Potential Impacts of 2008 Weather on Fire Blight in 2009

The 2008 apple growing season was a huge challenge for growers. Early frosts reduced crop size, and hail throughout the province during the summer also reduced yields. Research indicates that these weather patterns may impact tree susceptibility to fire blight next year.

Researchers at Ohio State examining impacts of differing crop loads found that crop load can increase tree susceptibility to fire blight in subsequent years. In a 3 year experiment on Golden Delicious (which is not considered to be very susceptible to fire blight), many of the trees which were completely deflowered were severely infected with fire blight.

When flowers were removed for three consecutive years, trees had elevated carbohydrate levels in their tissues. These excessive carbohydrates resulted in thicker leathery leaves, which provided an optimal growth medium for the fire blight bacteria. As a result many of the deflowered trees were killed by fire blight.

Young non-bearing trees that are growing rapidly also develop high levels of carbohydrates which may make them more susceptible to fire blight. Late spring frosts or freezing create similar conditions, making the trees more vulnerable to fire blight.

As a result, growers in Ontario that experienced frost damage that removed flowers should be aware of the increased risk for fire blight in 2009. Monitor carefully for fire blight cankers in the orchards this winter, and monitor weather conditions in the spring, to be ready with control measures where necessary, especially on susceptible cultivars.

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