Population dynamics of Erwinia amylovora on apple flower stigmas and effect of antibiotic treatment

Abstract: Fire blight, is caused by the bacterium Erwinia amylovora. Fire blight epidemics are typically initiated by blossom blight infections. Growth of E. amylovora to large population size on apple flower stigmas (106 to107 cfu per flower) typically precedes extensive outbreaks of blossom blight. We inoculated stigmas of 1-, 3-, and 5-day old flowers of field grown ‘Gala’ and ‘Fuji’ trees with a marked strain of E. amylovora and monitored populations over a 5-day period. We observed rapid increases in population size, for example from 103 to106 cells, within a single 24-hr period. As expected, older flowers did not support much E. amylovora growth. Apple growers in the U.S. have three antibiotic options to control fire blight: streptomycin, kasugamycin, and oxytetracycline. Streptomycin and kasugamycin are both bactericides while oxytetracycline is bacteriostatic. Since populations build over the course of bloom, the timing of when antibiotics are applied may aide in control of fire blight. We inoculated one day old flower stigmas with 1 × 103 cfu E. amylovora, and before or after inoculation, either streptomycin, kasugamycin, or tetracycline was applied to trees by air blast. Results indicated that streptomycin and kasugamycin suppressed bacterial populations when applied after inoculation. In contrast, oxytetracycline treatments initially suppressed populations, however, growth was recovered after 72 hrs.