An English elm

One of millions dead. Around 60 million in the UK, in fact.

Dutch elm disease is caused by a fungus which elm bark beetles spread from tree to tree. The fungus blocks the tree’s water-transport system, so the branches wilt and die.

Dutch elm disease first appeared in north-west Europe in the early twentieth century. Despite its name, the disease did not originate from the Netherlands, but from Asia, and does not only affect Dutch elms, either. It’s so called because Dutch scientist Bea Schwarz first isolated the fungus that was the cause of the disease, thus forever associating it with her country.

The first European epidemic, in the 1920s, was caused by the fungus Ophiostoma ulmi, and many countries lost many of their elms. Things calmed down but then, in the late 1960s, a new highly aggressive form of the fungus — Ophiostoma novo-ulmi — caused a second outbreak of the disease. This fungus had been imported into Britain on elm logs from Canada. By 1980, most mature English elms had died.

There are a few beacons of hope. The first is that the disease is self-limiting. When the bark beetles have killed off most of the mature elms that they rely on, so the beetle population declines and the disease virtually disappears. (Though when young elms grow, back come the beetles.)

The second is that there are pockets of survivors. In East Sussex, local authorities got together and established an extremely effective control zone. East Sussex now contains the only population of mature English elms in the world.

But as you can see from these recent pictures, which are of an East Sussex elm, the disease has lost nothing of its destructive power. No chemical control has worked in the long term, either with fungicides or pesticides.

So, if you see a healthy and happy elm, it might just be one of the trees that you could allow yourself to hug.