Dutch elm disease
This serious tree disease has affected millions of trees in Britain since the late 1960s. The fungus that causes the disease is spread from tree to tree by elm bark beetles that are native to the UK.
Hosts
All elms (Ulmus spp.) can be affected to differing degrees, as can related Zelkova species (although these are less severely affected than Ulmus). English elms (U. procera, U. glabra and U. carpinifolia) are among the more susceptible.
Symptoms
Sudden yellowing, then browning and shrivelling of the leaves on part or all of the tree, any time in summer or early autumn. Stripping the bark from affected branches will reveal brown streaks in the wood, visible as a broken or continuous circle of discolouration in the outer growth ring if the branch is cut across. Affected trees die back to ground level, but regrowth from suckers can occur and the tree is not completely killed.
Biology
The current epidemic is caused by the fungus Ophiostoma novo-ulmi. Prior to the 1960s, the disease occurred in the UK in a less severe form, caused by the fungus Ophiostoma ulmi, then known as Ceratocystis ulmi. Early research on the disease was carried out in The Netherlands, hence the epithet ‘Dutch’. In the late 1960s O. novo-ulmi was accidentally introduced to the UK from North America in infected elm logs imported for timber. At this time the distinction between O. ulmi and the more virulent O. novo-ulmi was not recognised and due quarantine precautions were not taken. The epidemic spread rapidly from the ports of entry and destroyed the adult elm population in the 1970s.
The damaging fungi are spread by native elm bark beetles, principally Scolytus scolytus. The beetles lay their eggs in the bark of dying elms and the larvae form galleries (networks of tunnels) under the dead bark. The fungi produce sticky spores on the walls of these galleries, so when the adult beetles emerge they are already contaminated with spores.
The adult beetles fly to healthy elms, usually above 4m (13-14ft), and feed on living bark. In so doing they introduce the fungal spores to the outer growth ring of the healthy tree's wood where the fungus grows and blocks the tree's water conduction. Affected branches wilt rapidly and do not recover.
Sucker regrowth means that elm is still a common plant, but it succumbs to re-infection after about 20 years when the plants become large enough to attract the beetles. This fungal plant disease is unusual in having an insect vector that can actively seek out the trees, ensuring a very high level of infection. There are, however, a few disease-free areas, scattered around the country, where the beetles have not yet introduced the fungus.
Control
Some hybrid elms with promising levels of resistance are now available: Ulmus ‘New Horizon’ (currently available to trade buyers only), U. americana ‘Princeton’ (Knoll Gardens; 01202 873931; enquiries@knollgardens.co.uk), Ulmus 'Lutèce' (Duchy of Cornwall Nursery; 01208 872668; sales@duchyofcornwallnursery.co.uk) and U. glabra ‘Clusius’ (Barcham Trees, Cambridgeshire; 01353 720748; sales@barchamtrees.co.uk). The parent species are not native elms and the hybrids have a different growth habit.
Further Information
The Woodland Trust and TROBI (The Tree Register of the British Isles) are also running a recording scheme of all elms that survived the last outbreak of Dutch elm disease; www.ancient-tree-hunt.org.uk. They are recorded as notable, veteran or ancient, according to their age or condition. Alternatively, contact David Alderman of TROBI (info@tree-register.org) or Jill Butler of The Woodland Trust (Autumn Park, Dysart Road, Grantham, Lincs NG31 6LL).
The establishment with most experience of the disease and its control and knowledge of the current breeding programmes is the Forest Research Station of the Forestry Commission at Alice Holt Lodge, Wrecclesham, Farnham, Surrey GU10 4LH.
An interesting report assessing resistant elms in the context of ensuring elm foliage as a food resource for the white letter hairstreak butterfly can be found at www.hantsiow-butterflies.org.uk/conservation.html#Elm520Report.