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Ulmus comprises 20–45 species, distributed across the temperate northern hemisphere and into subtropical Asia and Central America. Elms are deciduous or evergreen trees, or rarely shrubs. The branches may be more or less smooth, or have two flat opposite corky wings or longitudinal corky layers. The leaves are in two distinct rows; they have pinnate venation and each vein extends to the leaf margin, where it terminates in a tooth; the margins are serrate or double-serrate. There are two stipules at each node, membranous and caducous, leaving a pair of scars at the leaf base. Inflorescences are fascicled, racemose or cymose. The flowers are hermaphrodite and/or unisexual, pedicellate, appearing in spring before the leaves, or rarely in autumn or winter; the perianth is campanulate, four- to nine-lobed, the tepals membranous, the number of stamens equal to the number of tepals. Floral characters are generally not significant in elm taxonomy, and are poorly reported in the literature. The fruit is a flat, usually more or less circular samara, with the seed in the centre or towards the apex; samaras are usually notched at the apex (Sherman-Broyles et al. 1997, Fu et al. 2003).
Most interest in elms, dendrological or otherwise, focuses on their effect in the landscape as magnificent umbrageous trees – Tennyson’s ‘immemorial elms’, or the iconic English Elm, as described by H.J. Elwes (in Elwes & Henry 1913): ‘Its true value as a landscape tree may be best estimated by looking down from an eminence in almost any part of the valley of the Thames, or of the Severn below Worcester, during the latter half of November, when the bright golden colour of the lines of elms in the hedgerows is one of the most striking scenes that England can produce.’
There is much beauty in their detail too:
And whoever wakes in England
Sees, some morning, unaware,
That the lowest boughs and the brushwood sheath
Round the elm-tree bole are in tiny leaf
(R. Browning, Home-thoughts, from Abroad)
All the mystery and magic of spring is captured in that word ‘unaware’. Even before the leaves appear, the flush of flowers and the bright green of the samaras are welcome sights as the days begin to lengthen.
In their eclectic way, Elwes & Henry (1913) left the elms to the end of their magisterial if randomly arranged work, allowing them longest to study them, but even in books produced in a different order, Ulmus remains the last great dragon in the way of a dendrological survey. The modern view of the genus revolves around Dutch elm disease (see below), which colours all perspectives on it in the pallid yellowish green and brown of a stricken tree. So many trees have been lost – and with them not only their own aesthetic but the important wildlife habitats they represented. This is not the place to mourn their loss, however, but to look forward to options for their replacement.
Many of the complications associated with innumerable European taxa have effectively been swept away with the trees, but the taxonomy of the genus remains controversial and infrageneric affinities are unclear for many species. The highly reduced flowers and fruits yield little taxonomic information, and interspecific hybridisation is common (Wiegrefe et al. 1994). In addition, many elms reproduce asexually, allowing clones to survive indefinitely. The problem is exemplified by the case of the British elms, which comprise anywhere between one and six species and additional hybrids, depending on taxonomic viewpoint (Armstrong & Sell 1996). A sensible current view is that there are a limited number of species (U. glabra, U. minor, U. procera) with one frequent hybrid (U. ×hollandica) and several to many distinct clones of uncertain origin that are probably best treated as cultivars (Coleman 2002). A basic guide to the currently most widely accepted names of recognised Ulmus species and the synonymy associated with them is provided by Buchel (2000), but this does not include all names used by Bean (1981b) and other authors. Some synonyms – for example, U. carpinifolia (now U. minor) and U. japonica (now U. davidiana var. japonica) – are frequently used in the literature by breeders referring to certain genetic entities within the specific pool, and in such cases the name used by the breeder is also given in the accounts below.
