The City of Grande Prairie is reminding residents to inspect their trees due to a rise in elm aphids and fireblight disease.


The recent wet weather has caused a resurgence of fireblight disease in Grande Prairie’s urban forest.

Apple, pear, hawthorn and mountain ash trees are vulnerable so owners of these trees are strongly recommended to check for symptoms and prune them immediately if fireblight is found.

Symptoms of fireblight include:

•                 Sudden flagging and death of new growth, often black/brown and curled.

•                 Sunken cankers spreading down shoots and branches in front of the dying leaves.

To treat fireblight, prune out all infected branches 12-18 inches below the infection.  Disinfect pruning tools in between each cut with gas line antifreeze or a 10% bleach solution to prevent spreading the disease further throughout the tree.

Fireblight is legislated under the Agricultural Pests Act of Alberta and must be controlled wherever found.  It affects apples, ornamental crabapples, pears, hawthorn and mountain ash trees.  This disease spreads extremely quickly and may kill large portions of a tree in a single season if left untreated.

Over 10% of the City’s public tree inventory is susceptible to fireblight.  Parks Operations staff will be inspecting and treating city-owned apple, crabapple, pear, hawthorn and mountain ash trees throughout the summer to curb infection spread.

Visit the fireblight page on the City’s website for more information.

Elm Aphids

Aphids are reaching outbreak levels in elms across the city and, although they pose no risk to the tree’s health, they do cover the ground and anything else below them in a layer of sticky honeydew.

Residents with elm trees might be noticing their cars, driveways and decks covered in residue.  Elms that are fertilized and have a lot of lush growth are targets for these insects.  Once they complete their life cycle their population will decrease and the honeydew problem will also cease. 

For those with elm trees impacted by aphids, the honeydew can be washed off with high pressure water, which is also the best solution to controlling the aphids themselves.  Shoot the canopy of the elm tree with high pressure water every day for a week to knock the aphids off and drown them.  The trees will also benefit from extra water to replace what the aphids take.

Alberta has the last stand of Dutch elm disease free elms in the entire world and these trees are very important and valuable to our tree inventory.  The City’s elms are worth over $4 million so residents are encouraged to ignore the aphids if they can so they can continue to enjoy these wonderful and rare trees.

Visit the aphids page on the City’s website for more information.

For more information, contact