Fireblight in Crabapples and Flowering
Gail Ruhl, Senior Plant Disease Diagnostician,
Department of Botany & Plant Pathology, Purdue University
Symptoms of fireblight have recently become
more evident following several weeks of elevated temperatures. Fireblight
is caused by the bacterium Erwinia amylovora. It affects over
130 plant species in the rose family including apple, crabapple,
pear (including ornamental pear), mountain ash, hawthorn and
cotoneaster. It also occurs on Rubus species such as red and
black raspberry and thornless blackberry.
The bacteria overwinter in cankers formed during
the previous years’ infections and as “resident” bacteria
on plant surfaces. During wet spring weather the bacteria ooze
out of cankers and are spread by rainsplash, wind and insects to
blossoms. This causes “blossom blight.” The pathogen
can also infect succulent new leaves and shoots, especially tissue
that has been wounded by pruning, hail, or insects The diseased
shoots turn dark brown or black as though scorched by fire and
often exhibit a curled tip, referred to as a shepherd’s crook.
This is the symptom that has become most evident over the past
There are several cultural practices to help manage
fire blight. The pathogen is most infective on lush, new growth,
thus avoid any cultural practices, such as high nitrogen fertilization,
that stimulate rapid tree growth and excess branch proliferation.
Shoots can be pruned out in the summer, but do not prune in wet
weather. Make the cuts at least 12-18 inches below the lowest visibly
diseased tissue. Disinfect tools between each cut with 70% ethanol
or 10% bleach (if you use bleach be sure to clean and oil your
tools after use to avoid damage from corrosion). Infected branches
can be marked now (with something like colored yarn) and pruned
in late winter to remove dead shoots and cankers. As with summer
pruning make the cut below visible diseased tissue, at least 8-10
inches to be safe, and sanitize tools between cuts.
Chemical controls are available but are most effective
at bloom so this year we have passed the window of opportunity
for chemical protection; concentrate on use of cultural practices.
If fire blight continues to be a yearly problem, a copper-based
fungicide like Bordeaux or a fire blight spray containing streptomycin
sulfate can be applied. Follow directions on the label for application
procedures. Read the label carefully and apply only as directed.
Commercial growers refer to recommendations
in the Midwest
Tree Fruit Spray Guide (pdf file).