Known in the UK since 1927, this disease was considered relatively unimportant until an outbreak of a more aggressive strain (Ophiostoma novo-ulmi) occurred in the late 1960s, linked to the import of logs.
By 1976, nine-million elms had been killed in southern England out of an estimated population of 23 million. Infection rates were higher in English elm (Ulmus procera) than in wych elm (Ulmus glabra) or smooth-leaved elm.
Despite the high mortality of English elm in southern and central England, the root systems of hedgerow and coppice elms survived. Millions of suckers sprang up in the countryside in the 1980s following the deaths of mature trees in the first wave of the epidemic. These suckers then matured and a second wave of destruction from feeding by Scolytus scolytus was seen during the 1990s. A roughly 20-year cycle of the epidemic is likely as each new clonal generation of elms matures.
Elsewhere, the disease has spread more slowly through a combination of factors including climate, the mix of elm species present (wych elm predominates in the north and west), natural biological controls and geographic isolation of populations. Propagation of replanting stock from surviving trees in isolated populations has occurred but plants are not necessarily resistant clones.
Beetle colonies are a source of re-infestation and re-infection to maturing suckers in south and central England.
Crosses between various North American and Asian elm species have yielded named clones with varying degrees of claimed resistance to the disease. However, none exhibit the same growth forms as UK native species.
How to recognise it
Confirm that the yellowing foliage or wilting symptoms are caused by Dutch elm disease by scraping the bark from affected twigs or small branches to look for characteristic brown staining in the wood. The staining can also be seen by cutting cross-sections of twigs or branches.
Verticillium wilt rarely affects elms in the UK but can cause similar symptoms and will produce brown marks in the wood. In street trees, gas leaks and other forms of soil pollution may cause yellowing and wilting. Phytophthora infection of roots may also cause similar symptoms.
The first signs are wilting followed by a yellowing of foliage on part of the tree, often a single branch, and may appear as early as June, but more often from July onwards. This is followed by withering of foliage and affected branches die back from the tips, with young shoots wilting to form distinctive "shepherd's crooks", which can aid detection of the disease after leaf fall. In a severe attack the entire tree can die in a single season.
Sometimes, even with extensive foliage yellowing, only a few twigs die and the tree may recover if the infection is light. Twigs from infected parts of the tree show dark spots or streaks in the outer wood if they are cut across or if the bark is stripped.
In summer, adult female beetles lay eggs beneath the decaying bark of dying elms. After pupation, adults emerge, carrying spores of the pathogen, which grows and sporulates in the bark. The beetles fly off to feed on twigs of healthy elms, usually at least 4m tall. The feeding wounds become infected with the pathogen. The key period for infection is May and June, when the fungus spreads rapidly through the large springwood xylem vessels. Summerwood vessels are smaller and restrict the spread of the fungus.
The fungus can also spread through the connected root systems of hedgerow suckers and coppices and through root grafts formed between adjoining amenity trees.
The fungus grows in the tree in a yeast-like form and is spread in the sap in the vessels of the outermost annual ring. Infection causes the vessels to stop carrying water, partly as a result of the gums produced by the tree as a defence and that form the characteristic dark patches in the wood. The fungus also produces a wilt toxin (ceratoulmin).
If air temperatures are maintained at 17 degsC with five-to-seven hours of sunshine per day in the spring and summer months, more beetle generations can be produced that subsequently create a greater incidence of disease on trees.
Treatment: biological control
Wych elm bark harbours the Phomopsis fungus, a natural biological control of the beetle because it restricts feeding and breeding in the bark.
A product containing a suspension of spores of a Verticillium spp. (Dutch Trig(R)) has been available in Europe for a number of years that, when injected into the xylem of healthy trees, can help to induce disease resistance. This resistance can go a long way in preventing Dutch elm disease from successfully colonising the vascular system.
Other natural biological controls of the beetle include other species of fungi and predators such as woodpeckers. Researchers have investigated a naturally occurring virus-like organism that infects Ophiostoma novo-ulmi.
They are also interested in the possibility of locating potential bio-control agents in Himalayan forests, from where the disease is believed to have originated. There are currently no biological controls approved for use in the UK.
Treatment: cultural control
A number of Dutch elm-resistant cultivars of elm have been bred, most of them based on Asian elm species, such as Ulmus pumila. Asia is believed to be the centre of origin of the most aggressive form of the disease. However, these are very different in appearance from the English elm.
Trees bred in the USA and in Europe with good resistance include Ulmus 'Sapporo Autumn Gold', Ulmus 'New Horizon', Ulmus americana 'Princeton', Ulmus glabra 'Clusius' and Ulmus 'Lobel'. These all have different growth habits from native species.
Monitoring of trees during the beetle breeding season and sanitation felling has been successful, notably in East Sussex, where resort towns have been relatively isolated from infection sources.
Occasionally, diseased limbs can be removed from large trees to save the whole tree, but only if it can be done early in the infection and if the cut can be made far enough below the visible symptoms to escape the disease infection front.
Neighbouring street trees may be linked by root grafts through which the disease can spread. These may be separated by careful trenching.
Regenerating sucker elms are just as susceptible as the parent trees. They can be conserved by keeping them trimmed - prominent elms in the landscape are most likely to attract bark beetles.
Treatment: chemical control
Applications of insecticides containing chlorpyrifos (Dursban WG, Equity); dimethoate (Danadim Progress or lambda-cyalothrin (Hallmark WZT, EAMU 2008-2944)) to bark prior to beetle activity may help to deter them from feeding.
Fully updated by Dove Associates.
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