Apples, crabapples, and pears may frequently host either of the diseases bacterial blast or fire blight. Most landscapers are very familiar with fire blight, a bacterial disease caused by Erwinia amylovora. The symptoms include sudden, rapid death of stem tips resulting in “shepherd’s crooks” (bending over of stem tips), twig and leaf blight, and blossom blight. In wet conditions, droplets of bacterial ooze appear on the stems. The leaves, bark, and wood are discolored from the point of infection to the edge of the blighted tissues with no green tissue intervening. Commonly, we see bacterial blight in warm, wet weather.
Bacterial blast involves the bacterium Pseudomonas syringae but is caused by the presence of this bacterium only in freezing or near-freezing conditions. Blossom infection is most common, but green stems and leaves may be affected as well. Tissues turn brown or black as with fire blight, but symptoms may be spotty with green, unaffected tissues between affected tissues. There may be discolored sapwood, but this is often in streaks. On apple, crabapple, and pear, the cankered areas are often covered with a papery, flaky layer of bark. Wood underneath this layer is discolored. Pseudomonas is often present on plant material without causing noticeable injury under normal conditions. It is, however, known as an ice-nucleating organism. This means that it will cause freezing of water in plant tissues at or near freezing temperatures. The resulting injury is more severe than the injury from freezing alone. The Callery pears may host bacterial blast but are resistant to fire blight.
Earlier in the season, we saw several examples of bacterial blast on crabapple and pear. Although a frost is still possible, that is not likely, and we are hoping that we are finished with bacterial blast this year. Fire blight may be appearing now in some areas of the state. Watch for symptoms described above with droplets of cream- to bronze-colored bacterial ooze on the stems. For more on fire blight, consult Report on Plant Disease (RPD) No. 801, available on the Web http://www.ag.uiuc.edu/~vista/ horticul.htm.