Previously, I described several lethal tree diseases in North America that have, if not decimated populations throughout the tree’s distributions, certainly significantly reduced them over wide areas. The tree species discussed, American chestnut, American elm, butternut and flowering dogwood, all highly valued, succumbed to non-native, invasive pathogens introduced by man via shipped wood products or nursery stock or, by spores “hitch-hiking” onto traffic. Similar tree disease problems have developed in Europe although perhaps more recently and on a smaller scale than in North America. Once again, humans are responsible, and seeking to mitigate if not reverse the threats to prized trees brought about by this unwanted increase in species diversity. Discussed in this post are diseases introduced by man to the British Isles, but with implications for much of continental Europe.
Acute Oak Decline
This relatively recent introduction to the UK has infected mature oak trees, especially in the Midlands and southeastern England. One assessment suggests that “many thousands of trees are affected” though surveys are incomplete. The etiology of the disease is uncertain but probably involves bacteria found in dying trees whose dispersal is facilitated by bark beetles. Three different genera of bacteria are suspected to be causative agents. The disease affects the two native oak species of the the UK-sessile oak (Quercus petraea) and pedunculate or English oak (Q. robur), but, interestingly, has not (so far) extended to the several non-native oaks like turkey oak, black oak and red oak, all species of North American origin planted in the UK for forestry purposes. However, the disease agent does attack bilberry (a member of the genus Vaccinium, as are blueberries, huckleberries and the cranberry all native to North America), a beloved berry plant.
Oaks infected with acute oak decline exhibit dark fluid seeping from stem wounds. Mainly mature trees are infected and these can die in as little as 4 or 5 years following initial infection. There is hope that some oak trees may prove to be resistant or immune to these diseases based on natural genetic variability, as has been found in certain ash trees in relation to ash dieback in England.
Another disease, called “sudden oak death”, a major problem in North America caused by Phytophthora ramorum, a fungus-related “water mold”, has recently (2002) appeared in the UK, and is believed to have been accidentally imported probably from Asia, not from North America as initially suspected. But the first tree to be infected by P. ramorum was a mature scarlet oak (Quercus coccinea), a North American native. Other species in Germany and The Netherlands also have been infected. Sudden oak death is not synonymous with acute oak decline as it differs in symptoms and in pathogen.
The oak processionary moth
Named after the caterpillars’ habit of lining up “head-to-toe” when on the move, this moth has recently become established in southern England. The pest was imported from continental Europe in logs in 2006. The caterpillars are capable of stripping whole oak trees of foliage, sometimes killing them. In addition the larvae bear stinging hairs that cause a painful skin rash in humans who come into contact with them. Larvae also shed hairs into the air that can be toxic to humans and pets, especially dogs which are allergic to the hairs. The Forestry Commission says that the pest no longer can be eradicated in London, only controlled. The moth is native to southern Europe and central Asia where native predators apparently help limit the moths. In northern Europe, where predators are absent, this pest is presently exponentially increasing. In England the Forestry Commission is resorting to aerial spraying of woodland areas where the pest is beyond control.
In 2012 a fungal disease causing leaf loss and crown dieback in ash (Fraxinus excelsior and F. angustifolia) trees in Denmark and the UK was determined to be caused by Hymenoscyphus pseudoalbidus,an ascomycete fungus. This species was not distinguished until 2010 from the closely related H. albidus, a native saprophytic species that does not attack living tissues. It was until recently known as Chalara fraxinea. The pathogen was discovered for the first time in the United Kingdom in shipments of trees from The Netherlands. It only took months for the disease to spread beyond the imported nursery stock to 10 other locations, spurring alarm. By September 2013, the disease had spread to 500 locations in England. Ashes are valued in the UK for beauty and for their wood; they constitute about 5% of British woodlands. Ash dieback has “devastated” ashes in Denmark and is spreading throughout central Europe. In Denmark, 60-90% of native ashes have been lost. Ash dieback was first recognized in Poland in 1992, but its origin is unknown.
“Dutch” Elm Disease
A fresh outbreak of Dutch elm disease in England affecting the “English” elm (Ulmus procera) was reported in 2010. Twenty-five to 30 million elms had been killed beginning in the 1970s when a virulent strain of a fungal disease arrived with Canadian logs. The “Dutch” refers to laboratories where the fungus was identified. Tall “English” elms (now often called the Atinian elm to reflect the species’ wide European distribution) once dominated hedgerows and were one of the largest trees in Europe. Apparently elms were introduced into Great Britain centuries ago. In fact, they can be traced back to introductions by the ancient Romans who used them to prop up vines in Britain. Genetic studies verify that U. procera trees from Spain, Italy and the UK are genetically identical; in fact, they are clones of a single tree! This homogeneity is probably a factor in the tree’s disease susceptibility. Other species of elm such as U. glabra and U. minor have some genetic resistance to the disease but not U. procera. Infections occur when beetles feed on elm twigs, biting through bark thereby introducing spores to living tissues within. The Atinian elm has been saved from extinction by ruthless sanitation felling and pruning over the past 40 years. Still, only a few survivors are left in England aside from East Sussex and the Isle of Man. The year 2013 has been a bad one- Dutch Elm Disease has been spreading rapidly. Factors exacerbating the situation include hotter than average weather, the high costs of large-scale control efforts, and the recent imposition of fees for felling which encourages landowners to leave diseased trees (a source of infection) uncut.
The tree disease situation in the UK is of growing concern. Diseases caused by Phytophthora spp. in particular, are capable of quickly killing trees over large areas. A recently affected species is the Japanese larch (Larix kaempferi) which is commercially planted in England, Wales and Ireland. Infected larches were first detected in 2009 in England and more recently were found in Ireland.
In 2012, larvae of the Asian Longhorn Beetle which can kill oaks and willows was found in Kent (southern England) where 2000 trees were burned as a sanitation measure. Other threats are the chestnut blight (affecting the native Castanea sativa), plane (Platanus spp., a non-native street tree) wilt and the bronze birch borer, a small beetle that attacks native birch trees (Betula spp.) in both Europe and North America. One report suggests that there are a total of 15 tree diseases found in the UK, with another five not present but with potential to become a problem in the future. Some useful references among many others:
SCIENCE vol.338,p586 2 Nov 2012.