As is well known, Dutch elm disease (DED), caused by fungi of the genus Ophiostoma, O. novo-ulmi being the most virulent, is carried from tree to tree by bark beetles of the genera Scolytus (in Europe, and as an alien in North America and Australasia) and Hylurgopinus (in North America). The attractiveness of particular species to the beetle is an important aspect in transmission of the disease. For example, the European White Elm (U. laevis) is not favoured by the beetle, probably because of the presence of a triterpene (alnulin) in the bark (Martín-Benito et al. 2005), and therefore often survives unscathed in areas ravaged by DED, although it is extremely sensitive to the disease if inoculated (Heybroek 2000). Infection can also enter trees through interconnected roots – a pathway that may have been responsible for the rapid demise of many American street elms – and via contaminated tools and machinery. Death is caused by the tree’s vascular system becoming blocked, by the fungus and by gels and tyloses produced by the tree itself in reaction to the infection. Originating in Asia, where Ulmus species are largely resistant, first Ophiostoma ulmi and then O. novo-ulmi spread through the elm populations of Europe and North America in the twentieth century, resulting in the death of millions of trees. In Britain and continental Europe the ease with which the disease spread, and the almost total annihilation of mature elms from the landscape, is often ascribed to the fact that many of the trees were of clonal origin (or at least of limited genetic diversity); in North America, however, where most elms were genetic individuals grown from seed, it spread just as rapidly and was equally devastating in its effects on the landscape. A third species of fungus, O. himal-ulmi, was discovered in the Himalaya in 1993; there it is in balance with native elms, but it is potentially virulent as a source of disease in European and American species (Brasier 2000). Innumerable reviews have been published on the subject of DED, as an internet search will reveal, but a particularly authoritative and well-illustrated account is provided by an excellent website (www.dutchelmdisease.org) hosted by the University of Amsterdam (Universiteit van Amsterdam 2008). Another good source of information on DED and many other aspects of the biology of Ulmus is The Elms (Dunn 2000), the proceedings of an International Elm Conference held at the Morton Arboretum in Illinois. For those who read Dutch, a further resource is De iep of olm, karakterboom van de Lage Landen (The Elm, Characteristic Tree of the Low Countries) (Goudzwaard et al. 2008).
Dutch elm disease is a complex problem, and challenges anyone working with elms in any way. The response to the epidemic of O. novo-ulmi, in its European and North American races, has been intense and sustained, from research into its pathology to attempts to breed and select resistant cultivars. The devastation to countryside and cityscapes caused by the loss of the elms has provoked a huge emotional response which continues to spur efforts to find replacements, especially in North America – even to the extent that clones with the same growth habit as U. americana are the stated aim of some breeding programmes (Ware 2000). The history of efforts to breed DED-resistant elms has recently been comprehensively reviewed by Mittempergher & Santini (2004), and the following account is largely based on this work.
Mittempergher & Santini identify six major centres of activity, three on each side of the Atlantic. In Europe, work started in the Netherlands in 1928 after the first outbreak of DED caused by O. ulmi, and resulted in the release of some cultivars that were resistant to that species, but not to the later-arriving O. novo-ulmi. This programme, transferred to the Dorschkamp Research Institute, Wageningen, continued to select and release elms until 1992 when it was terminated on the retirement of its leader, Hans Heybroek, and from it came the cultivar ‘Nanguen’ (Lutèce), which promises to be the best replacement (to date) for Europe’s native elms. In Italy Lorenzo Mittempergher has directed the elm-breeding work of the Istituto per la Protezione delle Piante (Institute of Plant Protection), based in Florence, focusing on making selections suitable for Mediterranean conditions, while in Spain a team led by Luis Gil at the Universidad Politécnica in Madrid is also seeking suitable cultivars for local use.
In the United States, work by the US Department of Agriculture Agricultural Research Service (USDA/ARS) was begun in 1937, when U. americana started dying. Work has continued to the present day under the leadership of Alden J. Townsend, and by 2004, 11 clones had been released to the market from this programme – an indication of the long-term commitment needed when breeding big forest trees. An important step taken in this project (by the late Frank S. Santamour, Jr.) was to incorporate U. parvifolia into breeding work. At the University of Wisconsin, Madison, elm-breeding started in 1958 and continued until 2002, when it was terminated following the death of the long-term project leader Eugene Smalley. One goal was to seek resistant U. americana, the other to breed new entities from the Eurasian gene pool. The U. americana work resulted in the release of a group of six clones known collectively as American Liberty, but these have not proved to be as reliable as other selections. The Eurasian elm project, which involved acquiring material from Japan, and China shortly after it ‘opened up’ in the 1980s, was much more successful, producing ‘Sapporo Autumn Gold’ as its first release. The starting point for the ongoing programme at the Morton Arboretum, at Lisle near Chicago, was the recognition in 1972 of the resistant qualities of an exceptional tree already growing in the collection, later named ‘Morton’ (Accolade). This became the foundation of the breeding work commenced and continued by George H. Ware and his team, which has resulted in the accumulation at the Arboretum of the finest collection of elms in North America, if not anywhere. They have sought out and incorporated into the breeding programme many formerly (and often still) obscure Ulmus species from China.
