Monday, December 17, 2012

Ash dieback: the coastal spread

The map below shows some of the main places mentioned in this note. Chalara fraxinea now occurs in all the countries shown, often quite widely. Places in red are of particular significance in the spread of this fungal pathogen around the coast of the Baltic Sea, the North Sea and the English Channel as discussed in the text below.


Chalara fraxinea and its teleomorph Hymenoscyphus pseudoalbidus, both stages of the fungus that causes ash dieback disease, have now been recorded from most countries in northern and western Europe, from Norway to Italy and from Ireland to Russia. It was first noticed in Poland and Lithuania in the early 1990s, although one account says it was first recorded in Latvia (Flora Locale, 2012), and has spread, and continues to spread, rapidly in all directions.

According to Kraj, W., Zarek, M. & Kowalski, T. (2012) there are several strains of C. fraxinea in Poland: “strains in lowlands were characterised by smaller number of markers, smaller number of polymorphic loci and smaller intrapopulation genetic variability.” Most of the populations considered in that paper are in hilly or mountainous southern Poland, whereas the different lowland strain occurred in areas close to the Baltic Sea to the west of the port of Gdansk.

As has been widely reported, ash dieback is now established in Lithuania, Latvia, Estonia, Baltic Russia (Kaliningrad), Finland, Sweden, Denmark and Germany, as well as Poland and these are all countries with a Baltic shore. The fungus is also now widespread in southern Norway, just outside the Baltic.

In the south the disease has advanced steadily across central Europe as far as France, Italy, Romania and other countries.

By following the history of these advances it appears that there is a maritime dispersal route and an overland dispersal route for this fungus. The terrestrial route seems consistent with propagation mainly by airborne spores of the H. pseudoalbidus stage and maps produced in France illustrate this well.  In following the advance of C. fraxinea, it is important to try and discover the first reports in different areas as (a) the fungus spreads very rapidly and (b) once discovered far more people start looking for it. Also it has to be borne in mind that it may have been present for some years before being detected. Because of this and other factors, possible dispersal routes become unclear quickly.

The maritime dispersal route seems more complex than the overland and, while wind borne spores are likely to play a major part in the overland dispersal routes, there would seem to be some other vector involved such as bird migration or shipping on the maritime routes.

The disease has, for example, been a problem on several Baltic islands.  In Öland and Gotland (both Sweden) ash trees have been ‘ravaged’ by the disease, while in Finland it was first discovered in the Åland archipelago in 2007 some 170 km (100 miles) across the Baltic north from the nearest area known to be infected at a time when it had not been reported from the Finnish mainland (EPPO, 2007). Much shipping passes close to these various islands en route to places like Stockholm, Helsinki and St. Petersburg and, of course, they all have cruise liners calling  and ferry services to the mainland.

In Germany C. fraxinea was initially widespread in the north of the country (Schumacher, GALK) near the Baltic rather than in the south, while in Denmark it was first recorded in 2002 near Haderslev, a small port on the Baltic side of the Jutland peninsula. The following year it was found in Zealand and Bornholm, the latter a Danish island to the east of the main part of Denmark and well out in the Baltic Sea.  Haderslev is the home of a company called Euro-Timber that trades widely in timber products throughout the region, though this, of course, may have nothing to do with the spread of C. fraxinea. There is also a frequent ferry service from Swinoujscie in northern Poland to Bornholm island and Ystad in Skåne (Scania), southern Sweden, where ash dieback has ‘raged’.

The disease was first found in Norway in a nursery in the Østfold region in the most southerly part of the country in 2007.  There is a frequent ferry service from Hirtshals in northern Jutland, Denmark, to Kristiansand the capital of Østfold and, of course, much shipping passing through the Skagerrak from the Baltic to the North Sea.

Ash dieback was first reported from The Netherlands in 2010 (later than in many countries) in young trees in public parks in Bellingwedde in the far north east of the country close to the German border and about 15 km south of the coast at the Dollard (German: Dollart) an inlet from the Wattenmeer, part of the North Sea lying between the Frisian Islands and the coasts of The Netherlands, Germany and Denmark. The busy German port of Emden lies on the northern side of The Dollart 25 km or so north of Bellingwedde.

