If you’ve noticed blackened branches or scorched-looking leaves on your trees this spring, fire blight might be to blame. This aggressive bacterial disease (not actually a fungus!) affects many common trees in Colorado, especially apple and pear species. And if left untreated, it can spread rapidly and kill large sections of a tree.
Here’s what Colorado homeowners and land managers should know about fire blight, how to spot it, and what to do next.
Is Fire Blight a Fungus?
No, fire blight is not a fungus — it’s a bacterial disease caused by the pathogen Erwinia amylovora.
This is a common misconception because it spreads and damages trees in ways that can look similar to fungal infections. But unlike fungi, which produce spores, fire blight bacteria multiply and spread through tree tissue, wounds, and blossoms, especially during wet spring weather followed by warmth.
What Is Fire Blight?
Despite its name, fire blight is caused by a bacterium called Erwinia amylovora, not a fungus. It infects trees through blossoms, wounds, and natural openings, especially during cool, wet springs followed by warm weather — which we often see across Colorado’s Front Range.
Once inside the plant, the bacteria multiply quickly and spread through the tree’s vascular system, damaging tissues and causing the “burnt” appearance that gives fire blight its name.
Trees Most at Risk in Colorado
Fire blight primarily affects members of the Rosaceae family, including:
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Crabapple trees (very common in Colorado landscaping)
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Apple and pear trees (both ornamental and fruiting varieties)
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Mountain ash
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Hawthorn
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Serviceberry
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Cotoneaster
These trees are popular throughout the state for their spring blossoms and hardiness, which unfortunately also makes them frequent fire blight victims.
How to Identify Fire Blight
Look for these telltale signs:
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Scorched blossoms: Flowers wilt suddenly and turn brown or black.
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Shepherd’s crook shape: Infected twigs curl downward like a cane.
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Oozing cankers: You may see amber-colored ooze coming from infected bark.
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Rapid dieback: Entire limbs can brown and die within weeks of infection.
Often, fire blight spreads so quickly that a tree can go from healthy to heavily damaged in a single growing season.
Why It’s a Big Deal in Colorado
Colorado’s changing spring temperatures and periods of moisture can create ideal conditions for fire blight to thrive. Once the bacteria are present, pruning alone may not be enough — and improper cuts can actually spread the infection.
Because many homeowners in areas like Denver, Boulder, and the foothills plant susceptible species, entire neighborhoods can experience outbreaks that move from tree to tree.
A large portion of this crabapple tree was pruned away because of fire blight. Pruning away dead branches and leaves is not advised in summer time.
What You Can Do: Prevention & Treatment
1. Prune infected branches.
Cut at least 8–12 inches below visible infection. Sterilize pruning tools between cuts with alcohol or bleach solution to avoid spreading bacteria.
2. Avoid excessive nitrogen fertilizer.
Too much nitrogen encourages lush, new growth — which is especially vulnerable to infection.
3. Apply a bactericide.
Copper-based sprays can help, especially if applied during bloom. However, they must be timed correctly and applied by professionals to be effective.
4. Remove severely infected trees.
In some cases, tree removal is the best way to prevent fire blight from spreading to nearby trees.
5. Call a certified arborist.
Professional tree care companies in Colorado are equipped to diagnose, treat, and manage fire blight outbreaks safely and effectively.
6. Learn more about fire blight from Colorado State University:
Need Help with Fire Blight?
If you suspect fire blight is affecting your trees, don’t wait. Early intervention can save the tree — and keep it from spreading to others nearby. At Splintered Forest, our ISA-certified arborists offer plant health care services across Colorado’s front range, including fire blight identification and treatment.
Call us today to schedule a tree inspection or request a free quote.