I dare not predict what will happen to ash. The recent cycle of Elm Disease is too uncomfortable a precedent. Who would have foreseen in 1970 that 40 years on the geographical distribution of the various elms would be almost unaltered, but big elms would still be abundant only in Huntingdonshire, Cambridgeshire woodland, east Sussex and the Isles of Scilly?

What might replace ash? Hazel and oak no longer reproduce in existing woodland owing to introduced pests: grey squirrel for hazel, and for oak probably oak mildew. Birch is a likely replacement, considering how readily it replaced planted trees (of whatever species) in the twentieth century. Maple, hornbeam, hawthorn and sycamore are alternatives. But before we think about how to replace the ash tree in our landscape, much more needs to be understood about plant disease and our role in exacerbating the problem of how disease moves around the planet.

Ash Disease

Ash Disease is caused by the microscopic fungus Chalara fraxinea, which I have not seen. It inhabits leaves and twigs, which it damages by making a chemical called viridiol that is very toxic to ash. In summer it attacks leaves and produces spores called conidia, which form sticky masses and may be spread around the tree by rain to start more infections.

In 2009, the Horticultural Trades Association, representing responsible nurserymen, warned the Forestry Commission about the threat of Ash Disease; the Commission cited excellent legalistic reasons for doing nothing. Not until Ash Disease was noticed in England itself did the Forestry Commission react with belated promptitude. A great survey was got up, aided by new technology for rapidly matching the DNA of samples and thus distinguishing Ash Disease from similar but unrelated conditions. This revealed that H. pseudoalbidus was already present throughout Britain and Northern Ireland. Because of bureaucratic parochialism the survey was not continued into the rest of Ireland, although the fungus was there too.

By July 2013 the fungus – or, rather, something with DNA indistinguishable from that of the fungus – had been found in 549 sites. Of these, 24 were tree nurseries, 336 were in ‘recently planted’ ash (how recently is not stated), and 189 were in established ash trees in woods and in non-woodland sites. These have very different distributions. Recent plantings were randomly scattered through the area surveyed. Finds in established trees were concentrated in East Anglia and Kent, with a few near the east coast into Scotland. Some of these were in ancient woodland, where ash shoots coppiced two to four years ago seemed to attract the disease.

A simple explanation is that nurseries inadvertently imported infected plants from the Continent and spread them all over Great Britain and (at least Northern) Ireland; this agrees with reports of nurseries spreading the disease in other countries. The tree-planting fashion has brought Ash Disease throughout England, far into Wales and the Scottish Highlands, and very efficiently into Ireland.

Emerald Ash Borer

This is ‘one of the most feared beetles on earth’, but not in these islands, where British parochialism ignores pests and diseases until they have got here and the battle to contain them has been lost. Agrilus planipennis, a pretty little insect, about 8 mm long and iridescent green, is said to come from the Far East and to have got into North America in a shipment of Japanese car parts. It is a bark beetle: it lays its eggs on the tree; the grubs tunnel prodigiously between bark and wood, killing the tree.

Presumably in its East Asian home it came to terms, over millions of years of evolution, with the local species of ash, as Hylesinus has done in Europe. In America, Agrilus meets unfamiliar, susceptible species of ash and escapes the predators of its homeland – within 11 years of arriving in an area, the beetle’s population explodes and kills all the ash trees. Ash being one of the commonest remaining trees, the result is whole landscapes of dead ashes, especially in towns. The dead trees, too many for tree-fellers to get round to them, fall on cars and people’s heads. Human health is affected as people are suddenly deprived of their favourite trees and exposed to high temperatures and air pollution.

Emerald Ash Borer was first noticed in 2002. The authorities imposed quarantine, but to little effect. The nursery and firewood trades are said to have spread it to fresh areas; it has already killed more ashes than there are in the whole of Britain and Ireland. An industry has grown up for injecting or spraying ash trees with insecticide. This can probably save only a few specially significant trees, but gains time to work out a policy. Bee-keepers complain of insecticides getting into bees as they gather ash pollen. It is proposed to try biological control by introducing Chinese predators and parasites of the insect.

At the time of writing the beetle had jumped west to Kansas and east to New Hampshire. It seems only a matter of time before it jumps the Atlantic. There are already reports that it has reached Moscow westward through Siberia. What happens when Emerald Ash Borer reaches Britain?

Globalisation of pests and diseases

Plant diseases are not new. The ancient Romans sacrificed puppies to propitiate Robigo, the god or goddess of wheat rust. Much earlier, the Elm Decline in the early Neolithic was apparently due to Dutch Elm Disease. Was this related to the beginnings of agriculture? Did Neolithic people introduce the disease? Did agriculture help it to spread? Conversely, did farmers spread into north Europe because a disease had cleared land for them? Did Elm Disease trigger the Neolithic Revolution?

For thousands of years people have been moving plants around the globe. In 1787 William Bligh of the Bounty was sent to Tahiti to collect breadfruit plants to take to the Caribbean to feed slaves, but something nasty happened to him and the plants were thrown overboard. Four years later he tried again – successfully, until the slaves refused to eat the breadfruit. On voyages round Cape Horn, any parasites would probably die out or kill their hosts on the way.

