How cool temperatures affect the incidence and population growth of Erwinia amylovora on the apple stigma

Cool temperature (below 17.8C) effects on epiphytic Erwinia amylovora (Ea) populations on the apple stigma have never been fully described. Five apple cultivars Cameo, Gingergold, Royal Cortland, McIntosh, and Delicious were inoculated with three Ea strains (Ea 273, Ea 321, Ea 4001a) and phosphate buffer control at three continuous temperature treatments (11, 18, and 25C). For each temperature treatment, strains were inoculated on separate branches of the same tree replicate. Blossoms were collected over a 3-day period and the populations of bacteria were quantified with real-time PCR and a Taqman probe. The data were analyzed as a repeated measures experiment as both incidence (colonization) and population data. Temperature was recorded throughout the experiment and was used to calculate cumulative degree-hours (CDH). The colonization data were fitted with CDH and the best base was determined to be 10C. Cultivar did not have a significant effect on colonization but strain of Ea, day after inoculation, and the interaction cultivar x CDH did have a significant effect (P < 0.05). The best model for population of Ea on a stigma surface had significant main effects (P < 0.05) of CDH (base 5C) and strain of Ea. Colonization appears to be the rate limiting step in the orchard spread of Ea, so greater attention to modeling this aspect of fire blight epidemiology is necessary for the improvement of fire blight forecasters.