On both continents, other smaller and shorter-lived projects have resulted in the release of various cultivars in all categories. Successful releases from the main research institutes have complicated genetics, built up over decades of breeding and testing, and it is important that resources of knowledge and material are not jeopardised by cessation of funding – as, for example, in the cases of the University of Wisconsin and the Dorschkamp Research Institute (Mittempergher & Santini 2004).
For those with an interest in planting elms, the more or less resistant cultivars resulting from these programmes are effectively almost the only possibilities to consider, as DED has wiped out of the picture most native species and most of the old clones (with the notable exception of U. laevis), and the Asian species are very rare in commerce, especially in Europe. In taking account of these resistant cultivars we treat them here in groups according to the principal origin of their genetics (rather than their place of raising): see below, alphabetically within the sequence of accounts, ‘Asian elm hybrids and selections’ and ‘European elm hybrids and selections’. Many selections have two names, the usual cultivar name (for example, ‘Morton’), and a selling name (Accolade) often rendered in the form of a vernacular name (for example, Accolade Elm).
The urge to develop resistant elms led to a surge in interest in the Asian species during the 1980s and 1990s, and a number of species have been collected by both European and American breeders for research into their resistance and for use in breeding work (see, for example, Ware 1995c). These form the core of the accounts that follow, in the usual way, but it is probable that most of them will be seldom available to the ordinary grower. The Morton Arboretum has been especially active in obtaining material from the seed lists of Chinese botanic gardens and the Chinese Academy of Forestry, as well as via specially commissioned collections. In addition to the species described or cross-referenced below, the following taxa are or have been in cultivation there (Ware 2000, Morton Arboretum online catalogue 2008): U. changii W.C. Cheng, U. elongata L.K. Fu & C.S. Ding, U. harbinensis S.Q. Nie & G.Q. Huang, U. lanceifolia Roxb. ex Wall., U. microcarpa L.K. Fu, U. prunifolia W.C. Cheng & L.K. Fu, U. pseudopropinqua Wang & Li, and U. sukaczevii Andronov (probably a form of U. glabra). In many cases the sample of natural variation of a species in cultivation is very limited, small numbers of clones having been grown from the original accession. The Morton Arboretum is the International Cultivar Registration Authority (ICRA) for Ulmus, reflecting the long and sustained programme of research on the genus carried out there.
In Europe collections are maintained by some research institutions, but there is not much interest in planting elms among the wider public, as reflected by the paucity of entries in recent editions of the RHS Plant Finder. In the United Kingdom, one exception to this is in West Sussex, where the Brighton & Hove City Council maintain the National Plant Collection of Ulmus, comprising over 100 taxa distributed throughout their area (see the Council website for updated information on all aspects of this remarkable community conservation effort: Brighton & Hove City Council 2008). Another important project was initiated in 2001 by the Hampshire and Isle of Wight Branch of the charity Butterfly Conservation. The White-letter Hairstreak butterfly Satyrium w-album is dependent on elm as its larval food plant and has become a rare species in consequence of the loss of most of the mature elms in southern England. The Butterfly Conservation project aims to assess the potential of various elms, mostly hybrids, for their suitability for the butterfly and as landscape specimens, and has established a significant trial of new elm selections. A preliminary report is available (Brookes 2006), and assessment continues. In France, extensive trials of elms for disease resistance have been carried out by the Institut National de la Recherche Agronomique (INRA), with trial plantations in the Bois de Vincennes and elsewhere, from which much valuable information has been derived and which have led to the propagation and promotion of some of the most important new ‘European’ cultivars (Pinon & Cadic 2007).
