Higher dutch elm disease pressure due to a longer beetle season: is there still a future for elms in resistance class 3?

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MARKET & CURRENT AFFAIRS

Ronnie Nijboer, Thursday 18 September 2025

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No need to panic, but the issue deserves attention

After more than a century of breeding elms against dutch elm disease, we have the disease fairly under control. Climate change may upset this balance. Especially when it comes to elms in resistance class 3. There is no need to panic, but the issue does deserve attention and possibly additional policy

Dutch elm disease

Advantage

The great disadvantage of the elm has always been dutch elm disease. I dare say this is actually an advantage. My explanation: dutch elm disease is the first serious tree disease in Europe. The first wave in the Netherlands occurred around 1920. We have therefore had more than a century to work towards resistance. With more recent tree diseases we have not yet succeeded. With chestnut bleeding canker and ash dieback we are only at the beginning of the search. With the elm we are many steps further, with the resistance level of the introduced elm cultivars steadily increasing. Around 1970 the much more aggressive dutch elm disease fungus Ophiostoma novo-ulmi came from the US to England and quickly to the continent. Fortunately, our elm researchers led by Hans Heybroek already had promising varieties in the pipeline. Research showed that they were already quite resistant to this new strain. As a result, 'Dodoens', 'Lobel' and 'Plantijn' were released in 1973 by the forestry research station De Dorschkamp. In the years that followed, a steady stream of new introductions appeared with good and even high resistance. With 'Columella' as the undisputed champion. The various clones are often divided into classes, with class 1 not resistant and class 5 highly resistant to Ophiostoma novo-ulmi.

Ronnie Nijboer

Until recently, my experience was that the minimum acceptable resistance level was that of Ulmus 'Lobel'. This clone has been widely used as a reference clone in inoculation trials and almost always proved reasonably resistant. In practice, a 'Lobel' is occasionally affected by dutch elm disease, but never in large numbers. The mortality of grafted 'Lobel' trees due to delayed incompatibility is much greater, but that is another of my hobby horses. There is no time for that topic today.

Breda, good use of Ulmus 'New Horizon' (resistance class 5) as a shade tree

Until recently, my experience was that the minimum acceptable resistance level was that of Ulmus 'Lobel'

Semi-resistant

Elms such as Ulmus 'Lobel' are nowadays called semi-resistant. They fall into resistance class 3. 'Lobel' can safely be planted without a major risk of loss due to dutch elm disease, even in areas with high disease pressure. Read: areas with no or only moderate control. This applies particularly to areas with a cooler climate. Comparable to that of the Netherlands, Belgium, northern and central France and further Germany and Austria. In southern Europe the De Dorschkamp clones performed much less well. Not for nothing did the Italians start their own resistance breeding project in the 1980s. In the hotter and drier Mediterranean climate the growth of the Dutch clones is significantly poorer than in the cooler homeland.

Due to climate change, the beetle season in some years has already been extended by a month.

Climate change

Whether we like it or not, climate change is a fact. In recent years we could sometimes already sit on a terrace in March or April, and it is not unusual to still do so regularly in October. The growing season is therefore considerably extended, on average by two to three weeks in spring and two to three weeks in autumn. And the beetle season also easily extends by a month or more in some years. After pupating under the bark of the elms (breeding trees) in winter, the beetles wait until the temperature is about 20? is. Once this temperature is reached, they fly en masse, with about 20% early flight, 60% main flight and 20% late flight.

Ulmus 'Fiorente' in Loosdrecht (resistance class 4)

Extra beetle flights

The greatest risk lies in the higher autumn temperatures. Then there may be additional beetle flights, which are not yet included in our current dutch elm disease protocols. This may mean that after the last inspection round in September there are still beetle flights that can infect a large number of breeding trees unnoticed. Moreover, the main symptom of infection, leaf wilt, is much less prominent at the end of the growing season and more difficult to detect because this discoloration is hidden in the onset of autumn color. It therefore seems worthwhile in the coming years to experiment with an extra inspection round in early October. This could possibly eliminate unexpected breeding trees.

The main symptom of dutch elm disease infection is less prominent in autumn and harder to detect because of autumn coloration

What do we do with class 3?

Elm cultivars from resistance class 1 and 2 are in principle only suitable for green heritage management, meaning as replacements in existing situations and on a small scale for cultural-historical reasons. But what shall we do with elms from class 3? These are well-known and trusted names, with 'Lobel', 'Dodoens' and 'Plantijn' at the forefront. Given the increasing disease pressure from climate change, there is a considerable chance that class 3 clones will fail from dutch elm disease more often than before. I think we should already take this into account in our planting plans by not planting them at all or in much smaller numbers. This in favor of the good and highly resistant elms from class 4 and 5, because there are now plenty of those (see box).

Conclusions

Climate change in Europe is accompanied by higher temperatures. Due to warmer autumn weeks, elm bark beetles fly longer. This can lead to many extra breeding trees unnoticed. Because days with a temperature of circa 20? in spring occur considerably earlier, we may be surprised by early beetle flights. The extension of the beetle season will contribute to higher disease pressure, causing semi-resistant clones such as 'Lobel' and 'Dodoens' (resistance class 3) to be affected more often.

Dry and hot periods stimulate the imbalance between scion and rootstock in grafted trees. As a result, we are more often confronted with delayed incompatibility. It seems wise to say goodbye to elm cultivars from class 3 in favor of the highly resistant assortment from class 4 and 5. Of course, from now on we will only grow, buy and plant elms on their own roots!

Examples of cultivars per resistance class

The resistance level of the various elm cultivars is usually indicated with a number from 1 to 5. Class 1 is very susceptible to dutch elm disease and class 5 highly resistant.
Examples:


Class 1	Class 2	Class 3	Class 4	Class 5

'Belgica'	'Bea Schwarz'	'C. Buisman'	'Europa'	'Columella'

'Exoniensis'	'Commelin'	'Clusius'	'Fagel'	'New Horizon'

glabra	'Groeneveld'	'Homestead'	'Fiorente'	'Rebella'

minor	'Urban'	'Lobel'	'M. Glossy' TRIUMPH	'Rebona'

'Sarniensis'	'Vegeta'	'Pioneer'	'Nanguen' LUTECE	'Sapporo Autumn Gold'

.	.	'Plantijn'	'Plinio'	'Wanoux' VADA

.	.	.	'San Zanobi'	.

Note: The difference between class 4 and 5 is sometimes small. Newly introduced cultivars are first placed in class 4. If no spontaneous cases of dutch elm disease occur, they eventually deserve the designation highly resistant, meaning class 5.

European white elm

After manual inoculation, Ulmus laevis always becomes severely diseased and eventually dies, which would place it in class 1. But the European white elm has good field resistance (beetles do not like it), which usually allows it to escape dutch elm disease. Given recent reports of infection in Ulmus laevis, cautious planting is advisable. Mix it in rather than planting in large numbers.
This also applies to Ulmus 'Frontier'. After inoculation this cultivar shows very severe wilting reactions, even to the death of the crown. It then regenerates from the root with healthy shoots. How its field resistance will prove remains to be seen.

This article was previously published on 18 September 2025 on the website of Boomzorg.

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