Fire blight, a disease of apples and pears caused by a bacterium, first strikes by infecting flowers. During peak bloom in Iowa this year, conditions were near perfect for the disease to spread and cause symptoms. Also referred to as blossom blight, the symptoms appear as though the flowers were burnt, turning orange then black. Colorful ooze droplets may appear on the flower stem and nearby shoots will begin to turn orange and flag. The flagging shoot symptom is known as a shepherd’s crook due to the shape. Pears will turn directly black, whereas many apple cultivars remain orange for some time.
Besides damaging the flowers, this disease will kill immature apple and pear trees, sometimes within the same year as infection. The ooze droplets can spread from tree to tree as the apple shoots grow, increasing the damage.
Blossom blight symptoms will appear approximately 2 to 3 weeks post bloom. Swift removal of the blighted flowers will help save trees from further disease spread. If possible, remove at least 12 inches below the infected flowers and shoots for best results. The bacteria spread internally quicker than disease symptoms appear.
For summer of 2024, the Slack and Yuan fruit disease program received a grant to test for resistance in fire blight to streptomycin. If you used streptomycin and still had trees with active fire blight infections, please contact Suzanne Slack at slacksuz@iastate.edu for testing options.
For more information on fire blight, see the fire blight encyclopedia article.
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