Fire Blight - Be on the Look-out

Gail Ruhl, Sr. Plant Disease Diagnostician, Dept. of Botany & Plant Pathology, Purdue University

Fire blight, caused by the bacterial pathogen, Erwinia amylovora, affects over 130 plant species in the rose family. The bacterium infects blossoms in wet weather with the optimal temperature range of 65 to 86 degrees F. This disease will be a problem this year.

Symptoms of fire blight on Rosaceaceous hosts such as apple, crabapple, pear, mountain ash and cotoneaster include dieback and ‘firing’ of branch and leaf tissue. Look for water-soaked or wilted new growth that quickly turns brown to black and remains attached to the stem, as in the image.

Look-alike problems include frost injury, heat damage (fire from controlled burns around trees), chemical damage, and possibly Psuedomonas blast, a condition that occurs in near-freezing weather. In the lab, we confirm the presence of bacteria by observing bacterial streaming with a microscope. That tissue is cultured in the lab to confirm the bacteria isolated as Erwinia species.

Because the bacterium originates on old cankers and bark infection, removal of branches killed by fire blight is extremely helpful in disease control. This should be done when the tree is dormant or when weather is hot and dry. Do not remove infected wood in the spring. Disinfecting pruning equipment is an important step in reducing disease spread. Use 10% household bleach or similar disinfectant between cuts. Cut out infected branches, going 6 to 8 inches into the good wood to be certain all infected tissue is removed.

There is little benefit from any bactericide sprays this late in the season. However, 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

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Bacterial streaming observed with microscope

Fire blight on pear

Fire blight on ornamental pear

Fire blight on ornamental pear

Fire blight on crabapple

Fire blight on crabapple