Caused by the bacterium Erwinia amylovora. Most commonly affects apple, pear, pyracantha and other plants in the Rose plant family. It has been known to destroy large commercial orchards.
In our experience, outbreaks are triggered when trees are in flower and warm wet rainfall events occur. The pathogen spores activate during these conditions and honeybees and other pollinators spread it from tree to tree.

Infections can be carried onto your tree from an infected neighboring tree. Treatment strategies are complicated and can be expensive. The use of antibiotics and copper sprays can only slow the spread and several repeated applications are necessary. Anecdotally, foliar sprays of Soil Restore biofertilizer seem to provide a degree of protection. Summer pruning to reduce inoculum source and careful management of fertilizer applications is important.