Horticultural Maintenance Tips

Check Your Trees for Disease

Black Knot Disease

A proliferation outbreak of Black Knot Disease on trees has been observed in Olds. This dreaded fungal disease affects Apricots, Plums and Cherries in the Prunus genus. This is most prevalent in Schubert Chokecherry and Mayday trees here in Alberta although other cherry trees and plums can certainly be affected.
How to Identify Black Knot
It starts as olive green gall or swelling and over two to three years will grow into a bulky, black, tar like growth on the affected branch.  If  it is not dealt with it will spread to other parts of the tree internally and externally. When the gall is mature it releases huge amounts of spores in the spring that are airborne infecting other trees in your neighbourhood. Eventually the branch above the infection will die back. The tree will not die immediately but suffer a long slow death when not controlled and if the infection becomes extreme.
How to Control Black Knot
It is best to prune in late autumn after the leaves have fallen and right through the winter into early spring before you see any new buds starting to swell. It must be cut out of the tree at least 12 inches below the infected area.  Make  a proper pruning cut, do not leave  a stub. Make sure that the infected branch is disposed of properly, best to burn immediately, do not put it into the compost bin.
Please ensure all pruning equipment is sterilized following each cut, as the disease transfers easily.
If you must remove your tree, do not plant another Prunus species in the same spot. Make sure your trees are healthy so they are less likely to be infected by the spores floating through the air in spring. Consider hiring a certified arborist if you are not sure your tree is infected or not sure what to do.

Fire Blight

Fire blight, a highly destructive disease of several members of the rose family, is caused by the bacterium Erwinia amylovora. This bacterial organism is native to North America and is present throughout the continent where susceptible rose family hosts are grown. This disease can affect up to 130 plant species worlds wide.  In Alberta, this includes many highly desirable hardy ornamentals and fruit-producing species in the rose family This includes apple, crab-apple, (Malus Spp) cotoneaster (Cotoneaster Spp.), hawthorn (Crataegus spp.), pear (Pyrus Spp.) mountain ash, (Sorbus Spp.) raspberry (Rubus Spp.), saskatoon berry (Amelanchier spp.), plum and cherry (Prunus Spp.). It is particularly destructive to crab-apple, apple, and pear species that are non-native to Canada.

Fire blight causes the most damage when spring or summer weather is warm, humid, or rainy. Temperatures between 24 and 28 degrees Celsius and humidity above 60 percent allow the bacteria to reproduce and spread.
Fire blight outbreaks often happen after extreme weather events like hail, strong winds or heavy rain. These events can damage the tree and create wounds where the bacteria can enter.

  • The fire blight bacteria survive winter in cankers on the trunk or branches of infected trees.
  • In spring, during warm, wet weather, bacteria begin to multiply. The bacteria seep out of the canker through natural openings or cracks as a sweet, sticky liquid known as bacterial ooze.
  • Insects such as pollinators are attracted to this sweetness and carry the bacteria to wounds or flowers.
  • Bacteria can also be moved by splashing water from rain or irrigation, or on the hands and tools of gardeners.
  • The fire blight bacteria will live and multiply on the surface of leaves, twigs, flowers and immature fruit for a few weeks without causing symptoms.
  • Bacteria need a wound or a natural opening to infect the tree.
    • Many infections start when bacteria growing on flowers reach a certain population and enter the flower through natural openings.
    • Young shoots are infected through small wounds caused by insect feeding, wind whipping or rubbing of branches, blowing sand or other damage.
  • Once the bacteria have infected either blossoms or shoots, the bacteria can move into the branch through the vascular system of the tree. New branch cankers are formed by bacteria moving into branches in this way.
  • In highly susceptible cultivars, the bacteria can move into the main trunk of the tree and even the roots. At this point, the tree will die.

How to Identify

Infected blossoms Flowers and flower clusters appear water-soaked, then droop and shrivel, turning brown or black.

  • Brown to black flowers remain attached through the growing season.
  • Flower infections in Alberta is rare due to the cool temperatures during trees and shrubs blooming.
  • Young leaves and shoots wilt and turn gray-green. These wilted shoots bend downward forming the characteristic hook.
  • Leaves and infected shoots turn brown or black. Trees with multiple, infected shoots may appear scorched by fire.
  • Brown leaves hang downward. Often, they cling to the blighted twig through the growing season and remain attached to the tree into winter.
  • Fruit turns dark and shrivels into a mummy. This shriveled fruit may cling to the branch for several months.
  • Reddish brown-stained sapwood Bark on branch or trunk cankers appear sunken, dark and may be cracked or peeling. If bark is peeled back, brown staining of the sapwood can be seen.
  • Droplets of cream to light-yellow colored ooze is found along infected branches, shoots, or fruit during humid weather or after a rain.


