Dutch elm disease had a devastating impact on the UK’s treescape in the 20th century and its effects are still being felt today. However, for the country’s remaining population of veteran elms, a healthy future can be secured through yearly vaccination, as Ron Schraven from treecare company Bomendienst explains.

ELM trees have long played a major role in western culture. When agriculture first appeared in European forests, elm leaves were used for fodder, while its timber has been an essential resource for housing, furniture and ships for thousands of years. Today, we still benefit from its useful properties. Because of its tolerance to saline soil and harsh winds, we can still find many elms in coastal regions in Europe and North America. One of the most important elm landscapes in Britain is found on the Sussex coast. Its ability to adapt to stressful or harsh conditions, together with beautiful and not-too-dense canopies, makes it cherished along roads and in urban environments. Even in locations where planting spaces are limited by canals or buildings, elms usually flourish. As such, elms came to be viewed as ideal city trees.

Forestry Journal:

Dutch Elm Disease (DED)

Unfortunately, around 1920, elms in the Netherlands began showing sudden wilt that sometimes led to a rapid death, with the loss of over 30 per cent of the population. The outbreak’s impact in Great Britain was limited, with a mortality rate of around 20 per cent.

The research that followed, by Spierenburg and Schwarz, showed discolouration in diseased branches. They described the disease and identified the causal organism as Graphium ulmi. It is this early research, and the work of Westerdijk and Buisman, that gave the disease its international prefix ‘Dutch’ to ‘elm disease’.

Today, we know that Ophiostoma ulmi (and, since the 1970s, Ophiostoma novo-ulmi) is the cause of elms dying. The disease is mainly transmitted by the elm bark beetles Scolytus scolytus and S. multistriatus. These beetles are attracted by weakened and dead elms, breeding underneath the bark. As the pathogen forms fruiting bodies in breeding galleries, emerging beetles become stained with spores of DED and infect healthy trees by eating from fresh twigs and leaves.

Another way that DED is transmitted is via root grafts. Elms often develop root grafts with their neighbours, but transmission can be controlled by trenching and cutting roots.

The elm population of Britain consisted of about 30 million trees (mainly Ulmus procera, U. minor and U. glabra) and was distributed across the land, with over half concentrated in southern England. These elms were commonly found in hedgerows, city streets and rural landscapes. When Ophiostoma novo-ulmi was introduced around 1965, the outbreak was deemed a troubling but manageable problem. The spread of the disease began slowly, with flashpoints around Bristol and east of London, but more trees were dying as time passed. By 1970, official surveys had been carried out, and the devastating effect of this more virulent form of the disease became clear. Management programmes were introduced, but they did not have the desired effect. DED continued to infect trees and has been doing so ever since. Different epidemics have swept the land with varied intensity. As a result, swathes of English elm stands have disappeared. Fortunately, there are areas where the spread of the disease was restricted by sub-optimal conditions, intervention or lower host density. But these remaining elms, especially the veteran specimens, are becoming rarer.

Nowadays, elms represent an important scenic and cultural treasure for local residents and are important habitats for many rare British species, including the White-letter Hairstreak butterfly.

A major population can be found along the Sussex coast, where continued DED control programmes, with a lot of volunteers involved, have allowed thousands of mature elms to survive.

“It is important that we preserve our heritage for generations to come,” said Anthony Becvar, elm officer in East Sussex. “Currently, that means felling diseased trees to save others from becoming infected. The close proximity of Sussex to the coast of the English Channel results in cold winds from the sea making it a difficult area for the elm bark beetles to reach. These conditions, and our continued control programme, have resulted in one of the largest population of elms in the country.”


It seemed there was little that could be done to combat DED, especially when trees that had survived the first wave perished in the second wave by O. novo-ulmi.

Managers carried out the inspection, sanitation felling and replanting of trees, but the losses were immense and cities that were not willing or able to spend large amounts of money lost up to 15 per cent of their trees each year.

Using existing knowledge and building on the claim by Chester in 1933 that acquired immunity by plant vaccination was a possibility, the Dutch authorities launched the search for a defence against the new and more aggressive O. novo-ulmi. The aim was to find a (biological) agent that would induce an immunity to the disease. In the 1980s, several organisms were tested and, in 1989, a Verticillium fungus was found to trigger a mechanism in the tree and protect it against DED. This fungus was tested extensively in the vicinity of Amsterdam and showed the Verticillium isolate WCS850 was able to trigger the defence mechanism of the elm trees, protecting them from the disease while others in the area succumbed and died. Since then, Verticillium WCS850, known under the product name DutchTrig, has been an essential part of the integrated elm programme run by Bomendienst in the Netherlands.


The inoculated spores of Verticillium induce a response of the immune system of the tree and awaken its natural defence mechanism. With induced resistance up and running throughout the growing season, the elm is able to successfully fend off a DED infection. Since DutchTrig functions like a vaccine, the product has no curative properties and should not be used if the infection has already occurred or if the tree is root grafted with diseased (and/or already removed) trees in the area.

DutchTrig is applied annually in early spring, as soon as the leaves have begun to sprout and before any beetle infection of DED occurs. The vaccine is injected into the tree at breast height with the DutchTrig injection tool at every 10 cm of tree circumference, by pushing the chisel of the inoculation tool through the bark and releasing the exact amount of vaccine with one pull of the trigger.  

This inoculation system makes the application fast, safe and cost-efficient. It is possible to treat large numbers of trees every year. In the Netherlands, over 20,000 trees are treated annually in a period of six weeks in May and early June. Nationally, the success rate is steady, with only around 1 per cent of trees dying from DED since the early 1990s and, in the last five years, 99.5 per cent of the elms remaining.

The Conservation Foundation, which contributed to the preservation of elms through the Ulmus Maritime project, acknowledges that DutchTrig can be a new tool for managing Britain’s elm trees.

Director David Shreeve said: “The elm has been a cherished feature of our life and landscape for centuries. Many have disappeared over recent years, but this vaccine offers new hope for these magnificent trees and an opportunity to help protect the elm landscape on the Sussex coast for future generations to enjoy.”

Britain’s elm protection programmes have mostly relied on sanitation felling to help control beetle populations. Unfortunately, these measures are curative instead of preventative. DutchTrig, now fully licensed for use in the UK, offers a chance to protect Britain’s elms from DED and preserve them for the future.