By 1990, very few mature elms were left in Britain or much of continental Europe. One of the most distinctive
The largest surviving urban forest of elm trees in North America is believed to be in the city of
New York State
and adjacent areas, its cost made it uneconomical except in large cities where elms were considered valuable attractions.
Chemical
In the US, when Dutch elm disease spread away from the Atlantic coast, control focused on controlling the bark beetle by using insecticides such as DDT and dieldrin , which were sprayed heavily across all parts of elm trees, usually twice a year in the spring and again at a lower concentration in the summer. In its early years, it was generally thought by observers that pesticides did slow the spread of the disease across the United States[52] but as early as 1947, concern was raised that many bird species were killed in large numbers by ingesting poisoned invertebrates.[52] [53] In areas sprayed during the 1950s, local people observed birds such as the American woodcock , American robin , white-breasted nuthatch , brown creeper and various Poecile species dying. Biologist Rachel Carson consequently argued for improved sanitation and against spraying elms, which she saw as having been more effective in areas with earlier and greater experience countering Dutch elm disease.[54] Although modern critics of Carson have argued that the bird deaths were caused by other factors such as mercury poisoning in the soil,[55] spraying against elm bark beetles declined very rapidly after 1962, a trend aided by fungicides without dangerous side-effects being discovered for the first time after many years of research.[56]
Lignasan BLP (
carbendazim phosphate
), introduced in the 1970s, was the first fungicide used to control Dutch elm disease. This had to be injected into the base of the tree using specialized equipment, and was never especially effective. It is still sold under the name "Elm Fungicide".
Arbotect (
thiabendazole hypophosphite
) became available some years later, and it has been proven effective. Arbotect must be injected every two to three years to provide ongoing control; the disease generally cannot be eradicated once a tree is infected. Arbotect is not effective on root graft infections from adjacent elm trees. It is more than 99.5% effective for three years from beetle infections, which is the primary mode of tree infection.
Alamo (propiconazole ) has become available more recently, though several university studies show it to be effective only for the current season in which it is injected. Alamo is primarily recommended for treatment of oak wilt .
Multistriatin is a pheromone produced by female elm bark beetles, which can be produced synthetically. It has potential in being used to trap male beetles, which carry the fungus.
Biological
Because of the ban on the use of chemicals on street and park trees in the Netherlands, the
induced resistance.
[57] Trials with the American elm have been very successful; in a six-year experiment with the American elm in Denver, CO, annual Dutch elm disease losses declined significantly after the first year from 7 percent to between 0.4 and 0.6 percent;
[48] a greater and more rapid reduction in disease incidence than the accompanying tree sanitation and plant health care programs.
[58]
Preventive treatment is usually justified only when a tree has unusual symbolic value or occupies a particularly important place in the landscape.
Resistant trees
Research to select resistant cultivars and varieties began in the Netherlands in 1928, followed by the United States in 1937 (see Ulmus americana cultivars ). Initial efforts in the Netherlands involved crossing varieties of U. minor and U. glabra , but later included the Himalayan or Kashmir elm U. wallichiana as a source of antifungal genes. Early efforts in the USA involved the hybridization of the Siberian elm U. pumila with American red elm U. rubra to produce resistant trees. Resulting cultivars lacked the traditional shape and landscape value of the American elm; few were planted.
In 2005, the National Elm Trial (USA) began a 10-year evaluation of 19 cultivars in plantings across the United States. The trees in the trial were exclusively American developments; no European cultivars were included. Based on the trial's final ratings, the preferred cultivars of the American elm (Ulmus americana ) are ‘New Harmony’ and ‘Princeton’. The preferred cultivars of Asian elms are the Morton Arboretum introductions and ‘New Horizon’.[60]
Recent research in Sweden has established that early-flushing clones are less susceptible to DED owing to an asynchrony between DED susceptibility and infection.[61]
Testing for disease resistance
Elms are tested for resistance by inoculation with the fungal pathogen in late May when the tree's growth is at its annual peak. Clones raised for testing are grown to an age of 3 or 4 years. In Europe, the
University of Wisconsin
team, which drilled holes in the branches to simulate natural infection by the bark beetles feeding in the twig crotches, but results from this method were found to exaggerate the genetic resistance of the host. Consequently, tests were conducted on specimens in a controlled environment, either in greenhouses or customized plant chambers, facilitating more accurate evaluation of both internal and external symptoms of disease.
