Relation of Apple Flower Age to Infection of Hypanthium by Erwinia amylovora

Blossom age as related to hypanthial susceptibility to Erwinia amylovora is not well established, but is relevant to disease risk assessment. To test this, detached crab apple blossoms were maintained for various periods and at different temperatures before applying inoculum to hypanthia. Inoculum potential on hypanthia due to wetting was evaluated by subjecting detached stigma-inoculated blossoms (~10⁶ CFU per flower) to varying amounts and durations of simulated rain (or dew) at 14°C. Blossoms of varying age on mature 'Gala' apple trees were inoculated on hypanthia with 10², 10⁴, or 10⁶ CFU per flower. In the laboratory, susceptibility decreased with flower age at rates that increased with temperature. Wetness periods up to 12 h resulted in populations on hypanthia of <10³ CFU per flower; 24 h of wetness resulted in ~10⁴ or ~10⁵ CFU. A dose response was shown in the orchard, and regression curves indicated steepest decline of susceptibility during initial days after petal expansion. Disease models incorporating a blossom-age component may be effective because they indicate the potential for infection when temperatures favor rapid bacterial growth on stigmas within a window of high hypanthial susceptibility. Further investigation of these relationships could lead to advancements in determining fire blight risk.