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Fireblight is a plant disease of apples and pears caused by the bacterium Erwinia amylovora. It is native to North America where it resides in wild hosts such as crab apple and hawthorn. Over the decades, it has spread to other pome fruit producing regions in Europe, the Middle East, and New Zealand. But it is currently absent from South America, South Africa, East Asia, and Australia. Apple and pear orchards in the Pacific Northwest are at risk from this disease, which is even more challenging in other parts of the country.

A serious infection does not occur every year in every orchard, but constant vigilance is required. Because the most common infection route under Northwest conditions is through the blossom, monitoring and control is generally most critical at bloom.

The disease can be vectored in many ways, including honeybees, other insects, birds, rain, wind, and hail. Once a tree is infected, the bacteria travel downward in the tree’s vascular system and can kill the entire tree and can rapidly spread to infect an entire block. Thus, the economic ­consequences can be devastating.