Another variable is the composition of the inoculum; while an inoculum strength of 106 spores / ml is standard in both continents, its composition reflects the different Ophiostoma species, subspecies and hybrids endemic to the two continents. In Italy for example, two subspecies, americana and novo-ulmi , are present together with their hybrid, whereas in North America, ssp. novo-ulmi is unknown.[62] The differences in method and inocula possibly explain why the American cultivar 'Princeton' , displaying high resistance in the US, has often succumbed to Dutch elm disease in Europe.[63]
Hybrid cultivars
Inoculation of virulent strains of Ophiostoma in elm cambium, Dorschkamp Institute for Forestry and Landscape Planning, Wageningen , 1984
Many attempts to breed disease-resistant cultivar hybrids have involved a genetic contribution from Asian elm species that are demonstrably resistant to this fungal disease. Much of the early work was undertaken in the Netherlands. The Dutch research programme began in 1928, and ended in 1992. During those 64 years, well over 1000 cultivars were raised and evaluated. Still in use are cultivars such as 'Groeneveld', 'Lobel', 'Dodoens', 'Clusius' and 'Plantijn', although the resistance levels in these trees aren't high enough to confer good protection. The programme had three major successes: 'Columella' , 'Nanguen' Lutèce , and 'Wanoux' Vada ,[64] all found to have an extremely high resistance to the disease when inoculated with unnaturally large doses of the fungus. Only 'Columella' was released during the Dutch programme’s lifetime—-in 1987. Patents for the Lutèce and Vada clones were purchased by the French Institut National de la Recherche Agronomique (INRA), which subjected the trees to 20 years of field trials in the Bois de Vincennes , Paris , before releasing them to commerce—-in 2002 and 2006, respectively.
Asian species featured in the American DED research programs were the Siberian elm
'Rosehill' in the 1940s and 50s; the species last featured in hybridization as the female parent of
'Repura' and
'Revera' , both patented in 1993, although neither has yet appeared in commerce.
In
'Plantyn' as a safeguard against any future mutation of the disease.
[65] Two trees with very high levels of resistance,
'San Zanobi' and
'Plinio' ,
[66] were released in 2003.
'Arno' and
'Fiorente' were patented in 2006 and entered commerce in 2012. All four have the Siberian elm
U. pumila as a parent, the source of disease-resistance and drought-tolerance genes.
'Morfeo' was released in 2011; it arose from a crossing of the Dutch hybrid clone '405' (female parent) and the
Chenmou Elm , the latter a small tree from the provinces of
Anhui and
Jiangsu in eastern China, The '405' clone is a crossing of an English
U. × hollandica and a French
U. minor .
In the Netherlands a new program has been initiated. From the old proving grounds of the Dorschkamp Research Institute, 10 fourth-generation hybrids survive in a DED-ridden area. These have been tested and some have a very high level of resistance. At Noordplant Nursery new hybrids have been tested since 2013.
Species and species cultivars
North America
Results of artificial inoculation of Ophiostoma strains in elm cambium, Arlington Experimental Station, Wisconsin , 1987
Ten resistant American elm cultivars are now in commerce in North America. No cultivar is immune to DED; even highly resistant cultivars can become infected, particularly if already stressed by drought or other environmental conditions where the disease prevalence is high. With the exception of 'Princeton', no trees have yet been grown to maturity; trees cannot be said to be mature until they have reached an age of 60 years.
Notable cultivars include:
'Princeton', is a cultivar selected in 1922 by USDA in the early 1990s. As trees planted in the 1920s still survive, the properties of the mature plant are well known. However, 'Princeton' has not proven resistant in Europe, where the main vector of the disease—the larger elm bark beetle,
Scolytus scolytus —is capable of introducing far more fungal spores into the tree; many of the 50 trees planted at
Highgrove House in the south-west of
England in 2006 had died from Dutch elm disease by 2011.
[63]
Plant patent
6227). The oldest 'American Liberty' elm was planted in about 1980.
'Valley Forge', released in 1995, has demonstrated the highest resistance of all the clones to Dutch elm disease in controlled USDA tests.
'Lewis and Clark' = Prairie Expedition TM , released in 2004 to commemorate the bicentenary of the Lewis & Clark expedition, was cloned from a tree found growing in North Dakota which had survived unscathed when all around had succumbed to disease.
