Submitted to: Acta Horticulture Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2005
Publication Date: 2/1/2006
Citation: Pusey, P.L., Smith, T.J. 2006. Relation of pomaceous flower age to infection by Erwinia amylovora. Acta Horticulture Proceedings. v. 704. p. 127-129. Interpretive Summary: Fire blight, a serious bacterial disease of apple and pear trees, limits the production and international trade of pome fruit. The causal organism, Erwinia amylovora, becomes established on the stigmas of flowers during warm weather, and upon wetting by rain or heavy dew, moves to the floral cup where it invades through the nectary openings. The potential infection of flowers in relation to flower age has not been established. This information is important for accurate risk assessment needed by growers in deciding whether and when to apply control agents. Using 'Manchurian' crab apple trees in a greenhouse and 'Gala' apple trees in the field, experiments were conducted with pollinated and non-pollinated flowers of varying ages and in different temperature environments. It was found that disease incidence was relatively high only when flowers were 0 to 4 days old when inoculated with E. amylovora. These results support the use of a four-day degree hour accumulation, which is currently part of a fire blight risk assessment model used in the Pacific Northwest.
Technical Abstract: Potential infection of flowers by Erwinia amylovora in relation to flower age has not been established, but is relevant to fire blight risk assessment. This was investigated in conjunction with studies on stigma age and bacterial colonization. Crab apple flowers were collected daily from greenhouse trees, maintained by immersing cut pedicels in 10% sucrose, and incubated at 14 or 24C and 90% relative humidity. Half of the flowers were pollinated by hand. At 10 days, each flower was inoculated on stigmas (0.1-0.2 ÿl of 108 CFU/ml) or hypanthium (2.5 ÿl of 109 CFU/ml) and incubated at 24C. Stigma-inoculated flowers were evaluated for population size after 24 hours, and hypanthium-inoculated flowers were evaluated for disease after 5 days. A similar experiment was performed with mature 'Gala' apple trees in polyethylene enclosures. Flowers and buds were removed, leaving only newly opened flowers. Every 2 days, flowers were sampled, inoculated, and incubated, as described. Concurrently, additional flowers left on the tree were hypanthium inoculated, enclosed in polyethylene bag with added moisture for 48 hours, and evaluated for disease after 6 days. As reported previously, flower stigmas supported bacterial growth when inoculated at maximum ages from 4 to 10 days, depending on temperature and pollination. Disease incidence, however, was relatively high (60 to 100%) only when flower hypanthia were inoculated at ages 0 to 4 days; beyond 4 days, incidence declined rapidly. This was demonstrated with detached crab apple flowers held at a constant 14C and intact apple flowers at a mean of 15C. At a constant 24C, which is unrealistically high as a daily mean, disease incidence declined sharply when flowers were inoculated after the age of 2 days. Results support the four-day degree hour accumulation used in the Cougarblight risk assessment model.