As soon as fire blight is discovered, prune off infected branches 1 foot (12”) below the diseased sections. Diseased branches can be burned to prevent further infection. Do not compose or store for firewood. Dip pruning shears into a 10% alcohol or bleach solution between each cut to avoid transmitting the disease from one branch to another.
How to manage fire blight
Plant resistant tree varieties. 
There are no known trees or shrubs in the Rosaceae family that are completely immune to fire blight. Some varieties can defend themselves by limiting or slowing the spread of the disease. This gives the gardener time to prune out the infected branches before the infection reaches the main trunk of the tree.
Varieties are often ranked by their ability to resist infection and slow the progression of disease. Since new varieties are brought to market each year, check with a reputable nursery about the disease resistance characteristics of new cultivars.

Pruning Procedure

Twigs and branches infected with fire blight can be pruned out in order to prevent the infection from spreading to the main trunk.

Healthy trees should be pruned to maintain an open canopy. This allows air to dry all the leaves quickly after rain or dew. This is important since bacteria thrive and multiply best in warm, humid environments.

  • Prune diseased twigs and branches in late winter when the tree and bacteria are dormant. This reduces the amount of sucker growth from wounds and eliminates the chance of spreading infections between pruning cuts.
  • Make the pruning cut through healthy wood at least 8 inches below the discolored bark of a canker.
  • If fire blight is seriously damaging a cotoneaster hedge, cut the hedge to about six inches above the ground in late winter. If only a few stems are blighted, they can be removed as described above.
  • If pruning must be done during the growing season, sterilize pruning tools between each cut. To sterilize, spray the cutting blade with disinfectant or soak the cutting surface in disinfectant for at least 1 minute. Effective disinfectants include a 10% bleach solution and undiluted Lysol®.
    • A 10% bleach solution can be made by mixing one part household bleach to nine parts water. This equals 1.5 cups of bleach to 1 gallon of water.
  • Burn or bury infected cuttings. DO NOT compost or store for fire pits.
  • If the infection reaches the main trunk, the disease cannot be cured and the tree will eventually die. If this happens, it’s best to remove the entire tree along with the stump. This will eliminate one source of bacteria for the rest of the susceptible plants in the area.

Pesticide controls

  • Copper-based pesticide
  • Streptomycin sulfate
  • Serenade Garden Defense (Bacillus subtillis) Always follow the pesticide label directions attached to the pesticide container you are using. Be sure that the area you wish to treat is listed on the label of the pesticide you intend to use.

Dutch Elm Disease

Symptoms can first be seen in June and early July. Leaves will wilt and curl, turning yellow and brown in summer. Branches begin to die back. Brown staining can be seen on the side of the tree when the bark is peeled back.
Elms should be well watered from April to mid August. 
Dead branches and trees that provide beetle habitat should be removed. Since elm bark beetles are attracted to fresh tree wounds, pruning should be done between October 1 to March 31 when the beetles are not in their active stage.
Dispose of all elm wood you have by taking it to your local landfill or by burning, burying or chipping it. 
Dutch elm disease is caused by a fungus which blocks the tree's ability to conduct water and causes the tree to die.
The fungus is mainly spread by three species of elm bark beetles:
•    the smaller European, Scolytus multistriatus
•    the banded, Scolytus schevyrewi and
•    the native Hylurgopinus rufipes
Other elm bark beetles and root grafts with neighbouring trees can also spread the disease.
Elm bark beetles are attracted to freshly pruned, sick, weak, or dying elm trees. The larvae develop in the inner bark. Adults that emerge from infected trees will fly to healthy trees and infect them with the fungus, causing the disease to spread.
All species of North American elm are susceptible. The disease now occurs in most of the natural range of Ulmus americana from Manitoba to the Maritimes, except for Newfoundland and Labrador. Saskatchewan is considered partially infested with Dutch elm disease.
Elm Pruning Bans
It is illegal to prune elm trees  during periods when elm bark beetles are active. Pruning the trees creates wounds that can attract the beetles and cause the disease to spread.
The Alberta restriction on elm pruining is from April 1 to September 30.

Birch Leaf Miner

The following tips will help you maintain a healthy birch tree:
Roots of birch trees need a cool, moist, shady location. Proper site selection is crucial for a long, healthy existence.
Fertilizing is best done in early spring at the onset of the growing season. Lawn fertilizer applications around the tree may be sufficient.
Prune any dead wood and remove the smaller of any branches that rub one another. Birch tree pruning is best done after the leaves are fully developed (June to July).
During the growing season, provide water during prolonged drought conditions. Thoroughly soak the area under and around thetree at least once a week if no rainfall.

Download the Tree Pruning Brochure

Don't Move Firewood - buy locally

Many people don't realize that moving firewood can cause the spread of plant diseases. Residents are reminded to not move firewood and to buy it locally.

Moving firewood from places where regulated pests have been found can be a violation of the Plant Protection Act, with penalties of up to $50,000 and/or prosecution. Be aware of movement restrictions that may be in place before you move firewood or any other wood products.
To protect our forest resources and communities’ greenspaces, moving firewood outside of a regulated area is prohibited. As the invasive alien species present in Canada have different distributions, it is important to be familiar with the pests present in your area.
See this brochure from the Canada Food Inspection Agency