Mountain Ash Problems: Fire Blight and Leaf Spot

Your 10-year-old mountain ash in Calgary shows two distinct issues: (1) yellowish-brown puffy spots on leaves, and (2) fire blight symptoms after a lightning storm. They are unrelated.

Fire Blight (Erwinia amylovora)

• Branch tips blacken, curl into a “shepherd’s crook.”
• Spreads downward; cankers form on branches/trunk.
• Management: Prune 12+ inches below visible cankers in late winter (dormant season). Disinfect tools (10% bleach) between cuts. Avoid summer pruning if possible. Remove severely infected trees.
• Prevent: Avoid excess nitrogen, which promotes succulent growth. Blossom sprays (streptomycin) in spring if history of disease.

Leaf Spot (Fungal/Bacterial)

• Spots vary: tan/brown with purple margins, sometimes concentric rings or black fruiting bodies.
• Caused by various pathogens; favored by wet foliage, overhead watering, high humidity.
• Management: Rake and destroy fallen leaves. Prune dead twigs. Improve air circulation. Avoid overhead watering, especially late day. Fungicides (mancozeb, chlorothalonil) only as preventative in spring—ineffective once spots appear.
• Mostly aesthetic; rarely kills the tree.

Key Distinction

Fire blight attacks woody tissue (branches/trunk); leaf spot only affects foliage. Treat each accordingly.

Action Plan

1. Prune fire blight cankers now (if summer) or wait for winter—cut well below damage, disinfect tools.
2. Rake all fallen leaves this fall; dispose (don’t compost).
3. Water at base only; mulch to reduce splash.
4. Monitor next spring; consider preventative fungicide at bud break if leaf spot was severe.

Bottom Line

Your tree has two manageable issues. Sanitation and proper pruning are the primary controls. The tree can live a long life with care.