Although mature elms have essentially disappeared from the landscape across most of Europe and North America, they are not entirely gone. There are certain enclaves of varying size where DED has limited impact – for example, the area around Brighton in the United Kingdom, as noted above, where good populations of mature trees are defended by strenuous monitoring and control measures, and similarly protected populations of trees in Minneapolis and other cities in Minnesota (Stennes 2000). DED is absent from the Canadian province of Alberta (Alberta Government 2001–2008), where again there are rigorous controls in place. The elms of the Isle of Wight and the Channel Islands are also not affected (S. Andrews, pers. comm. 2008). In Europe elms remain very common in the form of regenerating suckers from the roots, persisting in woodland and hedgerows and sometimes developing into quite respectable trees – before the beetle eventually, apparently inevitably, finds them. These populations act as a genetic reservoir for the trees, from which they may regenerate should there ever come a time when they are able to escape the disease – but also for the pathogen and the beetles, as dead and dying trees are left standing and unattended to.
Dutch elm disease can be kept at bay on isolated or valued specimens by the use of systemic fungicides, usually by injection or infusion, which is much cheaper than the cost of removing a large mature elm tree and is thus a wise course for the preservation of trees in gardens and arboreta (Stipes 2000, Stennes 2000). Vigorously applied sanitary measures, including prompt removal of any infected material, severing of root unions between infected and healthy trees and the banning of imported elm timber for firewood, offer the only practical hope for control of the disease in larger populations – and authorities must be constantly vigilant. According to press reports, 370 mature elms were killed in Brighton in 2005, for example (see Brookes 2006).
Dutch elm disease is only the most severe of the pests and diseases that afflict Ulmus; Dirr (1998) was heretical enough to ask why elms have been ‘treated like royalty when they are so fallible’, going on to provide an alarming catalogue of other problems that can beset them. Among the worst, though principally affecting American native species, is the disease elm yellows, caused by a phytoplasma vectored by piercing insects, that causes rootlet death and progresses to kill the phloem and cambium. Also sometimes called ‘phloem necrosis’, it has been responsible for huge die-offs of U. americana in the past, and poses a continuing threat to that species even where DED is controlled (Sinclair 2000). In America it is of extremely rare occurrence in cultivated Eurasian taxa, but it is present in Italy, and although it seems to be of limited significance in native taxa it could be problematic in hybrids involving Asian species, which show greater susceptibility (Mittempergher 2000). A suite of insect pests feed on the leaves, including Japanese Beetle Popillia japonica, Elm Leaf Beetle Xanthogaleruca (Pyrrhalta) luteola, Elm Leafminer Kaliofenusa ulmi, cankerworms and weevils, causing unsightly defoliation and consequent debilitation. There are variations in the resistance of different clones to these pests, which must be taken into account when breeding and selecting new cultivars (Miller 2000). Elm Leaf Beetle is a particularly serious problem in North America and has unfortunately been introduced to Europe, although it is at present unknown in Britain (A. Brookes, pers. comm. 2008).
All these factors have implications for the success of newly planted elms and (particularly in North America) it will be advantageous to make local enquiries to find out which cultivars succeed best in a given area. With the unparalleled abundance of arboreta undertaking research and extension work in the United States, this information is much more readily available there than it is in Europe. In general, however, elms are tolerant of many growing conditions, though most require good drainage to be successful. It should also be remembered that ample space is required for these big, densely shady trees; one may imagine cases in which the demise of an elm too close to a house or shading a field could actually have been a very welcome event. Fertile clones can produce masses of seedlings that may be a nuisance, and they hybridise easily, as evidenced by the breeding work. Swarms of hybrid seedlings are potentially able to invade wild habitats, as they have in Spain and Italy, where the native U. minor Mill. has met with U. pumila (Collin et al. 2000).
The Butterfly Conservation trial in Hampshire (Brookes 2006) found that small, young specimens established much better than large specimens (over 4 m) supplied as bare-root standards, which required staking until they were properly established, by which time the smaller trees had overtaken them.
Propagation of selected elms is by cuttings, either of hardwood taken in winter and rooted with basal heat in a cool atmosphere, or as softwood in summer. Hormonal rooting preparations can be advantageous in both cases (Beckett & Beckett 1979, Mittempergher et al. 1991).
We shall probably never see the old, elm-dominated landscapes again, but the new hybrids give glimmers of hope that we will in the future be able to enjoy majestic elms in many places where they have been absent for some decades now. The lessons of the past must, however, be heeded. We must not depend solely on a few clones to supply our landscapes with elms – or any other genus of tree. Diversity must be maintained.
