Fire blight is a bacterial disease caused by Erwinia amylovora, which primarily targets plants within the rose family, known as Rosaceae. This pathogen affects fruit trees, particularly apples and pears, as well as many ornamental shrubs like hawthorn and cotoneaster. The disease’s name comes from the scorched, burned appearance it leaves on infected plant tissues. Because fire blight spreads rapidly, visual identification of the characteristic symptoms in the early stages is important for effective management.
Early Symptoms and Shoot Damage
The first visual signs of fire blight typically appear on the new growth of the plant during the spring and early summer. Blossoms often serve as the initial entry point, quickly turning brown or black and appearing water-soaked before shriveling. This “blossom blight” may rapidly extend into the supporting spur.
New shoots and young twigs are also susceptible, showing a sudden wilting that progresses to a dark discoloration, which is black on pears and often a rusty-brown on apples. A distinct visual cue is the “shepherd’s crook,” where the terminal end of the infected shoot bends over, resembling the curved staff. This curling occurs as the bacteria kill the tissue and the tip wilts.
The leaves on these infected shoots also turn brown or black, but they typically remain firmly attached to the branch throughout the season, creating the characteristic “fire-damaged” look. The rapid death of new growth causes the plant to appear scorched.
Appearance of Cankers on Woody Tissue
As the infection moves beyond the new growth, it manifests on older, woody parts of the tree, such as branches, limbs, and the trunk, forming cankers. These cankers are areas of dead, sunken bark that represent the chronic, overwintering stage of the disease. The bacteria survive the dormant season within the margins of these lesions.
The canker appears as a dark, discolored patch on the bark, often slightly sunken compared to the surrounding healthy tissue. The margin between the infected and healthy wood may be slightly raised or appear water-soaked when the canker is actively spreading. Peeling back the bark in an active canker area may reveal a distinct reddish-brown discoloration in the underlying inner bark.
Cankers reactivate in the spring, producing a sticky exudate known as bacterial ooze. Cankers on the main trunk are damaging because they can girdle the tree, cutting off nutrient and water flow, which leads to the death of the plant.
Visual Markers That Confirm Fire Blight
The presence of bacterial ooze distinguishes fire blight from other dieback issues. This ooze is a milky, slimy, or sticky liquid forced out of infected areas, including cankers and new shoots, especially during warm, humid weather. The color ranges from light tan or pale yellow to darker amber, darkening upon exposure to air and often leaving dark streaks on the bark.
The tenacious attachment of dead foliage is another key marker, as the blackened or brown leaves cling to the branches long after they would normally drop. This retention of dead tissue, combined with the shepherd’s crook, confirms fire blight infection.
The disease is confined to specific members of the Rosaceae family. Observing these susceptible host plants displaying the combination of scorched new growth, shepherd’s crook, and bacterial ooze confirms the diagnosis.