By Jill Calabro, Ph.D. Science & Research Programs Director AmericanHort and HRI

For the last 30 years every spring and summer, customers have been bringing their fire blight samples to Phelan Gardens and Earth Expressions Landscape in Colorado Springs. “Some years are worse than others but 2018 seemed to be an exception,” said company co-owner Mark Phelan.

“In normal years, we receive samples from our Colorado Springs customers that we believe to be fire blight at an approximate rate of one per week. Last year, in early June, we were receiving five to 10 fire blight samples per day!” Phelan recalled.

This phenomenon did not appear to be affecting solely the Colorado Springs area, rather it was across the entire Front Range. The Phelan Gardens staff concurred with the assessment of Boulder County CSU Extension Agent Carol O’Meara, posted June 6 last year, where she stated that “fire blight is rampant.”

To help reduce widespread fire blight infestation in coming seasons, CNGA asked Dr. Jill Calabro of AmericanHort and HRI to provide helpful information about this pest, including how to reduce risk factors of being infected by it.

Fight the (Fire) Blight

It’s common knowledge that certain bacteria can make people sick, but did you know that bacteria can cause plant diseases as well? They most certainly do! The first bacterial pathogen to be identified was anthrax, a disease affecting sheep and cattle, in 1876. Shortly after that, the first bacterial plant disease was confirmed — fire blight on apples and pears.

Fire blight is likely native to North America and has colonized most of the U.S. and Canada. While outbreaks are irregular, reports of devastation to apple, pear and other hosts were widespread in 2018, particularly in the western U.S. Areas in Colorado, Washington and California experienced extended favorable weather conditions, and fire blight flourished.

Fire blight is very difficult to control once established and can destroy entire orchards in a single season, if conditions are just right. Infected plant tissue cannot be cured. This is partly due to the fact that fire blight infections extend up to 2 to 3 inches in wood beyond any visual signs and symptoms. Infected tissue can (and should) be removed and destroyed, and clean plant tissue can be protected, but it is likely the tree will harbor the bacteria indefinitely. Disease management, as opposed to control, is key with fire blight.

If you’ve spent any significant length of time with a plant pathologist, you might have heard the term ‘disease triangle.’ It’s a model to help visualize how interactions between the plant, the environment and a pathogen result in disease development or not. For example, if a susceptible host is present (such as Snowdrift crabapple), but the pathogen (in this case a bacterium) is not present or active, then fire blight won’t be a problem. Likewise, if the environment is not conducive to disease (such as hot, dry weather), then fire blight won’t be an issue. Altering one or more of these three factors can impact fire blight development in a positive way for the grower.

Typical fire blight canker on pear. Note the darkened, sunken, water-soaked appearance.


Fire blight depends on favorable weather conditions associated with most springs: namely warm weather (between 65 and 85°F) accompanied by intermittent rains or high humidity. Throw in a little hail storm that damages tender shoots, leaving open wounds ideal for pathogen entry, and the environment is prime for infection. While ambient temperatures cannot be altered, humidity can be lowered with techniques such as pruning and weed management. These practices increase airflow, which will help lower disease susceptibility.


Fire blight infects more than 200 different species in the Rosaceae family, including crabapples, pears, apples, Pyracantha, and blackberry. Some cultivars are more susceptible than others, so planting cultivars more tolerant of fire blight will help minimize overall damage. Beyond that, the way the host is managed can lower disease severity, too. Fire blight prefers succulent growth; therefore, avoid actions that encourage new, vigorous growth, such as excessive nitrogen fertilization, high soil moisture, and aggressive pruning. These are all ways to manage the host to slow disease.


In the case of fire blight, this vertex on the disease triangle is toughest. Most plant diseases we know are caused by fungi, which tend to be easier to control with fungicides. Plant diseases not caused by fungi, however, pose a much greater challenge in terms of control. Humans and animals often rely on antibiotics for bacterial disease control, and in some situations, antibiotics are used in agriculture, such as citrus greening in oranges. Nursery production and landscape management situations are far less suitable for antibiotic use, so other control strategies are needed. Copper sprays are somewhat effective against the bacterium, especially to protect new, clean growth, but efficacy is limited under high-disease pressure.

Finally, removing the pathogen itself can help reduce inoculum in subsequent growing seasons. Since the pathogen resides in infected plants, cankered tissue should be removed in winter months, when the pathogen is not active, and then destroyed. When removing a cankered branch, find the lowest part of the canker, trace the branch to its point of attachment, and cut at the next branch juncture. In the case of large cankers on trunks, scrape down the bark around the canker to the cambium. Pay attention to wood color as it can be an indicator of infection. Wood directly underneath a canker is usually dead and turns a red color further out, eventually becoming red flecks. Beyond that, the tissue is healthy.

Fire blight is difficult to manage but not hopeless in most years. Management should focus on environment and host modifications as a means of reducing disease pressure. Cultural practices combined with mechanical strategies can help, but in some years (like 2018), almost nothing seems to help.

About the contributor: Jill Calabro manages all things science-y for AmericanHort and the Horticultural Research Institute (HRI), including HRI’s annual grants program. She also promotes HRI-supported research results and dabbles in regulatory advocacy to help ensure success of the green industry.