Final Thesis Defense Plant Pathology Ph.D. Degree Seminar: Katherine Dougherty
April 28, 2026 1:00PM - 2:00PM
Systemic migration and population dynamics of Erwinia amylovora: from initial infection to disease management in apple
ZOOM info: contact Lauren Andring
Date: Tuesday April 28th
Time: 1:00 pm in Food Safety and Toxicology room 162
Members of the Examining Committee and their Department:
- Dr. George Sundin - Plant, Soil and Microbial Sciences
- Dr. Kerik Cox – Cornell University, Plant Pathology and Plant-Microbe Biology
- Dr. Ray Hammerschmidt – Plant, Soil and Microbial Sciences
- Dr. Michelle Hulin – Plant, Soil and Microbial Sciences
ABSTRACT
Fire blight, caused by the gram-negative bacterium Erwinia amylovora, is a destructive disease that threatens pome fruit production worldwide. Young trees are particularly vulnerable to the shoot blight phase, where rapid downward spread of E. amylovora from infected shoot tips often creates girdling cankers that kill trees. To understand the systemic movement of this pathogen, we quantified and tracked E. amylovora migration through infected shoot tissues in field studies. Over two years of field experiments, E. amylovora cells moved through shoots at an average velocity of 4.2 cm/day. Scanning electron micrographs revealed extensive colonization and exopolysaccharide production in the cortical parenchyma, indicating this tissue as the primary migration pathway. Bacterial populations exceeded 10⁹ cells/g throughout 20-day sampling periods, though levels varied with environmental conditions. These findings prompted investigation into the impacts of weather conditions on E. amylovora growth during shoot blight development. We measured E. amylovora populations and lesion length across 27 experiments over three years using 'Gala' apple trees in two experimental blocks. Mixed-effect beta regression modeling revealed that maximum temperature and moisture (leaf wetness hours and precipitation) most strongly influenced both population growth and lesion development.
While systemic E. amylovora movement often begins during blossom blight, the mechanisms governing migration through nectarthodes remain poorly understood. To elucidate virulence mechanisms behind nectarthode infiltration, we constructed deletion mutants targeting genes potentially involved in floral movement: cheA (chemotaxis), flhDC1 (flagellum), scr (sucrose metabolism), amsG (amylovoran production), and dspE (type III effector). All mutants moved through the hypanthium as rapidly as wild-type strains, suggesting that E. amylovora employs passive movement through nectar to reach the pedicle and spread systemically. Recent studies indicate that combining a plant growth inhibitor (prohexadione-Ca; ProCa) with a plant defense elicitor (acibenzolar-S-methyl; ASM) can reduce shoot blight severity without compromising shoot growth or fruit set. We applied reduced rates of ProCa (140.1 g/ha) and ASM (70 g/ha) for three consecutive weeks to four-year-old Gala trees, then tracked E. amylovora populations and movement following inoculation. These treatments significantly reduced bacterial velocity, lesion development, and ooze formation. Collectively, these studies advance our understanding of E. amylovorapathogenesis and will inform evidence-based management recommendations to prevent fire blight-related tree mortality.
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