Watch for Tree Fungus This Fall
Contributed by Jolene Ottosen for the Chaparral Green Thumbs
As leaves turn and trees shed their summer coats, fall is the perfect time to check your trees and shrubs for signs of disease. Many fungal problems are easier to treat during the dormant winter season, so spotting them now gives you a head start.
Below are some common tree fungi found in Calgary, along with their symptoms and treatment tips. If pruning is needed, always use clean, sharp tools and disinfect between cuts. Bag and dispose of infected branches in your black garbage bin—never compost them at home or place them in the green bin. Fallen leaves, however, can go in your green bin since the City’s composting facilities reach high enough temperatures to kill most fungi (unlike home compost piles).
If a fungicide is recommended, check with a local arborist or garden centre to ensure you’re using the right product and applying it properly.
Apple Scab
• Symptoms: Brownish-green scabs on leaves and fruit; curled, shrivelled leaves; early leaf drop; cracked or deformed fruit.
• Treatment: Rake and dispose of fallen leaves and fruit. Apply fungicide in early spring before bud break. Prune to improve air circulation.
Black Knot
• Symptoms: Black, swollen growths (“knots”) on branches, common in Mayday, chokecherry, and other cherry trees.
• Treatment: Prune infected branches at least six to eight inches below the knot during late winter. Disinfect tools between cuts. Bag and trash (or burn) pruned material.
Bronze Leaf Disease
• Symptoms: Reddish-brown leaves that remain on the tree, especially in Swedish columnar aspens and tower poplars.
• Treatment: Prune infected branches in late fall or winter. Thin dense growth to improve airflow. Avoid planting aspens too close together.
Cedar Hawthorn Rust
• Symptoms: Yellow spots on young leaves that turn orange with black centres; lesions on undersides may cause early leaf drop.
• Treatment: Remove and dispose of infected leaves. Apply fungicide in early spring. Consider removing nearby junipers if rust keeps recurring.
Cytospora Canker
• Symptoms: Browning needles, branch dieback, and white resin on affected areas of stressed conifers.
• Treatment: Remove infected branches. Water trees during dry spells and avoid bark injuries. Space trees for good airflow and reduced humidity.
Fire Blight (Apple and Mountain Ash)
• Symptoms: Oozing cankers on branches and trunk; affects apple, pear, mountain ash, hawthorn, and cotoneaster.
• Treatment: Prune infected branches during dormancy, cutting eight to twelve inches below visible symptoms. Disinfect tools. Apply copper-based fungicide in spring to prevent spread.
Needle Cast (Spruce)
• Symptoms: Browning needles that drop, often with rows of tiny black bumps.
• Treatment: Apply fungicide in spring and repeat three to four weeks later. Improve airflow and avoid overhead watering. Use light mulch and plant in well-drained soil.
Septoria Leaf Spot and Canker (Poplar)
• Symptoms: Brown spots with lighter centres on leaves; yellowing and early leaf drop; bark may turn orange and ooze brown liquid.
• Treatment: Remove and dispose of infected leaves. Improve air circulation. Apply fungicide in spring. Support tree health with proper watering and mulching.
Final Tip: Healthy trees are more resistant to disease. Keep them watered, avoid bark damage, and give them space to breathe. If you’re unsure what you’re seeing, take a photo and check with a local arborist or garden centre—they’re usually happy to help.
As always, happy gardening!
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