In 2007, the Elm Recovery Project of the University of Guelph Arboretum in Ontario, Canada, reported that cuttings from healthy surviving old elms surveyed across Ontario had been grown to produce a bank of resistant trees, isolated for selective breeding of highly resistant cultivars.[67]
The University of Minnesota USA is testing various elms, including a huge now-patented century-old survivor known as "The St. Croix Elm" , which is located in a Minneapolis-St. Paul, MN suburb (Afton) in the St. Croix River valley—a designated National Scenic Riverway.
The slippery or red elm U. rubra is marginally less susceptible to Dutch elm disease than the other American species, but this quality seems to have been largely ignored in American research. No cultivars were ever selected, although the tree was used in hybridization experiments (see above).
In 1993, Mariam B. Sticklen and James L. Sherald reported the results of NPS-funded experiments conducted at
DED-resistant American elms.
Europe
Among European species, there is the unique example of the European white elm
sterols, which serves to make the tree bark unattractive to the beetle species that spread the disease.
[70]
In Europe the testing of clones of surviving
field elm genotypes have comparatively high levels of tolerance to DED. In Spain, for example, of around 5,000 native elms evaluated to 2013, some 25 genotypes (0.5% of those tested) fall into this category; and it is now hoped that the controlled crossing of the best seven of these (genetically and aesthetically) will produce
Ulmus minor hybrids with effective 'field resistance' and market appeal.
[72] Similar results are beginning to emerge in trials on surviving field elms in Greece.
[73]
United Kingdom
Much of the work in the United Kingdom is by the Forestry Commission's research arm, which has had Dutch elm disease on its agenda since the 1920s. In 1994 a Research Information Note (no 252) was published, written by John Gibbs, Clive Brasier and Joan Webber, and in 2010 a Pathology Advisory Note, as well as throughout the period a stream of more academic papers: notable results have been the observation that the progress of the disease through Scotland has been quite slow, and that genetic engineering has been tried to improve the resistance of the
English elm
.
In England the
English elm), as part of a scheme to return elms to city and countryside.
[74] [75] [76] [77] The Foundation was running two elm programmes: the 'Great British Elm Experiment' and 'Ulmus londinium', an elm programme for London – these use saplings cultivated through micropropagation from mature parent elms found growing in the British countryside: parent trees are monitored for disease, while saplings were offered free to schools and community groups, who are asked to monitor their trees' progress on the Foundation's online elm map; in London, places with 'elm' in their name were offered a sapling – in an attempt to find out why some elms have survived while others succumbed to Dutch elm disease. Both these projects have been discontinued.
The spread of DED to Scotland has focussed attention on a small number of wych elms U. glabra surviving in areas of high infectivity, prompting the Royal Botanic Garden Edinburgh to begin a programme of selecting trees, with a view to determining innate resistance (2009).[78] The Garden is raising and distributing in Scotland seedlings derived from controlled crosses of rare survivors in these areas (2023).[79] [80] [81]
In 2001–2004, English elm U. minor 'Atinia' was genetically engineered to resist disease, in experiments at Abertay University , Dundee , Scotland , by transferring antifungal genes into the elm genome using minute DNA-coated ball bearings.[82] [83] However, owing to reservations to GM developments, there are no plans to release the trees into the countryside.
Spain
In Spain, the Escuela Técnica Superior de Ingenieros de Montes,
Universidad Politecnica de Madrid , charged with discovering disease-resistant elms for use in forestry, has raised and patented seven cultivars of the field elm
Ulmus minor, although two have subsequently been found to have Siberian elm
U. pumila DNA, the species introduced to Spain in the 16th century. Although none have been released to commerce (2020), the clone
'Ademuz' , pure U. minor , has been imported into the UK since 2014, and widely planted there.
