An epidemic of Dutch elm disease in central and southern Britain

Dutch elm disease is one of the most serious tree diseases in the world. Bark beetles spread the fungus that causes the disease, which triggers foliage and tip dieback in all of Britain’s major native elms: English elm (Ulmus procera), smooth-leaved elm (U. carpinifolia or U. minor) and wych elm (U. glabra).

The disease first spread to Britain in the 1920s, when it killed 10-40% of elm trees. Although the initial epidemic died down, a more aggressive species of Dutch elm disease fungus was accidentally introduced into Britain in the 1960s.

An epidemic rapidly took hold of lowland central and southern Britain – where there were predominantly English elms – in the early to mid-1970s. By 1980, most mature English elm had died. Scattered escapes and pockets of mature elm occasionally survived, for example in Brighton and Hove, where the geographic situation has facilitated an effective and continuing sanitation control programme.

However, by the late 1980s the bark beetles used up most of the mature elms that they relied on for breeding material, so beetle populations declined and the disease virtually disappeared from many southern and south-western areas.

During this period enormous numbers of suckers grew from surviving roots of English elm and some smooth-leaved elm types, together with occasional young seedlings of wych elm. Also many small hedgerow elms that escaped the second epidemic matured, in some cases through careful husbandry, but often through an absence of hedgerow maintenance. Consequently a numerically massive and increasing elm resource developed, mainly of small to semi-mature U. procera, across much of southern Britain. From Essex to the Welsh borders the trees probably numbered many tens of millions.

In 1982, Forestry Commission research on the biology of O. novo-ulmi and disease transmission and its recent spread across Eastern Europe (Romania to Poland) was combined to produce a prognosis for the future of the disease and elm. This suggested that the disease would not decline in intensity in contrast to the first epidemic, caused by O. ulmi. Instead the new pathogen, O. novo-ulmi, would return in a continuing cycle to attack the following generation of small elms once they were large enough to support beetle breeding. Today we can see this happening in much of southern Britain.

The Forestry Commission established 20 elm plots across the south of England. Between 1980 and 1990 only about 1% of regenerating elms were killed annually, but disease reappeared on a significant scale after 1991. Around the Research Station in the Farnham-Guildford area, no trace of the disease was found during 1981 to 1987, then two separate infections were seen near Godalming in 1988. By 1990 new infections were scattered across the whole area. By 1994-5, substantial tracts of hedgerow elms 3-12 m high were dead or dying.

Pictured left are young hedgerow elms dying from Dutch elm disease at Jacobs Well, Guildford, Surrey, England

This pattern of renewed disease has now occurred across most of the old 1970s U. procera disease-outbreak areas. Indeed, the current disease situation is often remarkably reminiscent of the mid-1970s, except that the affected elms are much smaller. About 20 years separate each wave of disease from the initial outbreak.

Key points

• The regenerating sucker elms are just as susceptible to O. novo-ulmi as the parent trees from which they developed.
• The sudden resurgence of disease in the 1990s probably coincides with the return of the larger elm bark beetle, Scolytus scolytus, to the affected areas following its disappearance in the intervening period when little suitable breeding material was available. S. scolytus probably migrated back from neighbouring parts of Britain where it has survived. The smaller beetle, S. multistriatus, might actually be the first to return to an area, since it can use smaller-diameter branches as its breeding material. However, studies by Forest Research scientists show that S. multistriatus is a less effective vector of the disease, compared to S. scolytus.
  
• The best way to conserve hedgerow elms at present might be to keep them trimmed, because prominent elms are more likely to attract the bark beetles for feeding.

Dutch elm disease remains one of Britain’s most destructive tree disease. However, although further cycles of disease can be expected, elm will survive to provide a potential contribution to future landscapes.