Dutch elm disease

Dutch Elm Disease
David Elm with DED 2.jpg
Branch death, or flagging, at multiple locations in the crown of a diseased elm
Common namesDED
Causal agentsOphiostoma ulmi
Ophiostoma himal-ulmi
Ophiostoma novo-ulmi
Hostselm trees
Vectorselm bark beetle
EPPO CodeCERAUL
DistributionEurope, North America and New Zealand

Dutch elm disease (DED) is caused by a member of the sac fungi (Ascomycota) affecting elm trees, and is spread by elm bark beetles. Although believed to be originally native to Asia, the disease was accidentally introduced into America and Europe, where it has devastated native populations of elms that did not have resistance to the disease. It has also reached New Zealand. The name "Dutch elm disease" refers to its identification in 1921 and later in the Netherlands by Dutch phytopathologists Bea Schwarz and Christine Buisman who both worked with Professor Johanna Westerdijk.[1][2] The disease affects species in the genera Ulmus and Zelkova, therefore it is not specific to the Dutch elm hybrid.[3][4][5]

Overview

The causative agents of DED are ascomycete microfungi.[6] Three species are now recognized:

  • Ophiostoma ulmi, which afflicted Europe from 1910, reaching North America on imported timber in 1928.
  • Ophiostoma himal-ulmi,[7] a species endemic to the western Himalaya.
  • Ophiostoma novo-ulmi, an extremely virulent species from Japan which was first described in Europe and North America in the 1940s and has devastated elms in both continents since the late 1960s.[8][9]
Beetle feeding galleries on wych elm trunk
An infected English elm at West Point, NY, July 2010

DED is spread in North America by three species of bark beetles (Family: Curculionidae, Subfamily: Scolytinae):

In Europe, while S. multistriatus still acts as a vector for infection, it is much less effective than the large elm bark beetle, S. scolytus. H. rufipes can be a vector for the disease, but is inefficient compared to the other vectors. S. schevyrewi was found in 2003 in Colorado and Utah.

Other reported DED vectors include Scolytus sulcifrons, S. pygmaeus, S. laevis, Pteleobius vittatus and Р. kraatzi.[10] Other elm bark beetle species are also likely vectors.

Field resistance

'Field resistance' is an umbrella term covering the various factors by which some elms avoid infection in the first place, rather than survive it. A clear example would be the European White Elm Ulmus laevis which, while having little or no genetic resistance to DED, synthesizes a triterpene, Alnulin, rendering the bark distasteful to the vector beetles, obliging them to look further afield for more suitable elms. Another would be the inability of the beetles to see elms which did not break the silhouette. 'Weeping' elms are often spared infection owing to the beetles' aversion to hanging upside-down while feeding.

Mechanism

In an attempt to block the fungus from spreading farther, the tree reacts by plugging its own xylem tissue with gum and tyloses, bladder-like extensions of the xylem cell wall. As the xylem (one of the two types of vascular tissue produced by the vascular cambium, the other being the phloem) delivers water and nutrients to the rest of the plant, these plugs prevent them from travelling up the trunk of the tree, eventually killing it.

Signs

The first sign of infection is usually an upper branch of the tree with leaves starting to wither and yellow in summer, months before the normal autumnal leaf shedding. This progressively spreads to the rest of the tree, with further dieback of branches. Eventually, the roots die, starved of nutrients from the leaves. Often, not all the roots die: the roots of some species, notably the English elm, Ulmus procera, can engage in repeatedly putting up suckers which flourish for approximately 15 years, after which they too succumb.[8]

Other Languages
català: Grafiosi
čeština: Grafióza jilmu
dansk: Elmesyge
Deutsch: Ulmensterben
eesti: Jalakasurm
español: Grafiosis
galego: Grafiose
italiano: Grafiosi
magyar: Szilfavész
Nederlands: Iepenziekte
日本語: ニレ立枯病
norsk: Almesjuke
português: Grafiose
svenska: Almsjuka