Disease of the Month - Dutch Elm Disease

  Hort Snacks - March 2018
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 Ophiostoma ulmi

Crops Affected: Elm species

Disease Cycle:

  • Fungal pathogen is spread from tree to tree by 2 different beetles, native elm bark beetle (NEBB) and the smaller European elm bark beetle (SEEBB)
    • Both beetles are declared pests under the Alberta Pest Act
    • Emerging offspring are covered in the sticky spores, which are spread to healthy trees as they fly and burrow into new host trees
  • Spores that have been introduced into the host tissues germinate
  • The fungus grows and spreads, plugging the xylem tissues (tree's water conducting elements)
    • Infected branches wilt and die
    • Eventually the tree dies completely, usually within 1 or 2 seasons
  • Elm trees may exhibit the following symptoms
    • In early summer (mid-June to mid-July), leaves on individual branches may wilt, droop, followed by curling and browning
      • Brown wilted leaves remain on the tree
    • Later in summer, leaves droop and turn yellow (flagging), with leaves may drop prematurely
      • Late infections can be confused with normal, seasonal changes
    • Branches with smaller leaves than rest of the tree
    • Branches with no leaves
  • The sapwood will be stained brown
    • This is visible when the bark is removed from infected twigs
  • Evidence of insect pest vector
    • Beetle emergence holes, the size of the diameter of a pencil lead, and/or sawdust on the bark would be indicative of burrowing beetles
Elm tree with flagging

  • Monitor conditions of elm trees
  • Maintain healthy trees
    • Ensure that they have adequate water, nutrients, light and are not stressed
  • Prune trees to remove dead, dying or diseased wood
    • Avoid pruning elm from April 1 to September 30, which is when the insect vector would be attracted to pruning wounds
    • Completely remove dead or dying trees, including the stump
    • Dispose of pruned material properly (burning or burial deeper than 25cm) – do not chip
  • Avoid transport of elms or elm firewood from other areas (diseased areas)
  • Chemical treatment of infected trees is not effective and is very costly
The Disease Vectors
Native Elm Bark Beetle
Hylurgopinus rufipes

  • Native beetle
  • Small beetle (~1/8 inch, 2-3mm)
  • Adults bore galleries across grain; larvae bore perpendicular to adult galleries
  • Overwinters mainly as adult in galleries beneath the bark of healthy trees - can overwinter in any stage, except egg
Smaller European Elm Bark Beetle (SEEBB)
Scolytus multistriatus

  • Introduced pest
  • Small beetle (~1/8 inch, 3-4mm)
  • Adults bore galleries parallel to wood grain; larvae bore perpendicular to adult galleries
  • Overwinters as a larva, pupates and emerges as an adult in spring
Differences in breeding galleries
  • Characteristic pattern of the breeding galleries on the surface of the wood under the bark can be used to identify the two elm bark beetle species
  • Galleries that run parallel to the grain are caused by SEEBB
  • Galleries that run across the grain are caused by the NEBB
Native Elm Bark Beetle - adult
Smaller European Elm Bark Beetle – adult
Breeding Galleries – Left = SEEBB; Right = NEBB
Photos by: Natural Resources Canada, Canadian Forest Service
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on February 27, 2018.