Dutch elm disease

Dutch elm disease (DED), one of the more serious tree diseases in the world, acquired its name because so much early research on it was done in the Netherlands (Gibbs et al, 1994), rather than this being its source. Dutch elm disease, which may have originated in the Himalayas, is widespread throughout the natural distribution of the elm apart from China and Japan; it has even attacked exotic elms in New Zealand. Discovered in a number of European countries shortly after World War I, the first English record was made in Hertfordshire in 1927. This outbreak, which caused widespread deaths of elms in southern England, was caused by the insect-borne fungus Ophiostoma ulmi, probably brought into the country on infected logs from North America. In the 1940s the numbers of trees affected and the severity of the damage declined, though there were local 'flare-ups' from time to time.

In the late 1960s a new and far more severe epidemic developed. By 1980 it was estimated that it had killed over 25 million of the UK's estimated 30 million elms, causing losses throughout England and Wales and extending northwards into the central Scottish lowlands. The 1970s epidemic of DED in Britain resulted from the introduction of a very pathogenic form of the fungus (O. novo-ulmi) on diseased elm logs imported from Canada. This fungus exists as two subspecies, each of which has a different history of spread within the northern hemisphere. The outbreak also caused enormous damage in other European countries, south-west and central Asia and North America where more than half of the native Ulmus americana were killed. The 'Princeton' and 'Valley Forge' variants of this species are so resistant to DED that they are being planted again.

Much is now known about the onset of the disease. After infection, both species of fungus exist in the living tree as a yeast-like stage. This is spread through the tree in the sap of the vessels of the outermost xylem ring (the vascular tissue) and the fungus quickly blocks the water-conducting tissue of the tree. Soon after this elm twigs commence to wilt (hence why the disease is classified as a 'vascular wilt'). This directly relates to the activities of the fungus: fungal toxins are produced, some vessels fill with air as they are punctured by the mycelium, and tyloses (gummy extensions of the xylem walls) bulge into the cavities of the vessels giving rise to the dark streaks which can be seen in transverse sections of infected twigs. This is particularly damaging to the elm because it is a ring-porous tree, conducting all its water up the tree through just the outermost ring directly under the bark (in contrast diffuse-porous trees such as beech use the xylem of several years for water transport; see Thomas, 2000 for more detail). The virulent O. novo-ulmi can spread through a mature tree in

2 weeks. The fungus also produces mycelia in the bark of dead and dying elms and these give rise to two types of asexual fruiting structure. The sexual flasklike perithecia are formed in similar places but are fertile only if both the A and the B mating types are present. It is a combination of these spores that are carried between trees by the main disease vector - a beetle.

Where trees are connected by grafts, and in suckering elms, the fungus is transmitted directly from tree to tree. The main agents that transmit DED over any distance in Britain are, however, the large European elm bark beetle Scolytus scolytus, which reaches a length of 5-6 mm, and the small European elm bark beetle S. multistriatus that is about half as long and normally has only one generation per year. The larger of these two scolytids is the more important vector of the disease; the adult carries more spores than S. multistriatus, it can fly up to 5 km and is the only species that breeds successfully in Scotland and northern England. It also makes wounds that are more conducive to infection of the xylem. In the north of the UK it has a single generation each year, but in the south there are two and sometimes even a partial third generation. A very large number of mature beetle larvae can often be seen when the pupal chambers are revealed by paring away the outer bark of a recently dead tree. Figure 5.9 shows the progressive decline of a tree from initial attack by the vector beetle to severe infection by the fungus.

Wych elm Ulmus glabra (notably the variant 'Clusius') although not immune to DED, does show considerable resistance for a number of reasons, including the fact that it does not sucker (produce new shoots from the roots) as do most British elms and so is probably less liable to infection via root grafts, and it reproduces from seed and so each individual is genetically different. Its twigs are not as palatable to the beetles which spread the disease as those of the very vulnerable English elm U.procera (see Section 10.5.2), and the fungus Phomopsis rapidly invades the bark of newly dying wych elms providing a natural biological control of elm-bark beetles. The smooth-leaved elm U. minor ssp. carpinifolia of East Anglia is also resistant, as are some of the many regional forms of elm found in England.