After 1833, live plants were taken more securely in Wardian cases. With steamships, globalisation went up a gear as parasites survived faster ocean crossings. Three American grape parasites ‒ phylloxera, downy and powdery mildew ‒ came in the nineteenth century, and many others in the early to mid-twentieth.

There were even deliberate introductions. In 1868–9, Monsieur   Trouvelot, a French dissident living in Massachusetts, imported gypsy moths from Europe to teach them to be silkworms. He got no silk out of them, but let some escape: they got into the woods and now defoliate the trees on an 11-year cycle. The caterpillars are hated by foresters and gardeners, but they probably do less ecological damage than the succession of frantic and futile attempts that the authorities have made to ‘control’ them.

Who remembers the ‘Plant a Tree in ’73’ campaign? What happened to all the trees planted in 1973? How many are still alive 40 years on? I was suspicious at the time: was all that planting really necessary? Was it really a substitute for conserving native trees? As one forester, Richard Pawsey, writing in New Scientist, said at the time:

“The present enthusiasm for tree-planting . . . masks an almost total ignorance of how to keep them alive.”

Peter Sell, plant taxonomist, pointed out that what were sold as ‘native’ trees were often lookalikes from anywhere between here and Japan. Until recently, gardeners made it a point of honour not to grow native plants: bluebells in one’s garden must not be the beautiful and romantic native bluebell, but Spanish Bluebell, which gets into native woods via garden throwouts and displaces the native bluebell. Tree-planting, like muntjac deer and grey squirrels, was another aspect of Homo sapiens’s tendency to mix up all the world’s plants and animals regardless of consequences.

Planting went industrial. It entered a world of grants and tenders and contracts and subcontracts and work to be finished on time and money to be spent before the financial year’s end: an environment geared to the anthropology of bureaucrats and at odds with the ‘real’ world of trees and parasites. A subcontractor, required to produce so many oak seedlings and finding oaks did not bear acorns this season, goes to another country with more reliable acorns and cheaper labour: he brings in the oaks and any disease on them that is not too obtrusive. As if the depths of commercial frivolity had still not been plumbed, one hears of collecting seed in Britain, sending it to be germinated on the Continent, bringing back the seedlings (and any disease they may have picked up), and selling them as of ‘local provenance’!

Timber merchants’ websites reveal the same ‘coals-to-Newcastle’ attitude. Although log prices for ash in Britain are at a historic low, much of the ash timber sold in Britain is brought from America, and some from the Continent. (And much of the ash timber grown in Britain is sent to the Continent.) This exchange would not have contributed to the coming of Ash Disease, which begins on leaves, but it could easily let in Emerald Ash Borer. No doubt the regulations impose precautions, but they are unlikely to be completely effective. The future of ash in Europe ought not to depend on an American inspector being willing to stay on an extra half-hour on a Friday afternoon to finish the job.

Can trade kill trees?

Globalisation of diseases has become the top threat to the world’s trees and forests. Exotic diseases subtract ‘keystone’ species one by one from ecosystems, sometimes almost overnight, more efficiently than deer and much more efficiently than climate change. People are being urged to plant trees to store carbon dioxide. Why bother, if they succumb to disease, rot, and let the CO2 back into the atmosphere?

Intercontinental trade takes pests and diseases which had come to terms with their hosts through co-adaptation over millions of years, and introduces them to new, unadapted hosts. There is also the prospect, especially with Phytophthoras, that separated pathogenic species are brought together and hybridise to create new and aggressive pathogens.

There is an analogy with bees. Reader, you may think the humble bumblebee is indigenous, bumbling away to provide what scholastic writers call ‘ecosystem services’ for the human lords of creation, like a labourer earning the National Minimum Wage. In reality, she is imported from God knows where to pollinate tomato and strawberry crops. The volume of trade in bumblebees defeats the regulations that are supposed to keep out diseases, some of which affect hive-bees too. A recent investigation reports that most of the officially ‘parasite-free’ imported colonies carry parasites. This never-ending import of parasites appears to be a factor in the general decline of bees in Britain.

Can’t introduced tree diseases be controlled?

In the nineteenth century the three American vine diseases came within an inch of abolishing wine, anticipating the American Prohibitionists. Wine-growing survived, but in a permanently more complex and expensive form: grafting and chemical spraying are necessary to get a crop at all. Plant diseases affecting crops are dealt with by a combination of chemicals and plant breeding, neither of which works well with trees, especially wild trees. English Elm was a supertree, cloned by people for centuries – until it proved super-susceptible to the 1970s strain of Elm Disease.

Concerning Ash Disease, a report by the Department for Environment, Food and Rural Affairs (DEFRA) in 2013 proposes ‘developing resistance to the disease in the ash population’. This is not quite as absurd as it seems. Although plants do not have the immune system that enables vertebrate animals to acquire resistance to foot-and-mouth or leprosy, ash’s damage-limitation mechanism determines how far a Chalara infection progresses in the tree. However, to suggest that a Ministry, or any other human institution, can influence this process is a vain aspiration. (If it can be done, why didn’t East Europeans do it ten years ago?) Trees have a generation time of tens or hundreds of years: pathogens can run rings round them in evolutionary terms. The government, in its ill-informed optimism, expects the present ash trees somehow to be replaced by a new generation that is resistant to Chalara. Even if deer hold off, long before that happens the Emerald Ash Borer will have arrived and eaten whatever ash trees survive Chalara.

