How cool temperatures affect Erwinia amylovora populations on the apple stigma surface

The effect of cool temperatures (below 17.8 degrees C) on epiphytic Erwinia amylovora populations on apple stigmas has never been fully described. The most difficult situation is to determine whether antibiotic application is warranted for blossom blight after several cool days in the spring. Better knowledge of the effect of cool temperatures on epiphytic populations can aid the design of improved fire blight forecasting systems. In this study five cultivars (Cameo, Gingergold, McIntosh, Red Delicious, and Royal Cortland) were placed at three temperatures (10 degrees C, 17 degrees C and 24 degrees C). The blossoms were inoculated with 8.0 x 10(3) CFU drops of three E. amylovora strains (Ea 321, Ea 273 and Ea 4001a). There were five trees per cultivar for each temperature and the strains were inoculated on separate branches of the trees. The experiment was conducted twice. Blossoms were collected for three consecutive days and the bacterial populations were quantified with real-time PCR using a Taqman probe. The data were analyzed as a repeated measures experiment with both incidence and population as dependent variables. In general, the blossoms of some cultivars were more readily colonized by E. amylovora regardless of temperature. Gingergold and Royal Cortland blossoms were most frequently colonized. Ea 4001a colonized stigmas most consistently on all cultivars and at all temperatures. Populations were greatest, ranging from 10(6) to 10(8) CFU, on stigmas at 24 degrees C. Population increases on the stigma surfaces were observed at 10 degrees C but at lower populations, up to two days were required in some instances to become detectable at a range between 10(5) to 10(6) CFU. This study has shown that the effect of cool temperatures is not identical for all apple cultivars and E. amylovora strains. These factors should be taken into consideration when modeling the effects of temperature on E. amylovora populations.