Population Dynamics, Route of Infection, and Velocity of Systemic Spread of <i>Erwinia amylovora</i> in Infected Apple Branches

Fire blight, caused by <i>Erwinia amylovora</i>, is one of the most devastating diseases of apple and pear worldwide. Young trees are particularly susceptible to the shoot blight phase of the disease, and the rapid downward spread of <i>E. amylovora</i> from infected shoot tips throughout trees to the rootstock often results in the formation of girdling cankers that kill trees. We quantified and tracked the systemic migration of <i>E. amylovora</i> cells in field studies through infected shoot tissues to gain insight into the systemic movement of the pathogen. In 2021 and 2022, bacterial populations were monitored over a 20-day period in defined sections of 'Gala' apple shoots in replicated field experiments. <i>E. amylovora</i> reached populations >10⁹ CFU g^-1 and maintained high populations in shoot tissue throughout the 20-day sampling period under conducive environmental conditions. <i>E. amylovora</i> cells migrated through shoot tissue at a maximum of 49.5 cm at 5 days after inoculation (9.9 cm day^-1) and exhibited an average velocity of 4.2 cm day^-1. The rate of migration through the new growth was 5.4 cm day^-1, and further investigations using scanning electron microscopy did not reveal major obstructions at the bud scar. Microscopic examination of infected shoot tissue enabled us to detect prolific colonization and bacterial ooze formation in the cortical parenchyma. Our study refines the fundamental knowledge of <i>E. amylovora</i> systemic colonization during shoot blight and contextualizes previously divergent studies of colonization from the past 50 years.