The Purdue Plant and Pest Diagnostic Laboratory

What's Hot at the P&PDL for
June 8, 2010

Fireblight in Crabapples and Flowering Pear

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 several weeks.

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).



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Fire blight on pear

fire blight symptoms

Shepherd's crook

fire blight

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service