Possible earlier occurrences
The 'Elm Decline'
From analysis of fossil pollen in peat samples, it is apparent that elms, an abundant tree in prehistoric times, all but disappeared from northwestern Europe during the mid-Holocene period around 4000 BC, and to a lesser extent around 1000 BC. This roughly synchronous and widespread event has come to be known as the 'Elm Decline'. When first detected in the mid-20th century, the decline was attributed to the impact of forest-clearance by Neolithic farmers, and of elm-coppicing for animal fodder, though the numbers of settlers could not have been large. The devastation caused recently by DED has provided an alternative explanation. Examination of subfossil elm wood showing signs of the changes associated with the disease has suggested that a form of DED may have been responsible. Fossil finds from this period of elm bark beetles support this theory. A consensus today is that the Elm Decline was probably driven by both factors.[84] [85]
Historic period
A less devastating form of the disease, caused by a different fungus, had possibly been present in north-west Europe for some time. Dr Oliver Rackham of Cambridge University presented evidence of an outbreak of elm disease in north-west Europe, c. 1819–1867. "Indications from annual rings [a reference to the dark staining in an annual ring in infected elms] confirm that Dutch elm disease was certainly present in 1867," he wrote, quoting contemporary accounts of diseased and dying elms, including this passage in Richard Jefferies ' 1883 book, Nature near London :
There is something wrong with elm trees. In the early part of this summer, not long after the leaves were fairly out upon them, here and there a branch appeared as if it had been touched with red-hot iron and burnt up, all the leaves withered and browned on the boughs. First one tree was thus affected, then another, then a third, till, looking round the fields, it seemed as if every fourth or fifth tree had thus been burnt. [...] Upon mentioning this I found that it had been noticed in elm avenues and groups a hundred miles distant, so that it is not a local circumstance.
Earlier still, Rackham noted, "The name Scolytus destructor was given to the great bark beetle on evidence, dating from c. 1780, that it was destroying elms around Oxford."[86]
In Belgium, elm die-back and death was observed in 1836 and 1896 in Brussels , and in 1885–1886 in Ghent . In the later outbreaks the die-back was attributed to the elm bark beetle.[87]
It has been suggested that "for thousands of years elms have flourished in natural balance with the scolytidae , combating occasional infections of Dutch elm disease."[88]
Sir Thomas Browne , writing in 1658, noted in The Garden of Cyrus an elm disease that was spreading through English hedgerows, and described symptoms reminiscent of DED.[89]
See also
References
^ Schwarz, M.B. (1922). "Das Zweigsterben der Ulmen, Trauerweiden und Pfirsichbaume". Mededelingen Phytopathologisch Laboratorium, Willie Commelin Scholten . 5 : 1–73.
^ Buisman, C. (1928). "De oorzaak van de iepenziekte". Tijdschr Ned Heidemaatsch . 40 : 338–345.
^ "Dutch elm disease in Britain" . UK Forestry Commission. Archived from the original on 9 March 2018. Retrieved 6 June 2007 .
^ Dutch Elm Disease . Macmillan Science Library.
^ Smalley, EB (1963). "Seasonal fluctuations in susceptibility of young elm seedlings to Dutch elm disease". Phytopathology . 53 (7): 846–853.
^ Ascomycetes: Phylum Ascomycota , Biology of Plants, Seventh Edition , W. H. Freeman and Company, 2005.
^ Clinton, G. P., McCormick, Florence A., Dutch elm disease, Graphium ulmi ; New Haven, 1936
.
^ .
^ Johnson, O. (2011). Champion Trees of Britain and Ireland . Royal Botanic Gardens, Kew.
^ Ижевский, С.С.; Никитский, Н.Б.; Волков, О.Г.; Долгин, М.М (2005). Иллюстрированный справочник. жуков-ксилофагов – вредителей леса и лесоматериалов Российской Федерации (PDF) . Тула: Российская Академия Наук, Уральское отделение, Коми научный центр, Институт биологии. (Izhevsky, SS; et al. (2005). "An illustrated guide to the xylophagous beetles injuring forests and timber in the Russian Federation". Russian Academy of Sciences, Ural Branch, Komi Science Center, Institute of Biology. Tula). p. 165.
^ Clouston, B., Stansfield, K., eds., After the Elm (London, 1979)
^ Spierenburg, Barendina (1921). "Een onbekende ziekte in de iepen (An unknown disease in elms)". European Journal of Plant Pathology . 27 (5).
.
^ Heybroek, H. M. and Nijboer, R. (2013). Christine Johanna Buisman in Italy . p. 4–6. Private publication, Netherlands.
^ a b c Harris, E. (2017). The European White Elm, Ulmus laevis Pall. Quarterly Journal of Forestry , Vol. 111, No. 4, October 2017. p.263. Royal Forestry Society.
^ "Lutèce®, a resistant variety brings elms back to Paris" . All The News . Nantes , France: Institut national de la recherche agronomique (INRA). 15 April 2005. Archived from the original on 25 November 2006.
^ "Amsterdam, City of Trees" . DutchAmsterdam. 18 May 2011.
^ Amsterdamse Bomem Archived 2011-07-24 at the Wayback Machine
^ "The City and its elm population" . The Hague in the Netherlands . DutchTrig®. Archived from the original on 29 October 2013.
