Where did Dutch elm disease come from?
Contrary to what the name might suggest, Dutch elm disease (DED) did not originate from the Netherlands. The name actually reflects the fact that it was identified by Dutch scientists Bea Schwarz and Christine Buisman.
Most evidence suggests that DED originates from Asia, where many species of disease resistant elms can also be found.
At least two closely related species of fungi, Ophiostoma ulmi and O. novo-ulmi, cause the disease we call Dutch elm disease and the likelihood is that they have evolved in different parts of Asia. Japan is within the geographical centre of origin for O. ulmi.
What are the symptoms of Dutch elm disease?
Symptoms first appear in early summer when leaves at the tips of elm branches turn yellow and wilt. The affected shoots then die back and often form the shape of a ‘shepherds crook’.
As the disease progresses in an individual tree, more of the leaves in the canopy turn from yellow to brown. Towards the end of the summer most of the foliage becomes browned and withered.
If you peel the bark away from twigs which still retain yellow or browned leaves, the wood beneath the bark has dark streaks indicative of blockages in the water conducting vessels of affected trees.
How does Dutch elm disease spread?
The fungi that cause DED is transferred from diseased to healthy elms by elm bark beetles, which carry spores of the fungi on their bodies and, in so doing, spread the disease.
These tiny (2-5 mm long) dark brown beetles breed in the bark of dying and dead elms and, when a new beetle generation emerges from the bark in spring carrying the fungus, they transfer it to healthy elms as they feed on thin, sappy twigs high up in the tree canopy.
Once the DED fungus infects a tree, and if it reaches the roots, it may then spread to neighbouring trees via interconnected roots. This occurs most commonly with elms in hedgerows which may have connected root systems.
Why are some trees resistant to the Dutch elm disease?
Some elms have genetic resistance or field resistance to the DED fungi.
We don’t fully understand how genetic resistance arises, but we think that a combination of resistance mechanisms helps these elms to survive.
We know for example, that the structure of the water conducting system (xylem vessels) in the trees plays a critical part in limiting internal disease spread allowing resistant trees to survive infection.
In contrast, elm species with field resistance may lack genetic resistance but despite this often survive in areas where DED is widespread. In such instances, these elms tend to be less attractive to feeding elm bark beetles which spread the disease, and so escape the infection.
What effect has Dutch elm disease had so far on Britain’s elms?
We estimate that more than 60 million elm trees have been lost to DED in the UK since the 1970s.
However, although most mature elms have gone, young elms are still plentiful, as they regenerate from the roots of elms killed by DED or from the seed of others.
These young elms rarely survive more than 15-20 years before being struck down by Dutch elm disease again, but a new cycle of regeneration then begins again.
What can be done to protect against and treat Dutch elm disease?
The most effective action against DED is to remove infected elms before they become breeding trees for the disease-transmitting elm bark beetles.
Fungicide injection to cure infected elms can be effective but is no longer considered a cost-effective way of combatting the disease.
Another approach is to plant disease-resistant elms, as many clones have been released from reputable breeding programmes.
Should we be worried about Dutch elm disease?
Despite the vast number of elms killed by DED, they have shown themselves to be highly resilient.
Some estimates suggest there are more elms in the countryside now than there were before the current epidemic took hold in the 1970s, although few of the mature elms from that time still survive.
Find out more at Forest Research.
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Main image: Leaves of Elm (Ulmus Minor) with Dutch elm disease. © MAIKA 777/Getty