The first report of ash dieback in Britain was from a nursery in Buckinghamshire in 2012 on trees from The Netherlands that were said to be infected. This may be demonstrably true, but increasingly suggestions are being made that the pathogen has been present in Britain and elsewhere since well before its first report and, if this is correct, at least some nursery stock may have been infected from local woodlands after importation. The disease has spread quickly through The Netherlands and, although the Dutch phytosanitary authorities suspect that it was present before the discovery in Bellingwedde, it does seem to have made its initial entry close to the coast in the north of the country. The distance from there to East Anglia is not as great as some of the distances that C. fraxinea seems to have travelled over the Baltic Sea and this indicates that airborne spores might survive quite long and cold journeys, though if wind or birds are important as vectors it is difficult to explain why the disease seems to have jumped over intervening territory on the nearby mainland.

In Belgium the first report was from Silly in the province of Hainaut in 2010. This is some 75 km from the port of Antwerp and 91 km from the Belgian coast at Bruges thus its route into the country may have been overland from the east rather than via the North Sea fringe (the disease is established both in northern France, southern Germany and Luxembourg). The Belgians were sufficiently worried about the spread of the disease to undertake extensive testing of ash trees for C. fraxinea in 2009 in the province of Wallonia (Delhaye, Helson, & Chandelier, 2010).  No infection was found, but when the experiment was repeated in 2010, the fungus was present at several sites – a good illustration of the speed at which it can spread.

The situation in France presents some interesting data. The spread of ash dieback has been closely mapped by, among others, DRAAF (Direction Régionale de l'Alimentation, de l’Agriculture et de la Forêt).  In an information sheet published in June 2011 (DRAAF, 2011) they showed that the disease was widespread in north eastern France in 2010 but there was only one dot on the map in the Pas-de-Calais over 300 kilometres (186 miles) from the next nearest French site, though probably slightly closer to some of the Belgian sites. In the north east the disease spread south and west in 2012, while it was also detected in an increasingly large area around the Pas-de-Calais site in 2012.

However, it also turned up in 2012 as an apparently isolated occurrence 265 km (165 miles) south west of the next nearest French site on the coast of the Cotentin peninsula in the department of Manche in Normandy (Association des Communes Forestières de la Côte d’Or, 2012).  This map also shows how C. fraxinea advances relatively short distances overland and along a specific front.

The French coast on this part of the Cotentin is only some 28 kilometres (17 miles) from Jersey from where ash dieback was reported in November 2012. According to the Jersey Evening Post “The infected ash tree has been identified as one of a number imported into the Island from the UK in 2010 or 2011.”  If this is correct, it is just as likely that the fungus was brought in from the UK as having established itself by windblown spores from Normandy.  Indeed, the Normandy outbreak may have come from Jersey or Guernsey where the disease has also been found.  Again, if the trees were infected before they left for Jersey, the disease must have been in mainland Britain before February 2012, the date that is often given for its arrival.  It does no injury to logic to suggest that the outbreak in the Pas-de-Calais might have come from Britain, especially as the Forestry Commission are now suggesting that the disease has been in East Kent for some time.

While movement of plants undoubtedly exacerbates the problem of introducing pathogens to areas where they have not been previously recorded, there is a tendency to blame other countries for exporting infected stock. As one senior French forester said when asked how he thought C. fraxinea had arrived in north eastern France: “On suppose l’importation de plants contaminés.”

So far as the British Isles is concerned, virtually all the current records of Chalara from the wider environment (i.e. not from nurseries or recent plantings) are on the eastern side of the country, many close to the coast. While wind borne spores from mainland Europe may be responsible for many of these outbreaks, most are also near ports, places where there are many arrivals and departures of people and goods to and from other countries. This is particularly apparent in the area around Dover and Folkestone (Forestry Commission, 2012).