^ Research on Dutch Elm Disease in Europe , ed. D. A. Burdekin (London, 1983)
^ Brighton and Hove Council page on the city's elm collection Archived 2011-06-14 at the Wayback Machine (viewed 2 June 2010)
^ "Dutch Elm Disease (DED)" . Environment and Planning: Land and premises: Conservation: Trees & landscapes . Lewes District Council. 2009. Archived from the original on 5 July 2009.
^ Gupta, Tanya (11 November 2005). "How Brighton beat Dutch Elm menace" . BBC News, South East .
^ Isle of Man elms, geocomputation.org
.
^ prolandscapermagazine.com 24 February 2017
^ Coleman, Max, ed., Wych Elm (Edinburgh 2009)
^ edinburgh.gov.uk/info/20064/parks_and_green_spaces/256/trees_and_woodlands
.
^ 'Dutch elm disease management in Edinburgh' (2021); treecare.org.uk
^ Ulmus 'Nanguen' www.foretpriveefrancaise.com
^ PDF Archived 24 September 2015 at the Wayback Machine
^ Life , 11 September 1944, p. 58
^ Baulch, Vivian (20 December 2001). "How Detroit lost its stately elms" . Detroit News .
^ New York Times, 5 December 1989, nytimes.com nytimes.com/1989/12/05/science/new-varieties-of-elm-raise-hope-of-rebirth-for-davastated-tree.html?sec=health&spon=
^ Sherald, James L (December 2009). Elms for the Monumental Core: History and Management Plan (PDF) . Washington, D.C.: Center for Urban Ecology, National Capital Region, National Park Service . Natural Resource Report NPS/NCR/NRR--2009/001. Archived from the original (PDF) on 29 November 2010. Retrieved 14 October 2010 .
^ Beaucher, Serge (Autumn 2009). "Québec, terre des ormes" . Contact (in French). 28 (1). Laval University.
^ CFIA annual pest survey report. 1999 Summary of Plant Quarantine Pest and Disease Situations in Canada (report available upon demand at the Canadian Food Inspection Agency: http://publications.gc.ca/site/eng/9.831610/publication.html )
^ "Dutch Elm Disease" . Alberta Agriculture and Rural Development. Archived from the original on 21 May 2018. Retrieved 14 December 2014 .
^ "Hydro contractors snubbing Winnipeg elm-pruning ban, group suspects" . CBC News . 26 July 2017. Retrieved 26 July 2019 .
^ Agriculture, Alberta; Forestry (11 April 2018). "Elm pruning ban now in place" . Alberta Farmer Express . Retrieved 26 July 2019 .
^ a b "Pruning ban on elm trees starts April 1" . CTV News Regina . 25 March 2019. Retrieved 26 July 2019 .
^ Semeschuk, Darci. "Majestic Elms marked for removal" . Souris Plaindealer . Retrieved 26 July 2019 .
^ Release, Stopded (14 November 2016). "Elm pruning ban over until March" . Alberta Farmer Express . Retrieved 26 July 2019 .
^ "Winnipeg is home to the largest Urban Elm Forest in North America - Winnipeg | Globalnews.ca" . Global News . Retrieved 9 August 2022 .
^ "Elm Bark Beetle Control Program" (PDF) . City of Winnipeg. 2009.[permanent dead link ]
^ a b Rumbolt, Colin (17 November 2009). "Dutch elm vaccine tested in Winnipeg" . the Manitoban .
^ "Dutch Elm Disease" . Biosecurity New Zealand. 26 May 2008. Archived from the original on 5 December 2012. Retrieved 1 October 2012 .
^ "Elm disease strikes out south" . Manukau Courier . Fairfax NZ News. 18 August 2013.
^ Auckland's elms, bts.nzpcn.org.nz/bts_pdf/ABJ58(1)2003-38-45-Elms.pdf
^ .
.
.
.
.
^ About Dutch Trig® Archived 2010-11-17 at the Wayback Machine
^ "Dutch Trig - United States - Distributors" . Archived from the original on 15 June 2013. Retrieved 5 August 2013 .
^ "Elm Tree Lawn Begins New Life" . Scripps College News . Scripps College . 14 April 2008. Retrieved 18 February 2021 .
OCLC
7347020445 . Retrieved 7 February 2021 .
Based on the ratings, the preferred cultivars of American elm were 'New Harmony' and 'Princeton', and the preferred cultivars of Asian elm were The Morton Arboretum introductions and 'New Horizon'.
