Fire blight

Note on taxonomy: The pathogen causing the destructive disease known as "Fire blight" is the bacterium Erwinia amylovora. The term Lactobacillus amylovorus is not the accepted scientific name for this plant pathogen. This guide focuses on the control and biology of the actual causal agent, Erwinia amylovora.

The causative agent of fire blight is Erwinia amylovora, a gram-negative bacterium that belongs to the Enterobacteriaceae family. It is a highly destructive quarantine pest. The pathogen colonizes the xylem, producing substances that cause systemic wilting and tissue necrosis throughout the host plant.

The disease primarily affects members of the Rosaceae family, including apple, pear, quince, hawthorn, mountain ash, and cotoneaster. Pear trees are particularly susceptible, often showing rapid, devastating symptoms that can kill a mature tree within a single growing season if left untreated.

Symptoms are distinct: blossoms wilt and turn black, followed by the appearance of "shepherd’s crook" on young shoots. Leaves turn brown or black but remain attached to the branches. In humid weather, amber-colored bacterial ooze may be visible on infected branches, stems, and fruits, which is a hallmark of active infection.

Fire blight thrives under warm and humid conditions, especially during the bloom period when temperatures exceed 20°C. Primary transmission is mediated by pollinators (bees and flies), as well as splashing rain, wind, and contaminated pruning tools. The bacteria overwinter in cankers on the woody parts of the tree.

Management requires a strict sanitation program to minimize the inoculum load:

• prune and destroy infected branches at least 30 cm below the visible necrotic tissue;
• sterilize tools with 70% ethanol or sodium hypochlorite solution between every cut;
• minimize excessive vegetative growth through careful nitrogen fertilization management;
• apply copper-based bactericides during dormancy and bloom, following local regulations.