There is only half an instance in Europe of a disease of wild trees being controlled, let alone exterminated. Chestnut blight almost destroyed the chestnut trees (Castanea sativa) which had been a major food source in southern Europe. In the 1960s it ceased to be a problem, not because anyone did anything, but because God raised up a fungal virus which crippled the fungus and made it incapable of damaging the tree. The Apennines are full of huge trees that once were five-sixths dead. On Mount Athos, the Holy Mountain in north Greece, the monasteries depend for their livelihood on coppiced chestnut woods, an isolated population of chestnut only recently reached by the disease. In 2001, the monks (who are keen on technology) were busy inoculating their trees with virus-infected fungus. This is half an instance, because all attempts to get the virus going in North America have failed.

What can be done?

1.   Recognise the problem. Whether or not Ash Disease turns out to be quite as bad as was anticipated in 2012, it is not an isolated problem. Tree disease has struck half-a-dozen times, and each time is still treated as happenstance, rather than as part of a wider pattern, as enemy action. Governments throw a little money at each separate disease after it has arrived. They are ill-suited to deal with the wider problem, because each government encounters only one new tree disease; when the next disease arrives it will be a new government which will treat it as a new problem and will not learn from last time or look forward to next time. The public, faced with a depleting landscape, regards depletion as normal. Since the last Elm Disease a new generation has grown up to accept the absence of big elms as normal – even ecologists fail to notice or study places where big elms survive or are returning.

2.  As John Gibbs, the great tree pathologist, has pointed out, it is no good reacting to known plant diseases: that battle has already been fought and mostly lost. What is needed is to forestall diseases that have not yet got here or are still unknown. For ash trees, the latest year in which to react to Chalara was 1995. The real threat is now not Chalara but the Emerald Ash Borer.

3. Don’t use climate change as a let-out for inaction. If global warming were the underlying cause, then each hot summer would see tree diseases from the south extending their range northward. That is not the pattern: unknown diseases suddenly appear, usually from west to east or east to west, regardless of weather or climate.

4.  Make use of being an island. The Isles of Scilly out in the Atlantic still have a full complement of great elms. Banning imports before the event might not have kept Ash Disease out of Britain, but probably would have kept it out of Ireland. Chile is, in effect, an island, isolated by the ocean, the Andes, and the Atacama Desert, and (I am told) is determined to remain so: it stringently forbids commercial imports of plants and soil, especially in order to protect its pre-phylloxera grapevines.

5. Get real. Stop letting the anthropology of commerce overrule the practical world. Stop treating plants (and bees) as mere articles of trade, like cars or tins of paint, to be made and brought in industrial quantities from anywhere. Importing a million cars does not imperil the cars that are already here, but trees are different. Nobody has to import trees commercially: it is only an artefact of how business happens to be conducted. What matters is volume. My little Christmas tree from an Alpine holiday will not do much harm. But a commercial supplier, importing a million container-grown hawthorns from Ruritania (as though there were no hawthorns in Britain!), inevitably imports a thousand tons of Ruritanian soil and whatever is in it. However thoroughly the Customs, or a responsible nurseryman, inspect the consignment, they cannot detect a microscopic pathogen when they do not know in advance what to look for. If it is ash trees, imported in winter, they will not detect all Chalara even if they do know what to look for. Trees should be imported only in small numbers for special reasons, with precautions that are impractical with commercial shipments.

6.  Plant fewer trees, more expensive trees, wider apart, and take proper care of them. Stop making tree-planting a default option, as in the Scots proverb: ‘Ye may be aye stickin’ in a tree; it’ll be growin’ while ye’re sleepin’. This casual mindset needs to be changed. It would be disastrous if the death of ashes were made the pretext for a massive replanting, bringing in more foreign stock and more foreign diseases. The pros and cons of every planting need to be formally assessed, including the risk that planting trees will kill existing trees. Tree-planting, like chemicals, is not risk-free: if not used sparingly it will lose its effectiveness.

7. Revive the science of tree pathology. Although the underlying problem belongs to anthropology rather than science, the understanding of tree diseases has been scandalously neglected in Britain. (I except the recent revival at Bangor University.) I was taught tree diseases in Cambridge Botany School by Denis Garrett and John Rishbeth. I read research papers and passed examinations; although my career has been in other directions I have maintained a lifelong interest. Times have changed. Garrett and Rishbeth retired, Cambridge University failed to replace them, and their expertise was lost. My contemporary was John Gibbs OBE, who became head of the Forestry Commission’s pathology department and retired in 2001. Botany turned into Plant Sciences, of which tree pathology was not one. I understand there are about a dozen of us left in Britain. I am one of the last survivors of a Critically Endangered Species. I belong in the Zoo.

Oliver Rackham died in February 2015, a year after publishing The Ash Tree, from which this article is abridged.

oliver rackham