USDA-ARS, Tree Fruit Research Laboratory, 1104 N. Western Ave., Wenatchee, WA 98801
Detached crab apple flowers were used as an experimental model to investigate the effect of relative humidity (RH), free moisture, and water potential Ψw on the interaction between Erwinia amylovora and pomaceous flowers. Flowers were maintained at 24°C with the cut pedicel submerged in a sucrose solution. The bacterium multiplied on inoculated flower stigmas at between approximately 55 and 100% RH but not in the floral cup (hypanthium) until the RH was higher than 80%. To study the effect of free moisture, stigma-inoculated flowers were kept wet for different periods. Flowers became diseased only with wetting, and incidence was high (77%) even when water application was immediately followed by a 52-min drying period. In other experiments with hypanthium-inoculated flowers, RH or sucrose concentration in holding vials was varied to affect Ψw of flower nectar and ovary tissue. Population size of E. amylovora in the hypanthium increased with nectar Ψw following a sigmoidal curve (R2 = 0.99). Disease incidence and severity, however, were more closely related to ovary Ψw (R2 = 0.85 and 0.91, respectively) than to bacterial population size (R2 = 0.25 and 0.67, respectively) as fitted to the quadratic equation. Maximum disease incidence and severity occurred at an ovary Ψw above -2.0 MPa, and disease severity continued to increase above -1.0 MPa. These results were confirmed with detached flowers of Delicious apple and d'Anjou pear. A practical implication is that disease might be partly managed in arid climates by limiting soil irrigation water during bloom and early fruit set.