Dutch elm disease was first recorded in 1919 in Wageningen in the Netherlands: elms suddenly dropped their leaves and died after a few weeks. It is caused by the sac fungus ophiostama ulmi or the even more aggressive fungus ophiostoma novo-ulmi. The fungal spores are distributed by the elm bark beetle. They penetrate the trees’ water‑carrying systems and block their water supply. As a result, their crowns dry out.
Dutch elm disease had catastrophic consequences for the European cultural landscape: up to one million trees were destroyed. The elm is a unique tree with a cultural history stretching back to antiquity. It was prized for its valuable wood and was a popular street and park tree for many centuries. The elm grows up to 35 meters tall and can live for several hundred years. The ‘Hahnheim elm’ (Rheinhessen, Germany) was treasured for 700 to 900 years – until it fell victim to Dutch elm disease in 1978.
The battle against Dutch elm disease lasted many years until Prof. E. B. Smalley from Madison University in Wisconsin, USA cultivated resistant varieties in 1958 from Asian, European and American elm varieties. At the time of the strongest sap stream, he artificially infected the trees with the fungus and tested their resistance. This enabled him to filter out highly-resistant varieties that are suitable for growing in Europe. These are sold by a few specialist European companies under the trademark RESISTA®.
“RESISTA® elms are not immune to the sac fungus, but they survive Dutch elm disease as though it were a cold,” explains Mark Schneekloth, Managing Director of Clasen & Co. “In the year after the infection, there are no further traces of the problem. Our RESISTA® elms are extremely resistant and adaptable and therefore make ideal trees for streets and parks.”