The disease is known as ‘Dutch’ because important early research on it was carried out in the Netherlands.
It is caused by the fungus Ophiostoma novo-ulmi which was accidentally introduced to the UK from the USA in the late 1960s on imported elm logs. Prior to this, northern Europe already had a milder form of DED caused by another related fungus, Ophiostoma ulmi, and for some time it was not realised that the fungus in the UK was different. O. novo-ulmi is not native to the USA and its true origin is unknown.
The fungus is spread by elm bark beetles, particularly Scolytus scolytus. Beetles breed in dead and dying elms, including those killed by the disease, where the larvae tunnel in the bark and outermost wood, forming galleries. The fungus produces sticky spores in these galleries, which contaminate the newly hatched adult beetles as they emerge. They then fly to healthy elms, where they feed on young bark and introduce the pathogen into the water-conducting tissue (xylem) of the tree. The fungus grows in the xylem, blocking water flow and causing rapid wilting and death. It can spread rapidly down rows of hedgerow elms through root grafts formed between adjacent trees.
The beetles tend to attack mature trees over 20-years-old, and therefore the first wave of the disease in the early 1970s was followed by a lull while the trees regenerated from suckers. But these regenerated trees have in turn succumbed. The disease has not quite reached the northern limits of elms and some remain in Scotland.