In-Orchard Population Dynamics of Erwinia amylovora on Apple Flower Stigmas

Populations of the fire blight pathogen Erwinia amylovora Ea110 on apple flower stigmas were tracked over the course of apple bloom in field studies conducted between 2016 and 2019. In 18 of 23 experiments, flower stigmas inoculated on the first day of opening were found to harbor large (10⁶ to 10⁷ cells per flower) populations of E. amylovora when assessed 3 to 5 days postinoculation. However, populations inoculated on stigmas of flowers that were already open for 3 days did not reach 10⁶ cells per flower, and populations inoculated on stigmas of flowers that were already open for 5 days never exceeded 10⁴ cells per flower. During this study, ≥10-fold increases in E. amylovora stigma populations in a 24-h time period (termed population surges) were observed on 34.8, 20.0, and 4.0% of possible days on 1-, 3-, and 5-day-open flowers, respectively. Population surges occurred on days with average temperatures as high as 24.5 and as low as 6.1°C. Experiments incorporating more frequent sampling during days and overnight revealed that many population surges occurred between 10:00 p.m. and 2:00 a.m. A Pearson's correlation analysis of weather parameters occurring during surge events indicated that population surges were significantly associated with situations in which overnight temperatures increased or remained constant, in which wind speed decreased, and in which relative humidity increased. This study refines our knowledge of E. amylovora population dynamics and further indicates that E. amylovora is able to infect flowers during exposure to colder field temperatures than previously reported.