In sum, the forms of Chalara fraxinea that have travelled overland from Poland, and maybe other parts of eastern Europe, generally seem to have advanced a relatively short distance each year that is consistent with establishment by wind borne spores, whereas the near-the-coast occurrences often seem to have made long hops apparently without settling on places in between. This could mean that there is a vector for C. fraxinea and some other plant diseases apart from the wind, birds and other fauna, and imports of infected material. It seems possible that shipping, and particularly cruise ships ferry services, may be the carriers of spores even when there is no mature infected material from the host plants on board. The environment on these vessels may help spores to retain viability in some way. This could also explain why Chalara fraxinea seems to have an affinity with islands, rapidly spreading to Bornholm, Öland, Gotland and the Åland archipelago in the Baltic, as well as the British Crown Dependencies of Jersey and Guernsey all of which are dependent on ferries and welcome cruises and other shipping services. 

Also the isolated 'wider environment' outbreak in the Buckie area of Scotland shown on the Forestry Commission's map of 11 December 2012 might be due to shipping "Buckie Shipyard offers a wide ranging capability to the marine industry , including new build, conversion, refit and repair of ferries, tugs, workboats, yachts, pilot boats, MOD vessels, small cruise vessels, diving vessels, lifeboats, fishing boats and fish farm cages."

There is a growing view that Chalara fraxinea originally came from the Far East and there are, of course, shipping services, from there to the Baltic. The Fesco Baltorient Line, for example, runs cargo vessels from the Far East to St. Petersburg and other European destinations.

Another curious aspect of the disease is that the map of Britain produced by the Forestry Commission shows sites where the infection has been confirmed in recently planted sites (including ?  nurseries) scattered very widely from Cornwall to northern Scotland and in Wales and Northern Ireland.  In France, however, most of the country seems to remain untouched, though the disease is advancing southwards on a broad front with hardly any egregious records.  In other words it appears to be widespread in British newly planted sites but not in French.

Oliver Rackham (2012) also seemed to think, having considered the options, that there was something about the expansion of Chalara fraxinea that we did not quite understand: “Many reported outbreaks, in various countries, are linked to the nursery trade, both within and between countries. Ash seedlings grown in nursery conditions are evidently very susceptible to infection, and can convey it long distances or to islands that it might not otherwise reach. However, occurrences like that in Ashwellthorpe Lower Wood, not obviously linked to any nursery, show that this is not the only explanation. Ascospores blowing from the Continent are a possibility — they are small enough to get that far. However it seems unlikely that a single spore, after a wind journey of hundreds of miles, would have enough inoculum potential (as Dennis Garrett, the great plant pathologist, taught me in the 1960s) to start an infection, though maybe a mass of conidia on the foot of a bird might do so on rare occasions.”


Association des Communes Forestières de la Côte d’Or (2012),

Delhaye, N., Helson, M., Chandelier, A. (2010) La chalarose du frêne : premiers foyers en Wallonie.[chalarose].pdf

DRAAF (2011) Forte progression géographique et des dégâts de la chalarose du frêne dans le Nord de la France

EPPO (2007) Ash dieback in Europe and possible implication of Chalara fraxinea: addition to the EPPO Alert List. EPPO Global Database Num. Article: 2007/179 Year: 2007 Month 09.

Flora Locale (2012) Ash dieback, tree planting and the plant trade.

Forestry Commission (11 December 2012) Chalara fraxinea Map 2b. Confirmed infection sites$FILE/UK_outbreak_map-12-12-11_Map2b.pdf (n.d.) Editorial on ash dieback

Kraj, W., Zarek, M. & Kowalski, T. (2012) Genetic variability of Chalara fraxinea, dieback cause of European ash (Fraxinus excelsior L.). Mycological Progress, February 2012, Volume 11 (1): 37-45

Rackham, O. (2012) Ash Disease: the present state of knowledge or ignorance. 9 November 2012.

Schumacher, J. (2010) Essterfte door Chalara fraxinea.