Fire Blight in Fruit Trees: Spotting and Stopping an Outbreak

What Fire Blight Looks Like

Fire blight is a bacterial disease of apple, pear and their relatives, and the treatment is pruning. Cut infected wood out 4 to 8 inches below the canker in the dormant season, or 8 to 12 inches or more below visible symptoms in dry weather during the growing season. Clean the tools, burn the prunings, and hold back nitrogen. University of Illinois Extension says antibiotic sprays cannot cure the disease, only prevent it, so the sprays that exist are timed to bloom.

The classic sign is at the shoot tip. University of Wisconsin Extension describes tips that bend into the shape of a shepherd’s crook while the leaves turn dry and brown. Droplets of ooze sometimes appear on the shoots, and the color varies by source: milky white, orange or brown in Wisconsin’s description, whitish tan in Illinois’s. On branches, University of Illinois Extension describes cankers that are sunken and dark brown to purple.

The bacterium is Erwinia amylovora. Besides apple and pear, Wisconsin and Illinois list crabapple, quince, hawthorn, mountain ash, serviceberry, cotoneaster, pyracantha, raspberry and Callery pear as hosts, so an ornamental in the next yard can be the source.

Why It Spreads So Fast in Spring

The bacteria overwinter at the margins of cankers, according to Wisconsin. In spring they ooze out, and Illinois notes that insects are attracted to the ooze and carry it, along with splashing rain and wind. Open blossoms are the main entry point.

Warmth and moisture set the pace. Illinois reports that the bacteria begin multiplying at around 65°F, and at 70°F they can double every 20 minutes. Wisconsin states that they multiply rapidly on blossoms when rain, fog or relative humidity above 60 percent comes with temperatures above 65°F. University of New Hampshire Extension gives 75 to 82°F with humid or rainy weather as the conditions that favor repeated cycles through spring and summer.

Wounds open a second route. Wisconsin warns that severe outbreaks often occur about 5 to 10 days after hail or strong winds.

How Far Below the Damage to Cut

Washington State University notes that the bacteria can migrate well beyond the visible symptoms, so every source asks for a margin. They do not agree on its size, and the season changes it.

Season	Cut this far below	Source
Dormant	4 to 6 inches below the canker margin	Wisconsin, New Hampshire
Dormant	6 to 8 inches below the canker	Illinois
Growing season	6 to 12 inches below visible symptoms	Illinois
Growing season	8 to 12 inches below the infection	Utah State, New Hampshire
Growing season	At least 12 inches below visible symptoms	Wisconsin
Growing season	12 to 18 inches, into two-year-old wood	Washington State

The Washington State figure is its best-practice standard for orchards and comes from field trials. In the New York trials it reports, every tree that received no pruning treatment died, and none of the trees that had infections removed died. Cutting matters more than the exact margin.

When to Prune: Dormant Season or Summer

Illinois calls the dormant season the best time, because pruning then avoids spreading bacteria to healthy parts of the plant. Winter is when to remove cankers left from last year, alongside the other cuts in our late-winter pruning guide.

Waiting has a cost during an active outbreak. New Hampshire advises removing and burning infected twigs as soon as they are detected, especially in early summer. Wisconsin recommends inspecting trees weekly from about a week after bloom through July. Both Illinois and New Hampshire say to make summer cuts in dry weather.

Keep summer pruning to the infected wood. Utah State University Extension warns against heavy pruning in early summer because it stimulates succulent growth, which is very susceptible to blight.

Some growers leave a stub and remove it in winter. Washington State tested the practice of leaving a 5-inch stub, and in two of five experiments the longer stub reduced the number of cankers on structural wood. Very young trees are a separate case. Infections move quickly through them, and Washington State recommends removing and destroying the whole tree. If you are shaping a replacement, see pruning a young apple tree.

Cleaning Tools and Disposing of Prunings

Wisconsin’s method is to soak pruning tools for 30 seconds in a solution of 1 part bleach plus 9 parts water. New Hampshire lists the same 10 percent bleach solution, with a reminder to rinse and oil tools afterward, and also accepts 70 percent rubbing alcohol.

How often to disinfect depends on the season. Utah State says to disinfect between cuts when pruning out active fire blight in summer, and that pruners need not be sterilized between cuts during the dormant season. Washington State’s trials add a useful detail: when cuts were made 12 to 18 inches beyond visible symptoms, skipping sanitation did not significantly increase new symptoms in nine of nine experiments. With a generous margin the trials found no measurable penalty for skipping sanitation, and cleaning the blade still costs little.

Take the prunings away from the tree. New Hampshire’s instruction is to burn infected twigs and branches.

Lowering the Risk Before Next Bloom

Utah State describes succulent new growth as very susceptible to blight. Wisconsin advises avoiding excessive nitrogen, especially later than July 1, and Illinois warns against overfertilizing and excessive pruning. Wisconsin also recommends planting on well-drained sites with a soil pH of 5.5 to 6.5.

Sprays are preventive. Wisconsin describes copper compounds applied when buds show 1/4 inch of green tissue, and streptomycin at 3- to 4-day intervals during bloom. Illinois limits streptomycin to no more than three or four applications in a season. Check your state extension service for products labeled for home orchards, and follow the label. Copper is one of the materials compared in our guide to organic fungicides.

Variety choice does the most over the long run.

Fruit	Listed as resistant	Listed as susceptible
Apple	Liberty, Empire, Enterprise, Gold Rush, Winecrisp, Redfree, Freedom, William’s Pride	Gala, Fuji, Jonathan, Jonagold, Cortland, Rome, Idared
Pear	Magness, Moonglow, Maxine, Summercrisp, Harrow Delight, Harrow Sweet	Bartlett, Bosc, Anjou, Clapp’s Favorite

The resistant names come from the Illinois, Wisconsin and New Hampshire lists, and the susceptible names from New Hampshire. The lists do not always agree. Wisconsin places Idared among resistant apples while New Hampshire lists it as susceptible, so check the list from the extension service nearest you before buying.