Fire blight

Pathogen and disease type. Fire blight is a devastating bacterial disease caused by Erwinia pyri, a phytopathogenic bacterium that attacks the vascular system of plants. It is classified as a quarantine organism in many countries due to its high transmission rate and potential to kill trees rapidly. The bacteria colonize the host's xylem, producing enzymes and toxins that cause wilting and necrosis of the plant tissues.

Host plants and severity. While the disease is named for its impact on pears, it affects many species in the Rosaceae family. The severity of Erwinia pyri infection is extreme, often resulting in the rapid death of blossoms, shoots, branches, and eventually the entire tree. In commercial orchards, this pathogen can lead to massive economic losses, necessitating strict management and immediate containment of infected areas.

Symptoms and indicators. The characteristic sign of the disease is the "scorched" appearance of infected plant parts. Blossoms wither and blacken, followed by the wilting of terminal shoots, which often bend into a distinct "shepherd's crook" shape. Leaves turn dark brown or black but remain attached to the tree. During wet weather, an amber or milky bacterial ooze may be observed exuding from cankers on the bark or from affected shoots.

Development and spread. Transmission primarily occurs through pollinators (bees), rain splash, wind, and contaminated pruning equipment. The pathogen thrives in warm, humid conditions, typically when temperatures are between 18°C and 28°C during the blossoming period. The bacteria overwinter in cankers on the branches and trunk, becoming active with the onset of spring and spreading into new growth through blossoms or wounds.

Control and prevention measures. Integrated management is essential, focusing on the sanitation and removal of infected wood, cutting at least 20-40 cm below visible symptoms. Proper tool sterilization is mandatory to prevent cross-contamination.

• Use resistant rootstocks and cultivars.
• Avoid excessive nitrogen fertilization to limit succulent shoot growth.
• Apply copper-based sprays as preventative treatments during the blossom period.
• Monitor the orchard regularly for early signs of infection.