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Effect of flower age and antibiotic treatment on the population dynamics of Erwinia amylovora on apple flower stigmas

Suzanne Slack: Michigan State University

<div>Fire blight, a disease of pome fruit, is caused by the gram-negative bacterium <em>Erwinia amylovora. </em>If left unchecked, fire blight can decimate an orchard in as little as one season. Pathogen growth on flower stigmas is a critical first step prior to the initiation of blossom blight symptoms. <em>E. amylovora </em>populations can attain sizes of 1x10<sup>7</sup> colony forming units (CFU) per flower if conducive environmental conditions occur. We examined the growth under field conditions of a marked <em>E. amylovora</em> strain on stigmas of flowers previously open for 1, 3, or 5 days. In 1 day open flowers, populations could increase 3-4 logs in 3 days with temperatures as low as 12C. Flowers open 3 days prior to inoculation also supported similar increases in population, however the response was cultivar dependent. Flowers inoculated after being open for 5 days did not reach carrying capacity, and the population crashed by the end of the sampling period. The results indicate that younger flowers are more conducive for <em>E. amylovora</em> population growth. Antibiotics were also applied before or after inoculation on 1 day old flowers and populations were tracked for 4 days. The results indicate the effectiveness of different antibiotic chemistries was dependent on weather and inoculation timing. From these data, we are developing a better understanding of how flower age, antibiotic timing, and weather influence <em>E. amylovora </em>population dynamics and flower infection.</div>