An Introduction to Dutch Elm Disease

(DED)

Dutch elm disease (DED) is caused by the fungus Ophiostoma novo-ulmi. It is a type of disease known as a ‘vascular wilt’ because the fungus blocks the vascular (water transport) system, causing the branches to wilt and die.

It is spread by Elm Bark Beetles, particularly Scolytus scolytus and damage is usually seen in summer and early autumn.

This photo shows a mature Guernsey Elm Ulmus.x minor 'Sarniensis' at Preston Park in Brighton, taken in 2017.

DED can be seen on the tree pictured on the right – look for the patch of browning leaves in the middle of the foliage.

DED is known as ‘Dutch’ because important early research on it was carried out in the Netherlands.

A form of the disease was first identified in Britain in 1926, but this was caused by a different fungus known as Ophiostoma ulmi and was mild in its effects, killing only a small portion of Elm trees across the country. It’s thought the reason this strain wasn’t as devastating was because the trees natural defenses were able to fight against it.

In the late 1960’s, a stronger fungus Ophiostoma novo-ulmi, was accidentally introduced to the UK from Canada on imported Elm logs and this strain was much more devastating. Ophiostoma novo-ulmi is not native to Canada and its true origin is thought to come from Asia.

The disease first appeared in Tewkesbury, Gloucestershire, followed by Norton near Chichester, West Sussex and then Colchester in Essex. It continued to spread rapidly across Britain throughout the 70’s and 80’s, with millions of Elm being lost.

Symptoms

From early summer, the first symptoms of Dutch elm disease is exhibited by premature yellowing and wilting leaf clusters, typically seen in the higher portions of the elm canopy.

These leaves eventually turn brown in colour and infected shoots die back from the tips, spreading down the branches.

As the health of the tree continues to deteriorate thanks to the spread of the fungus, twigs begin to lose their leaves, until the tree is completely bare.

It is from this point that the bark within the Elm becomes dry enough and favourable for Elm Bark Beetles to move in to lay their eggs.

Another more advanced diagnostic sign of infection is the occurrence of twigs developing hooked, curved or turned down tips. Please note that this symptom can be produced by other influences other than an infection.

Yellowing and wilting leaves Browning, shriveled leaves

Leaf loss, leaving bare twigs Hooked, curved, turned-down twig tips

Vector/Transmitter of Dutch elm disease

Scolytus scolytus - the Large Elm Bark Beetle.

The spread of Dutch elm disease is aided by Elm Bark Beetles of the genus Scolytus, due to their life-cycle.

There are two Elm Bark Beetle species in the UK, Scolytus scolytus, the Large Elm Bark Beetle and Scolytus multistriatus, the Small or European Elm Bark Beetle.

Be careful not to mistake them for the Furniture beetle, which is similar in appearance.

The Large Elm Bark Beetle accounts for most DED cases in the UK, as it’s far more successful at burrowing into the Xylem (conducting tissue) of the tree to induce infection.

The beetles like to breed in dead and dying elms, including those killed by the disease. This is because active Xylem vessels in healthy bark have a sap pressure that is too high for the Elm Bark Beetles to burrow through and they would instead be flushed out.

Their grubs (larvae) tunnel in the bark and outermost wood, forming galleries. The DED fungus produces sticky spores in the bark, which contaminate the newly hatched adult beetles as they emerge. Emergence is often triggered by periods of warm weather.

The fungal spores of Ophiostoma novo-ulmi will be attached to the backs of the beetle and will also be present within the digestive track.

From the end of June until the middle of July, the beetles will fly to healthy Elms, with Scolytus scolytus preferring older trees of a diameter of 70mm and above.

The Small Elm Bark Beetles Scolytus multistriatus prefers to target smaller, younger trees with a diameter of anywhere from 25mm and above, including saplings.

Once there, the beetles feed on young bark and introduce the pathogen into the Xylem of the tree. The fungus grows in the Xylem, blocking water flow and causing rapid wilting and death.

An entry hole caused by the Elm Bark beetle

Bark removed to clearly show the hole caused by beetle.

Once a female Elm Bark beetle has burrowed into the tree, she will lay her eggs. Sometimes the females will send out pheromones which other Elm Bark beetles will pick up on and respond to.

