ISHS Acta Horticulturae 704: X International Workshop on Fireblight
RELATION OF POMACEOUS FLOWER AGE TO INFECTION BY ERWINIA AMYLOVORA |
| Authors: | P.L. Pusey, T.J. Smith |
| Keywords: | fire blight, Malus, apple, crab apple |
| 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 24°C 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 μl of 108 CFU/ml) or hypanthium (2.5 μl of 109 CFU/ml) and incubated at 24°C. 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 14°C and intact apple flowers at a mean of 15°C. At a constant 24°C, 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. | |
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