A genus of between twenty and thirty species of deciduous (rarely subevergreen) trees, some of the largest size, natives of the northern hemisphere in both the Old and New Worlds, but not occurring in N. America west of the Rocky Mountains. Leaves alternate, toothed, usually oblique and sometimes very unequal at the base. Flowers produced in clusters or short racemes from axillary buds, either on the naked shoots in early spring, or on the leafy ones in autumn. They are very small, consisting of a green or red-tinged perianth with four to nine (usually about five) lobes. Stamens usually equal in number to the perianth-lobes, their purplish or reddish anthers being usually the most conspicuous feature of the flower. The fruit is most characteristic, being a flat, membranous, semi-transparent disk called a samara, enclosing the single seed in a cavity at the centre or towards the apex, where it is slightly or deeply notched.
On a healthy, unlopped adult tree the greater part of the foliage is borne on short-shoots of determinate growth, and the leaves on these, in particular the upper ones (distal and subdistal leaves) characterise the species, variety or hybrid. Leaves on suckers and water-sprouts (epicormic shoots) are of a juvenile type, and often very different from the adult ones, not only in shape and size but also in indumentum, being often hairy even when the adult leaves are glabrous. Also uncharacteristic are the leaves of lammas-shoots and on 'proliferating' shoots.
The genus has been grouped into sections largely on the basis of flowering-time, and on characters of the perianths, inflorescence and samaras. Foliage does not serve to distinguish the sections reliably.
sect. Ulmus (Madocarpus). – Flowers short-stalked, borne in clusters before the leaves expand. Perianth with four to seven equal lobes. Samaras rarely ciliate. The most important group. Apart from the N. American U. rubra all are natives of the Old World. Those treated here are:
Europe, S.W. and C. Asia: U. carpinifolia (also in N. Africa) and U. angustifolia, U. coritana, U. plotii, included in U. carpinifolia by some authorities; U. elliptica; U. glabra; U. procera; U. pumila (also in N.E. Asia and the inner Himalaya).
Himalaya: U. villosa; U. wallichiana (for other Himalayan elms see Melville and Heybroek, op. cit. in Bibliography).
E. Asia: U. davidiana and the closely allied U. japonica; U. laciniata; U. macrocarpa; U. wilsoniana.
sect. Blepharocalyx. – Flowers on slender stalks of unequal length, opening before the leaves expand. Perianth deeply divided. Samaras ciliate.
Ulmus americana (N. America); U. laevis (W. Eurasia).
sect. Chaetoptelea. – Flowers before the leaves, on slender stalks, arranged in racemes. Samaras downy and ciliate.
Two N. American species: U. alata; U. thomasii.
sect. Microptelea. – Flowers opening in autumn. Perianth deeply divided. Leaves rather leathery, falling late. Samaras glabrous.
Ulmus parvifolia (E. Asia); U. crassifolia (N. America).
sect. Trichoptelea. – Flowers opening in autumn as in the preceding section, but borne in pendulous racemes. Samaras hairy.
Ulmus serotina (N. America).
The present revision has been prepared under the shadow of the new and lethal outbreak of the Dutch elm disease, which has killed many of the elms that ornamented the parks and gardens of this country and, far worse, destroyed or defaced some of our loveliest rural landscapes. The contents of the present treatment are substantially as they were in previous editions, though it must be sadly acknowledged that many of the elms described, including some of the largest and finest, will never be planted again, and it is only to be hoped that resistant substitutes will eventually become available. The taxonomy of the British elms of the U. carpinifolia complex is a controversial matter, but it is one that in the main concerns countryside elms rather than those planted for ornament. The taxonomic position of a cultivar is also a matter of dispute in many cases, but controversy can here usually be avoided by placing the cultivar-name directly under Ulmus (cf. P. S. Green, op. cit., in Bibliography). For example, the name of the Jersey elm – 'Sarniensis' – could be placed under U. minor (if that is to be taken as the earliest name for the field elm), U. carpinifolia, U. angustifolia or U. × sarniensis. This instability of nomenclature is avoided if it is called Ulmus 'Sarniensis'. The same device can be used in presenting the names of the many probably hybrid cultivars that have in the past been placed under U. procera or U. carpinifolia, or confusingly shuttlecocked between them. See also U. × hollandica.
