Dutch Elm Disease | Epidemiology Control Dutch Elm Disease Value & Utilization

    EPIDEMIOLOGY

    The first adequately described observations of Dutch elm disease (DED) originate from France and the Netherlands, and date back to 1919 and 1921, respectively {[309],[310]}. By that time, the disease had affected most of Belgium and the Netherlands, as well as parts of northern France and Germany. It is therefore thought that DED arose in around 1910 in the area between the Seine river (France) and Brabant (the Netherlands). To begin with, DED was called “elm disease” or “elm death” by a number of European authors {[344],[498],[632]}. However, since only the Dutch performed extensive research on the wilt disease in those early years, it became known as “Dutch elm disease.”

    Photo 65: Christine Buisman
    (Courtesy of H. Heybroek, Wageningen, The Netherlands).

    After the first notes on excessive elm dying by Barendina G. Spierenburg, the cause of DED was discovered by Marie Beatrice Schwarz and Christine J. Buisman: a fungus currently named Ophiostoma ulmi {[309],[497],[499],[876]}. This pathogen was relatively non-aggressive, and many elms survived its spread across the Northern hemisphere. Nevertheless, of the 1,228,000 Dutch elm trees extant in 1930, thirteen years later (i.e., in 1943) 421,000 had been felled because of the disease {[323]}. Most European countries lost 10-40% of their elm trees {858}. After the first outbreaks in northwest Europe, DED rapidly spread eastward across the continent and into southwest Asia. By around 1925/1930, the disease was present in the United Kingdom, Austria, Czechoslovakia, Poland, Romania, the former Yugoslavia, Switzerland, Italy, and North America. After 1930, DED reached Bulgaria, Hungary, Portugal, Russia, and Kazachstan {[794],[858]}. O. ulmi was introduced into central Asia from Europe in the 1930s {[35]}. After a period of high disease incidence, during the 1940s the number of losses in Europe declined; however, probably as a result of the high DEDsusceptibility of U. americana, no decrease in the percentage of infected trees was observed in America at thattime {[858]}.

    Man has accidentally played a major role in the worldwide distribution of DED {229}. Several outbreaks of the disease have occurred along major transport routes of firewood and timber in Europe and the USA. It was man who gave elm bark beetles free rides on planes, trains, and cars, thus enabling DED transmission from one continent to another {877}. The importation of elm wood with attached bark is a likely mode of entry for DED to a new country or continent. It is thought that the introduction of the disease into the United Kingdom, the USA and, more recently, New Zealand occurred in this way {858,875,878}. In addition to the transport of infected elm logs, planting policies, cultivation, and management of the elm host population directly influenced the development of the DED pandemics, e.g., the planting of numerous susceptible elm trees connecting formerly isolated elm stands after the first epidemic in the Netherlands created an excellent environment for the outbreak of a second one {794}.

    Brasier {35} suggests that the second DED pandemic caused by different races of the aggressive O. novo-ulmi began as early as the 1940s. Outbreaks of the disease started simultaneously at two locations, i.e., the Moldavia-Ukraine area in eastern Europe (Eurasian race, EAN), and in the southern Lakes area of North America (North American race; NAN).

    The EAN race reached Romania in the early 1950s. Subsequently, it spread westward across the Ukraine to the former Yugoslavia, Hungary, and northern Italy, and eastward to Volgograd. By the 1970s, the EAN race was present in Czechoslovakia, Poland, Denmark, Germany, and Iran {35}. O. novo-ulmi EAN migrated steadily across Europe, reaching the Netherlands in the mid-1970s/beginning of the 1980s {794,858}. Probably as a result of a separate importation, the EAN race was introduced into southwest and central Asia in the 1970s/1980s. Currently, the EAN race is found in Russia, Eastern Europe, Italy, Greece , Ireland, Scandinavia, the Benelux countries, and Denmark {858}.

    After its arrival, the NAN race spread across the USA into Canada, reaching the east and the west coast by the 1970s/1980s. During the 1960s and 1970s, the O. novo-ulmi NAN race originating from the Windsor, Ontario area in Canada was introduced into the United Kingdom. It has since spread to the Netherlands, France, Spain, Sweden, and several other west European countries. {35,858}. Currently, O. novo-ulmi NAN and EAN coexist in a number of geographic regions in Europe, e.g., Italy. In 1989, the NAN race was first found in New Zealand. Probably the fungus was introduced into this country from western Europe {875}. Replacement at a rate of approximately 10% each year occurred at every location where the non-aggressive O. ulmi encountered the more aggressive and more fit O. novo-ulmi {879}.

    The second DED pandemic caused by O. novo-ulmi resulted in the wide-scale destruction of most mature elm trees in Europe, e.g., in the United Kingdom alone, over 25 million elms were killed. In America, the effects of the second DED pandemic were less obvious, due to the great susceptibility of the American elm even to the non-aggressive O. ulmi.

     In the Netherlands, DED control was regulated by law between 1977 and 1991. A national eradication campaign reduced the loss of elms to an average of 1% a year by the end of the 1980s. Unfortunately, the number of diseased elms has steadily increased since the law was abolished in 1991{794}.

    China has long been considered the center of origin of DED {35}. However, during an extensive search for O. ulmi s.l. in that country in 1986, the fungus could not be found. In 1995, Brasier and Mehrotra {139} reported the endemic presence of O. himal-ulmi (a third Ophiostoma species able to cause DED) in the western part of the Himalayas. In this region, there appears to be a balance between the fungus, the bark beetles, and the local elm hosts. Perhaps the origin of the species O. ulmi and O. novo-ulmi should also be searched for in the Himalayas {858}.

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