Fireblight is a contagious disease affecting apples, pears, and some other members of Rosaceae.

The causal pathogen is Erwinia amylovora, a bacterium. Pears are the most susceptible, but apples, crabapples, quinces, hawthorn, pyracantha, and some other rosaceous plants are vulnerable.

A fireblight infection is systemic. It spread downward from the point of infection to the roots of the plant. The only effective treatment is to prune off the affected branches. The rest of the plant can be saved if the blighted wood is removed before the infection spreads to the roots. Regular sprays of the antibiotics streptomycin or terramycin or certain biological controls consisting of beneficial bacteria can prevent fireblight from infecting new trees. Incorrect use of such sprays has led to streptomycin-resitant bacteria in some areas.

The bacteria are spread by wind, raindrops, and via by birds and insects. Honeybees can carry the disease to flowers.

The disease spreads during hot, wet weather. Blossoms and new growth are the most susceptible portions of the tree. Overpruning and overfertilization (especially with nitrogen) can lead to watersprouts and other midsummer growth that leave the tree more susceptible.

Fireblight is not present in Australia.