Bancroft, Helen. – 'Notes on the Status and Nomenclature of British Elms', Gard. Chron., Vol. 96 (1934), pp. 122, 139, 208, 244, 298, 334, 372.
Clapham, A. R., Tutin, T. G., and Warburg, E. F. – Flora of the British Isles, ed. 1 (1952), pp. 715-24.
Elwes, H. J., and Henry, A. – Trees of Great Britain and Ireland, Vol. VII (1913), pp. 1847-1929.
Fontaine, F. J. – Dendroflora No. 5 (1968), pp. 37-55.
Green, P. S. – 'Registration of Cultivar Names in Ulmus', Arnoldia, Vol. 24 (1964), pp. 41-80.
Hadfield, Miles. – British Trees (1957), pp. 226-47.
Hillier and Sons. – Hillier's Manual of Trees and Shrubs, Ed. 3 (1973). pp. 398-402.
Jackson, A. B. – 'The British Elms', New Flora and Sylva, Vol. 2 (1930), pp. 219-29.
Jobling, J., and Mitchell, A. F. – Field Recognition of British Elms. Forestry Commission Booklet 42 (1974).
Meikle, R. D. – British Trees and Shrubs (1958), pp. 148-55.
Melville, R. – Ulmus, in Stace, ed., Hybridisation and the Flora of the British Isles (1975), pp. 292-9.
Other studies by Dr Melville are cited in the text.
Melville, R., and Heybroek, H. M. – 'The Elms of the Himalaya', Kew Bulletin, Vol. 26 (1971), pp. 5-28.
Mitchell, A. F. – A Field Guide to the Trees of Great Britain and Northern Europe (1974), pp. 247-54 and pl. 22.
Moss, C. E. – 'British Elms', Gard. Chron., Vol. 51 (1912), pp. 199, 216, 234.
– – The Cambridge British Flora (1914), Vol. II, pp. 88-96.
Richens, R. H. – 'Studies on Ulmus'. A series of regional studies: Part I (E. Anglia), Watsonia, Vol. 3 (1958), pp. 138-53; Part II (S. Cambs), Forestry, Vol. 31 (1959), pp. 132-46; Part III (Herts), ibid., Vol. 32 (1959), pp. 138-54; Part IV (Hunts), ibid., Vol. 34 (1961), pp. 47-64; Part V (Beds), ibid., Vol. 34 (1961), pp. 181-200; Part VI (Fenland), ibid., Vol. 38 (1965), pp. 225-35; Part VII (Essex), ibid., Vol. 40 (1967), pp. 185-206.
– – 'Variation, Cytogenetics and Breeding of the European Field Elm', Annales Forestales (Anali za Sumarstvo), Vol. 7 (1976), pp. 107-41. An invaluable survey, with an extensive bibliography.
Ross-Craig, Stella. – Drawings of British Plants, Part XXVII (1970), Ulmaceae, plates 1-6.
Schneider, C. – Plantae Wilsonianae, Vol. III (1916), pp. 238-65.
Wyman, D. – Trees for American Gardens, Ed. 2 (1965), pp. 461-73.
Wilkinson, G. – Epitaph for the Elm. 1978.
DUTCH ELM DISEASE
One of the most important factors influencing the elms in Britain in the 20th century has been Dutch elm disease. This disease spreads rapidly through elm populations causing widespread wilting and death of trees. It was initially found in Western Europe soon after the first World War and was given its name following the invaluable early research carried out in Holland. It was first positively identified in Britain at Totteridge in Hertfordshire in 1927 but was almost certainly present some years earlier. The disease rapidly reached epidemic proportions in southern England and during the 1930s caused widespread deaths of most species of elm growing in the region. By 1936 the epidemic had reached its peak and thereafter the disease declined both in numbers of trees affected and in the severity of the symptoms on individual trees. During the subsequent two decades local flare-ups occurred but the disease came to be regarded as an endemic problem of no great consequence.
