DOSAGE-CONTROLLED STUDIES OF EELGRASS WASTING DISEASE: PHENOLOGY OF INFECTION AND FACTORS MODERATING VIRULENCE

Abstract

Coinciding with increased anthropogenic stressors and climate change, many marine diseases are on the rise. Understanding the epidemiology of marine disease that affect ecologically, economically, and culturally important species is a priority, but also a challenge given the nature of marine systems. In the first chapter of this thesis, we conduct a dosage-controlled inoculation method to enable quantitative investigations of aspects of phenology of eelgrass wasting disease infection, namely timescales and descriptors of characteristic infection, timescales of immunodynamics of newly established EGWD lesions and growth consequences of infection to the host over time. Observations of developing lesions showed that lesions first became visible 2 days post pathogenic exposure and leaf tissue can become totally necrotic in as soon as 12 days and furthermore suggested that the outcome of infection is largely contingent on the genotype of the host. Growth of plants that have been exposed to the pathogen are significantly lower than control plants by 12 days. Immuno-response as measured by induction of phenols is significantly higher in treatment plants by 12 days into the time series, and phenolic production is positively correlated with higher disease severity by the end of the experiment at 20 days. The second chapter investigates factors affecting severity of infection, namely pathogenic isolates, pathogen dosage, temperature, and light. Severity of lesions on eelgrass varied among the 3 different isolates inoculated in laboratory trials. Disease severity increased with pathogen dosage from 104 to 106 cells ml−1. In a dosage-controlled light and temperature 2-way factorial experiment consisting of 2 light regimes (diel light cycle and complete darkness) and 2 temperatures (11 and 18°C), L. zosterae cell growth rate in vitro was higher at the warmer temperature. In a companion experiment that tested the effects of light and temperature in in vivo inoculations, disease severity was higher in dark treatments and temperature was marginally significant. Although the details of these studies are specific to infection of the eelgrass Zostera marina by the protist Labyrinthula zosterae, it provides a general modeling framework for studying the role of within-host disease dynamics in other increasing marine diseases. Our work with controlled inoculation methods evaluate EGWD host-pathogen interactions narrowing the knowledge gap of phenology of infection and factors affecting virulence of a widespread and historically devastating disease.