylem to above-ground parts ^[52][95].

In Sweden, it was observed that tree health deteriorated rapidly within two years (from healthy to severely damaged to death) ^[7][63]. However, many specimens (in older age classes) improved, which was also confirmed by observations in Denmark, where ash trees with larger stem girths were less affected. This could indicate that older trees were able to overcome H. fraxineus infections and survive ^[7][63].

It has also been found that ash clones that develop buds earlier (which correlates positively with their health) are less susceptible to infection by H. fraxineus ^[23][7], and earlier discolouration and shedding of leaves in autumn also showed a positive correlation with health ^[5][26]. Susceptible individuals were characterised by prolonged growth in the summer ^[49][93].

Laboratory studies have shown that the fungus can produce toxins called viridiols ^[3][15][57][28,69,100], but these have not been found in samples of shoots and wood from infected trees ^[15][69], nor has it been demonstrated that such substances can cause necrosis of ash tissues ^[58][101]. Hymenoscyphus fraxineus, rather, has not been found to infect forest trees other than ash or to occur as an endophyte in living roots, shoots and leaves ^[42][87], although there is a risk of infection from related plants such as the olive ^[52][95], since they both belong to the Oleaceae.