However in the late 1960s fresh serious outbreaks of Dutch elm disease were reported from various localities in southern England. It soon became clear that a second epidemic had started on a greater scale than in the earlier outbreak. Surveys of disease incidence were instituted in 1971 in southern Britain and by 1977 about 50 per cent of the total elm population of 23 million elms present at the beginning of the survey period had been killed.
When the disease was first identified in Europe there was considerable argument as to its cause; however it soon became clear that a fungus, now called Ceratocystis ulmi (Buism.) C. Moreau, was in fact responsible. It took several years before further research demonstrated that the elm bark beetles Scolytus scolytus (Fab.) and Scolytus multistriatus (Marsh.) were the vectors which transmitted the disease from dead or dying elms to healthy trees. These bark beetles emerge from the bark of diseased elms carrying spores of C. ulmi on their bodies and often feed in the crotches of twigs on healthy elms. The feeding wounds can thus become infected with the disease and it subsequently spreads through the vascular system of the tree. The characteristic symptoms of the disease, including wilting, yellowing or browning of leaves and dieback of parts or the whole of the crown then appear. At the same time, dark brown streaks often develop in the outermost annual ring of the stem and these provide a useful diagnostic feature which can be seen when the bark is stripped away or the stem cut across.
The disease cycle is completed when the mature bark beetles invade the bark of diseased elms in order to breed. Egg-laying female beetles, together with the larvae which subsequently develop, tunnel out characteristic insect galleries within the bark. The disease can also be transmitted from one tree to its neighbour through connecting roots and this method of spread is particularly important in hedgerow trees.
Research undertaken during the course of the second epidemic in the 1970s has revealed much information about the origin of this outbreak and of the variability of the causal fungus. It was discovered that a very pathogenic, aggressive strain was responsible for the new outbreaks whereas a different, non-aggressive strain occurred in locations where infection had survived from the earlier epidemic. Detailed studies have demonstrated that the aggressive strain was probably introduced into Britain on elm logs imported from Canada and that this strain was responsible for the second and more devastating epidemic.
Much research has been directed towards finding methods of controlling the disease including trials with insecticides to kill the bark beetles and with fungicides to protect the trees from fungal invasion. None of these materials have so far proved particularly effective and the only proven method of slowing down the spread of the disease has been by sanitation felling. Experience both in the United States and in Britain has shown that an intensive programme of felling diseased trees and subsequent destruction of the bark in which the bark beetles breed can keep the disease at a relatively low level. Such sanitation felling programmes must however be initiated at an early stage in the epidemic for the task can soon become too great for the available resources.
In the longer term it may be possible to select or breed varieties of elm which are resistant to the disease and which are also suitable for planting as landscape or urban features. Resistant elms have been recognized, particularly among the Asiatic species, and breeding programmes are now in progress in Holland and USA. However, it may be some years before elm hybrids or cultivars, resistant to Dutch elm disease in its aggressive form, are available for widescale commercial planting.
Contributed by Dr D. A. Burdekin, Forest Research Station, Forestry Commission, Alice Holt.
A notable addition to the bibliography (page 636) is:
Richens, R. H. – Elm. Cambridge, 1983. With 152 text illustrations, many of them reproductions of paintings and engravings. The fruit of some thirty years of research, this work deals with the elms of Britain in all their aspects, with chapters on botany, history from the earliest times, vernacular names, artistic and literary associations etc., and also surveys the elms of England county by county. It is sad that this fine work should appear at such an inauspicious time.
Dutch Elm Disease. Dr D. A. Burdekin has provided the following amendments to bring his contribution up-to-date:
Page 637 para 2. The last sentence should read: Surveys of disease incidence were instituted in 1971 in southern Britain and by 1980 over 90 per cent of the total elm population of 23 million elms present at the beginning of the survey had been killed.
Page 638 para 3. The second sentence should now be replaced by: None of the insecticides has proved particularly effective, though one fungicide (thiabenzadole) injected into individual trees has shown considerable promise for the protection of, and to some extent cure of disease in, valuable specimens. Nevertheless, the only proven way of slowing down the spread of the disease on an area basis has been by sanitation felling.
Page 638 para 4. The last sentence should read: Although one or two disease-resistant clones (such as 'Sapporo Gold') are available, it may be some years before elm hybrids and cultivars, resistant to Dutch elm disease in its aggressive form, are planted on a wide scale.