Mountain Ash Tree
(Sorbus spp.)
Mountain Ash is valued for berries, flowers, and fall color, but it can struggle in hot Front Range conditions. Common issues include fire blight, borers, aphids, leaf spot, and drought stress.
Seasonal Pest & Disease Calendar
| Problem | Type | Active Season |
|---|---|---|
| Fire blight | Bacteria | Spring–Summer |
| Leaf spot | Fungus | Spring–Summer |
| Scale insects | Insect/scale | Spring crawlers; visible year-round |
| Aphids | Insect | Spring–Fall |
| Borers | Insect/borer | Summer |
| Sunscald / heat stress | Abiotic | Summer–Fall |
| Winter injury | Weather | Winter–Spring |
Pest & Disease Guide (Homeowner-Friendly)
Fire Blight
Common symptoms- Blackened shoot tips
- Wilted blossoms
- Prune infected tissue
- Sanitize tools
Leaf Spot
Common symptoms- Leaf lesions
- Early drop
- Clean up leaves
- Improve airflow
Scale Insects
Common symptoms- Bumps on stems
- Canopy thinning
- Dormant oil
- Treat crawler stage
Aphids
Common symptoms- Honeydew
- Leaf curling
- Rinse foliage
- Encourage predators
Borers
Common symptoms- Exit holes
- Dieback
- Reduce stress
- Protect trunk
Heat Stress
Common symptoms- Scorch
- Early fall color
- Deep watering
- Protected planting location
Winter Injury
Common symptoms- Tip dieback
- Bark injury
- Prune deadwood in spring
- Avoid late fertilization
When to Worry vs. When It’s Cosmetic
When to Worry (action recommended)
- Repeated fire blight
- Heavy dieback
- Borer activity
These can lead to major decline in warm Colorado sites.
Mostly Cosmetic (monitor only)
- Minor leaf spots
- Light aphids
- Some scorch in heat
These often improve with watering and seasonal care.