The brood gallery which contains the beetle eggs will run parallel with the trees surface. Once the eggs hatch, each larva or grub will begins to feed by making their own side-tunnels. These will be at right angles from the brood gallery and they will extend outwards and widen as the larvae grow and feed on the inner bark.

The larvae will go on to make a separate chamber to pupate within and when they are ready, the newly hatched beetles will find their way out, leaving behind a hole in the bark.

A second brood emerges around August and go on to attack further trees, although positive identification of DED during the latter part of the year becomes much harder, since the leaves all change colour during the autumn and the infection may go unnoticed until the following spring.

White beetle larva (pictured left) and larval feeding tunnels present

Spot the Elm Bark Beetle! Pictured middle-left, surrounded by the imprints of the larval feeding tunnels

Roots and logs: alternative movement of Dutch elm disease

Another way that the DED fungus is spread from one Elm to another is through adjoining root systems.

The disease can spread rapidly down rows of hedgerow Elms through root grafts formed between adjacent trees.

The roots of felled or fallen Elm trees that have been infected with DED also remain a risk, as the disease can lie dormant in the root system for up to 30 years.

This means if any re-growth appears from this source, this too will be infected and will eventually die.

The transportation of logs for fire wood is the biggest man-made cause of spreading the disease. Many people won’t be aware if their fire-wood is Elm, or know if it has been previously infected.

The advice for anyone using and transporting Elm logs is that the bark should be stripped off and burnt beforehand. This ensures any Elm Bark beetle infestation is destroyed and prevents any beetles from colonising again.

Other threats to Elm:

Elm Zigzag Sawfly was confirmed in Sussex near Burgess Hill during 2018. This latest threat to Elm has the potential to defoliate trees, as a result of the larvae’s vigorous feeding habit.

Larval feeding pattern on elm leaf

Even Dutch elm disease-resistant trees are at risk from Elm Zigzag Sawfly.

If you find Elm Zigzag Sawfly larval feeding damage on Elm leaves, you should report your sightings with supporting photos.

For more information, further photos of feeding damage and where to report your sightings, visit:

www.forestresearch.gov.uk/tools-and-resources/pest-and-disease-resources/elm-zigzag-sawfly/

This will contribute to the understanding of how Elm Zigzag Sawfly is spreading and distributed across the UK.

Reporting DEDIt’s very important to report any suspected cases of Dutch elm disease at the earliest opportunity, along with photos if possible, so that the relevant authorities can take action to prevent further spread of DED.

Brighton and Hove: (City Council) Arboriculturists, Email: arboriculture@brighton-hove.gov.uk and/or call 01273 292929.

East Sussex: East Sussex County Council, Dutch elm disease officer, Anthony Becvar, Email: dutchelmdisease@eastsussex.gov.uk or 01273 335087 / 0345 60 80 190. Please visit the following East Sussex County Council page to report your sightings online, photos and information, here: https://www.eastsussex.gov.uk/environment/woodlands/dutchelms

West Sussex: If Dutch elm disease is found between Shoreham and East Worthing, please report to Adur District Council, as this area forms part of a Dutch elm disease control buffer zone. www.adur-worthing.gov.uk. If you live in West Sussex, please report any DED sightings to your local council or local tree wardens where available.

This Wych Elm is over 100 years old and can be found in Queen’s Park, Brighton.

It is protected from Dutch elm disease thanks to active management by Brighton and Hove City Council and sits within a DED control zone.

Butterfly Conservation is the UK charity dedicated to saving butterflies, moths and our environment. We provide advice on conserving and restoring habitats. We run programmes for more than 100 threatened species and are involved in conserving thousands of sites and reserves. www.butterfly-conservation.org

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The Sussex Branch is run entirely by volunteers. To find out more about our conservation work, please visit: www.sussex-butterflies.org.uk/about

Elm Bark Beetle images © Alister Peters. All other photos and text © Jamie Burston, White-letter Hairstreak Species Champion,

Butterfly Conservation – Sussex Branch. Text assisted by Alister Peters, Peter Bourne, Rob Greenland and Neil Brothers.