THE EFFECT OF APPLE BLOSSOM AGE ON POPULATIONS OF ERWINIA AMYLOVORA ON THE STIGMA SURFACE

Blossom infection forecasts and effective antibacterial sprays provide the best available control of blossom blight, the stage where orchard inoculum levels can rapidly increase and facilitate further severe infections. It has been shown that blossom age has an effect on colonizing bacteria population size on an apple stigma, but it is unclear if cultivar also has an effect. To test these effects, a greenhouse experiment with 4 replications was carried out on five apple cultivars (Ace Spur Delicious, Cameo, Gingergold, McIntosh and Royal Cortland). In each replication, stigmas of 2 blossom positions (king and side) and 3 blossom ages at inoculation (1, 3 and 5 days) per cultivar were inoculated with 1.2 × 10⁴ CFU of Erwinia amylovora. Each blossom age and position combination was a separate set of 4 trees. Blossoms were collected from each tree for 5 consecutive days and bacteria populations were estimated by dilution plating. The data were analyzed as incidence across the whole data set using logistic regression and then the subsets with counts greater than 10² CFU/ml were analyzed with mixed models. It was found that cultivar had a significant effect (p>0.05) for colonization of the apple stigma; but there was no significant effect for population. Blossom age had a significant effect on both colonization and population (p>0.01). Blossom position was not a significant effect for colonization or population levels, but the interaction with blossom age was significant for population levels (p=0.025). Days after inoculation were also significant for both colonization and population (p>0.005). These results show that blossom colonization and population are not necessarily controlled by the same factors. This may have implications for improving models to forecast blossom blight.