Oak Decline

In Winnipeg, oak decline is a disease of bur oak. Visible symptoms include dieback of leaves and the major limbs at the topmost part of the tree’s canopy. Contrary to popular belief, oak decline is not caused by the two-lined chestnut borer. The insect is a secondary stress factor, and it attacks trees that are already in decline or under stress. You can help prevent oak decline (and the subsequent infestation of the two-lined chestnut borer) by protecting your oak trees against stress factors such as drought, flooding, and root damage due to construction activity. Practices to reduce stress to your oak tree and to prevent attracting two-lined chestnut borer include:


  • Monthly thorough watering with a showerhead-like hose nozzle or deep root feeder during long periods of drought.
  • Applying a thin layer of mulch on the surface of the soil around the tree to conserve soil water and slowly release nutrients.
  • Removal of sediments moved over the roots by flood waters.
  • Installation of drainage tiles to lead water away from oaks in constantly waterlogged soils.
  • Vertical mulching to aerate compacted soils.
  • Installation of a “Tree Protection Zone” enclosing an area to the extent of the drip line, in the least, to avoid soil compaction over the roots during construction activities.
  • Application of a thick layer (around 30cm) of mulch temporarily in areas where a Tree Protection Zone cannot be set up during construction activities to disperse pressure of heavy machinery and people moving over the soil.
  • Sanitation, the removal and burning or burial of dead oak trees and branches, to prevent two-lined chestnut borer beetles from using the material for broods.

A bur oak tree showing signs of decline.