Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/9/2007
Publication Date: 1/1/2008
Citation: Pusey, P.L., Smith, T.J. 2008. Relationship of apple flower age to infection of hypanthium by Erwinia amylovora. Plant Disease. 92(1):137-142. 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, colonizes the stigmas of blossoms during warm weather, and rain or heavy dew facilitates its movement to nectary openings where infection occurs. The relation of disease incidence to flower age has not been established, but is likely important for accurate disease risk assessment. This relationship was evaluated with detached crab apple flowers in the laboratory and mature ‘Gala’ apple trees in the orchard. Disease incidence decreased with flower age at a rate dependent on temperature. In the field a dose effect was also indicated. The information could lead to improvements in fire blight risk assessment models, allowing orchard managers to determine with greater confidence the effective timing of protective sprays.
Technical Abstract: Blossom susceptibility to infection by Erwinia amylovora in relation to flower age has not been established, but is relevant to fire blight risk assessment. Detached crab apple flowers were maintained with cut pedicels in 10% sucrose for various periods and at different temperatures before directly inoculating hypanthia. Similar inoculations were performed with mature ‘Gala’ apple trees in 2005 and 2006 using inoculum doses of 102, 104 and 106 CFU per flower. Both experiments demonstrated that flowers become less susceptible to invasion of the hypanthium as flowers age. Flower susceptibility in the laboratory was also shown to decrease more rapidly as temperature increased, and a dose effect was indicated in the orchard. Disease models are based on bacterial growth on stigmas in response to temperature prior to wetting and transferal of cells to the hypanthium. The Cougarblight model uniquely involves a 4-day temperature evaluation, the success of which cannot be entirely explained by growth of E. amylovora, as bacteria are reported to increase on stigmas for longer periods. The importance of rapid bacterial multiplication over the first few days after flower expansion appears critical because this is when flowers are most susceptible to invasion of hypanthial tissues.