Fire Blight

Erwinia amylovora, the bacterium that causes fire blight, over-winters in old cankers and oozes out of cankers in spring. The bacteria are then spread by insects, human activity (i.e., pruning, bud pinching, etc.), and by splashing rain. Blossoms are the most susceptible host tissue, so preventing blossom infection is critical for controlling fire blight. After bloom, bacteria can infect growing shoot tips and fruit, especially if inoculum is abundant because of failure to control blossom blight.

Fire blight only attacks actively growing trees! This is an essential principle for understanding fire blight and formulating control measures. In general, both the establishment of new infections and the extension of visible infections ceases when trees set terminal buds and are no longer producing new blossoms or leaves.

Almost all apple and pear cultivars will develop fire blight infections if they are exposed to high levels of inoculum and to weather conditions that favor severe infection during bloom. However, under less favorable conditions, the following factors dictate the incidence and severity of infections:

Geographic location: Risks of fire blight are greatest when in years when bloom periods are warm and wet.
Tree age: The greatest losses occur when fire blight becomes established in orchards that are 1-8 years old. Trees that have settled into full bearing are less vegetative and therefore suffer less damage than vigorously growing immature trees.
Cultivar susceptibility: Almost all pear cultivars are highly susceptible. Among apples, Honeycrisp, Gala, Jonagold, Lady Apple, Ginger Gold, Paulared, Cortland, Mutsu, Idared, and Rome are among the highly susceptible cultivars that are still popular in the Northeast. Many of the cider apple cultivars (those grown to produce hard cider) are especially susceptible.
Tree nutrition: High nitrogen levels can contribute to rat-tail bloom that lingers into late June and also extends the period of terminal growth, thereby enhancing tree susceptibility.

Fire blight can be completely controlled in most orchards if orchard managers are vigilant and utilize multiple control tactics.