Individual elms are usually capable of regrowth from root suckers or from epicormic/adventitious buds (pre-existing/brand new buds) at the base of the trunk (see Fig. 5.10). However, once these shoots are large enough to attract the attention of the beetles, they are inevitably reinfected. Native elms in the UK are probably doomed for the foreseeable future to go through repeated cycles of growth and above-ground death, remaining as understorey shrubs. However, bear in mind that the widespread elm decline of Neolithic times may well have been due to Dutch elm disease (Girling and Greig, 1985; Perry and Moore, 1987) and the elms did eventually recover!

Life Cycle Dutch Elm Disease
Figure 5.9 Stages in the infection of an elm tree and the life cycles of Dutch elm disease (DED) fungus Ophiostoma ulmi or O. novo-ulmi and its scolytid beetle vector.

(a) Adult beetles emerging from the bark of dead and dying elms in spring or summer carry spores of the causal fungus.

(b) Beetles feed in the twig crotches of healthy elms and introduce fungal spores into the wood.

(c) The infected regions wilt and diseased twigs show characteristic streaks or spots.

(d) Trees weakened by DED become breeding sites for beetles.

(e) The larvae form galleries by burrowing beneath the bark.

(f) The fungus fruits in the pupal chambers, discharging spores.

(From Gibbs, Brasier and Webber, 1994. Research Note 252. Forestry Commission. © Crown copyright material is reproduced with the permission of the Controller of HMSO and Queen's Printer for Scotland.)

Several different approaches, involving both the fungi and the beetle vectors, have been employed in attempts to counter DED. Though advances have been made and much learnt, none have been very successful. The use of insecticides to protect healthy trees from beetle attack was ineffective. Use of attractants in beetle-trapping operations which could have led to biocontrol

Figure 5.10 Part of the complex mosaic of a climax woodland. Black 'bootlaces' (rhizomorphs) of honey fungus Armillaria mellea run over the trunk of an elm killed by Dutch elm disease Ophiostoma novo-ulmi, which lies surrounded by dog's mercury Mercurialis perennis, common nettle Urtica dioica, lesser celandine Ranunculus ficaria and occasional cleavers Galium aparine. Suckers derived from the dead tree grew nearby. Buff Wood, Cambridgeshire, UK 1982. (Photograph by John R. Packham. From Packham et al., 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall. With kind permission of Springer Science and Business Media.)

Figure 5.10 Part of the complex mosaic of a climax woodland. Black 'bootlaces' (rhizomorphs) of honey fungus Armillaria mellea run over the trunk of an elm killed by Dutch elm disease Ophiostoma novo-ulmi, which lies surrounded by dog's mercury Mercurialis perennis, common nettle Urtica dioica, lesser celandine Ranunculus ficaria and occasional cleavers Galium aparine. Suckers derived from the dead tree grew nearby. Buff Wood, Cambridgeshire, UK 1982. (Photograph by John R. Packham. From Packham et al., 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall. With kind permission of Springer Science and Business Media.)

using fungal or bacterial preparations that have worked with other insects was abandoned as the beetles are not susceptible to such agents, although phero-mone ('external hormone') traps proved to be useful in monitoring beetle populations in sanitation control areas. Injection with a soluble formulation of the fungicide thiabendazole that proved to be successful in combating DED in valuable trees in the early stages of infection is too costly for commercial application. Investigations continue into the virus-like 'd-factors' (mycoviruses) that have a deleterious effect upon both Ophiostoma ulmi and O. novo-ulmi. It now seems that hybridization between O. ulmi and O. novo-ulmi, and the consequent rapid genetic diversification, may have helped the disease at its peak by impeding the spread of these d-factors through the fungus population. Moreover, O. novo-ulmi is made up of two subspecies, ssp. novo-ulmi and ssp. americana, with the latter being the more virulent, but hybrids may be more virulent still due to enhanced resistance to d-factors (see Konrad et al., 2002).

(Indeed, the newly discovered 'alder' Phytophthora may be a product of this hybrid vigour in pathogenicity.)

Some Asian elms, such as Ulmus pumila and U. parvifolia, although quite different in appearance to British elms, have high resistance to the fungus and may eventually be saved by the use of genetic engineering to introduce disease resistance. Complete immunity to DED is, however, unlikely ever to be achieved as the disease keeps changing; this is a battle where constant watchfulness and ingenuity will be required in the centuries to come.

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  • maunu polvi
    What is ecology of dutch elm disease?
    3 years ago

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