Ash die-back Introduction Ash trees make up about 5% of woodland in Britain but are now under attack from a fungus, Chalara fraxinea. The Forestry Commission has identified over 300 infected sites across the UK. Spores from the fungus could spread from the east of England and Scotland to eventually cover Britain in some cases carried on people's shoes and clothes. Ash die-back disease could spread naturally across the UK by around 20 miles a year, infecting most of the country's 90m ash trees within a decade. Human movements could speed up the process considerably. Invading species According to botanists, more than 30 alien insects, fungi, bacteria, viruses, diseases and invasive plants, are expected to reach Britain and other European countries in the next few years. Many of them have the potential to devastate native populations. Some could arrive on imported trees and shrubs from Brazil or in soil from China or Africa, others via wooden ornaments and even packaging crates. They are likely to behave differently in new environments In fact, there have been more outbreaks of diseases in the past decade than in the whole of the twentieth century. Some of these organisms move into new niches when they move into an unknown habitat. Plants have little or no resistance to them and outbreaks can happen very quickly. Spread of the fungus The cases of ash die-back in East Anglia and Kent are thought to have spread by the wind from the continent. The location of infected sites offers some hope that the disease can be contained. Forestry groups say the government was notified of the fungus as early as 2009, when disease broke out in France, having spread through the Baltics, Scandinavia and eastern Europe (see Table 1). It could also have come into the country via infected trees imported by the horticultural trade.
Table 1: Chronology of ash die-back 1992 The fungus behind ash die-back, Chalara fraxinea, emerges in Poland. 2009 The Horticultural Trades Association warns the government that the fungus, now widespread in Denmark, could spread to the UK, and calls for an import ban. 2010 Disease confirmed in the Netherlands and Belgium. February 2012 Summer 2012 October 2012 Infected trees detected in the UK for the first time, in a nursery in Buckinghamshire. Outbreaks of ash die-back identified at plantations and nurseries across England and Scotland, from County Durham to Berkshire. First cases in the wild identified in Norfolk and Suffolk. The government imposes an import ban. UK forests 'under unprecedented threat from a range of disease' The UK's forests are under unprecedented threat from foreign pests and diseases. Some could die out in months and do no damage but others could spread rapidly, as Dutch elm disease did in the 1960s. The ash die-back fungus is just the latest invader to pose a serious threat to UK trees: more than three million larch trees, mainly in the West Country, Wales and Scotland, as well as thousands of mature oaks and chestnuts, have been felled in the past three years to prevent similar fatal plant diseases from spreading out of control. Plant pathogens are on the rise globally and Britain is particularly susceptible because of its increasingly warm, wet winters and because it is a centre of world trade. Easy access to plants from around the world has encouraged gardeners to buy millions of exotic plants, many of which can arrive diseased. Some plants can only enter Britain with passports, but the majority of diseases are only identifiable in laboratories. The fashion for instant landscapes and the planting of very large trees increases the risk of diseases spreading. There is a greater demand for more exotic and bigger plants. Some plants are shipped in from China or elsewhere with 1,000 litres of soil around them. That soil is full of billions of bacteria. The diseases and pests can arrive by several routes. In March, more than 250 live larvae of the Asian longhorn beetle, which can kill oak and willows, were found in trees in Kent. More than 2,000 trees had to be felled and burned. It was thought to have entered in wooden packaging for Chinese stone. There are currently about eight organisms in the British Isles that are a real concern to botanists. In the 1960s and 1970s it was Dutch elm disease, which killed 30 million trees; in the 1990s it was a new Phytophthora which affected alders along riverbanks. Plant experts are particularly concerned about the Oak processionary moth, which arrived in west London in 2009 and has now developed two major
populations. It has the potential to spread anywhere there are oak trees. Caterpillars from the Oak processionary moth can cause serious defoliation and weaken oaks to the point where they are prone to other diseases. The Dothistroma needle blight affects a range of conifer species, and threatens commercial forests by significantly reducing timber yields. In 2012 scientists also discovered a fungus that afflicts sweet chestnut trees in orchards sourced from France. A new and undiagnosed pathogen appears to be affecting Scots pine. Ecological value of ash trees The ash die-back fungus could be disastrous for Britain's ash trees with serious knock-on ecological consequences. It could change the countryside very significantly, and parallels with Dutch elm disease of the 1970s have been made. It causes leaf loss and can lead to the death of the tree. It has affected between 60% and 90% of ash trees in some areas of Denmark and is becoming widespread throughout central Europe. If ash trees suffer large scale declines, some of the country's rarest species could be at risk of being lost from Britain. As well as 80 common insects, at least 60 of the rarest insect species in the UK have an association with ash trees these are mostly rare beetles and flies. Scarce species could become even scarcer and may even be lost. Ironically, many of the rare species associated with ash depend on the dead or dying branches of old trees, but if infected trees are ultimately cleared away then even these species will suffer. Ash is also important for many lichens and mosses that grow in its bark, and its seeds are an important food for wood mice. Economic value of ash trees The UK government values the benefits of our woods and trees at about 1.2bn a year. The consequences of ash die-back could be catastrophic, not only for the environment but for the UK economy too. The ash crop, once worth millions of pounds, is now useless. Some nurseries infected with the fungus have been removed from the official map of the outbreak, after nursery owners complained that being identified might hurt their business. There is no longer any market for ash trees. Potential solutions To date, there is no chemical that kills the fungus and over 100,000 ash trees have been destroyed so far in England and Scotland. Trees cannot be vaccinated, and the airborne disease would be too expensive to treat chemically. However, there is a small chance that Britain's ash trees may have a resistance to the disease. If this is so, by the next season there could potentially be resistant forms of ash growing, though as very small saplings. Ash reproduces quite quickly. A small number of trees survived the very intense epidemic in
Denmark, where 90% were infected, so there must be hope here. If it is possible to slow its spread of the disease, it may be possible to find those trees with genetic resistance to the disease. One of the more radical suggestions is to have rigorous border checks on the billions of trees and plants imported into Britain and Europe from around the world every year for parks, gardens, woodlands and forests. Other measures expected include forcing nurseries to attach plant passports to all imported plants; a new system to ensure all plants are safe before they enter Britain; tighter rules on suppliers; restrictions on the public bringing in plants from abroad; and a radical overhaul of the European plant protection system. Mature trees will not be burned, because they are important for other wildlife and may help identify resistant strains. Websites Defra tackling the rising danger of imported diseases killing trees and plants Steve Collin of the Norfolk Wildlife Trust, who first discovered Ash die-back in Norfolk What ash die-back means for UK woods Download ash die-back guide from the Woodland Trust Activities 1 What is the fungus causing ash die-back? 2 Where did it come from, and how did it get into the UK? 3 How many ash trees have been felled as a result of ash die-back? 4 What can be done to reduce the impact on fungal infestations on ash trees?
Suggested answers 1 The fungus that is attacking ash trees is called Chalara fraxinea. 2 It most probably came from Denmark, by spores carried by the wind. Alternatively, it could have been brought in by an infected plant as part the UK s horticultural trade. 3 Over 100,000. 4 It may be that a small number of trees survive the fungus because of natural immunity. These could be used to develop disease-free ash trees. There could be border checks on the billions of trees and plants imported into Britain and Europe from around the world. Plant passports could be issued to all imported plants, and there could be restrictions on the public bringing in plants from abroad.