^ Ghelardini, L. (2007) Bud Burst Phenology, Dormancy Release & Susceptibility to Dutch Elm Disease in Elms (Ulmus spp.) . Doctoral Thesis No. 2007:134. Faculty of natural Resources and Agricultural Services, Swedish University of Agricultural Sciences, Uppsala, Sweden
^ Mittempergher, L; Santini, A (2004). "The history of elm breeding" (PDF) . Investigacion Agraria: Sistemas y Recursos Forestales . 13 (1): 161–177. Archived from the original (PDF) on 11 February 2017. Retrieved 9 February 2017 .
^ a b Brookes, A.H. (2013). "Disease-resistant elm cultivars, Butterfly Conservation trials report, 3rd revision" (PDF) . Lulworth UK: Butterfly Conservation. Archived from the original (PDF) on 29 May 2014.
^ Institut National de la Recherche Agronomique. Lutèce, a resistant variety, brings elms back to Paris [1] , Paris, France
^ Santini, A.; Fagnani, A.; Ferrini, F.; Mittempergher, L.; Brunetti, M.; Crivellaro, A.; Macchioni, N. (2004). "Elm breeding for DED resistance, the Italian clones and their wood properties" (PDF) . Invest Agrar: Sist Recur for . 13 (1): 179–184. Archived from the original (PDF) on 26 October 2007.
.
^ "Elm Recovery Project" . Guelph , Ontario , Canada: University of Guelph Arboretum . Archived from the original on 22 November 2019. Retrieved 22 November 2019 .
)
.
(PDF) on 28 June 2007.
^ Screening European Elms for resistance to 'Ophiostoma novo-ulmi' (Forest Science 2005)
[2]
^ ‘Spanish Clones’ (Oct. 2013) resistantelms.co.uk
^ Δoκιμή ανθεκτικότητας ελληνικών γενoτύπων πεδινής φτελιάς (Ulmus minor) κατά της Oλλανδικής ασθένειας , Σ. Διαμαντής και X. Περλέρου (:Resistance test of Greek Field Elm against Dutch Elm Disease, by S. Diamantis and H. Perlerou) [3]
^ Fifteen source-trees in England cloned for the Conservation Foundation's 'Great British Elm Experiment'; conservationfoundation.co.uk
^ Update on new clones in the Conservation Foundation's 'Great British Elm Experiment'; conservationfoundation.co.uk/the-great-british-elm-experiment-update
^ " "Super tree" from Northamptonshire helping to fight Dutch Elm Disease and repopulate woodlands" . northamptonchron.co.uk . Archived from the original on 5 March 2016.
^ " 'Young elms return to London', conservationfoundation.co.uk" . Archived from the original on 2 December 2013. Retrieved 16 November 2013 .
^ Coleman, Max, ed.: Wych Elm (Royal Botanic Gardens Edinburgh, 2009; ISBN 978-1-906129-21-7), p.76
^ Russell Blackstock, The Sunday Post (Dundee), 6 August 2023, p.8
^ 'Scottish Plant Recovery: Next gen elms', Botanics Stories (RBGE Personal & Project Stories), stories.rbge
^ Clarkson, R. & Coleman, M. (2022) 'Propagating healthy mature elms that have survived in areas severely impacted by Dutch elm disease: a scoping study for a wych elm project in Scotland.' Royal Botanic Garden Edinburgh.
^ 'First Genetically Modified Dutch Elm Trees Grown', unisci.com
^ resistantelms.co.uk, FAQ 'Disease Control'
^ Coleman 2009 , p. 17
^ "The mid-Holocene Ulmus decline: a new way to evaluate the pathogen hypothesis" . Archived from the original on 28 September 2011. Retrieved 4 November 2011 .
^ Oliver Rackham, The History of the Countryside (London 1986), pp. 242–243, 232
^ Meulemans, M.; Parmentier, C. (1983). Burdekin, D.A. (ed.). "Studies on Ceratocystis ulmi in Belgium" (PDF) . Forestry Commission Bulletin (Research on Dutch Elm Disease in Europe) (60). London: HMSO: 86–95. Archived from the original (PDF) on 15 February 2017. Retrieved 15 February 2017 .
^ Vaclav Vetvička, Trees and Shrubs (London 1985)
^ Oliver Rackham, The History of the Countryside (London 1986), p.242-3
Further reading
External links