A growing body of evidence supports the observation that marine disease outbreaks, especially those caused by opportunistic pathogens, are increasing in frequency and severity. One genus of such pathogens, Labyrinthula, has been identified as the causative agent of seagrass wasting disease, an epidemic that has historically plagued seagrass beds around the world. It is suspected that pathogenicity is intimately linked to the ability of the host to initiate defense responses, but a lack of compelling evidence prevents any meaningful application of preliminary observations. This body of work investigated the roles of host genotype, host immune status, and environmental stressors in dictating the susceptibility of Thalassia testudinum (turtlegrass) to seagrass wasting disease, through two investigational studies. The first, a lab-based study, addressed the deficit in empirical methods through the development of techniques that measured: 1) Labyrinthula loading in host tissue through a novel qPCR-based assay and 2) immune status in the seagrass host via four immune biomarker assays, measuring peroxidase (POX), exochitinase (EXOC), polyphenol oxidase (PPO), and lysozyme (LYS) activity. These methods were used to analyze turtlegrass individuals exposed to 1) abiotic stressors alone or 2) abiotic stressors followed by pathogen-challenge, in a controlled laboratory setting. The qPCR assay successfully quantified pathogen loading in seagrass tissue with high specificity. All four biomarkers were constitutively active in host tissue, but expression was largely unaffected by the chosen abiotic stressors. There were significant positive relationships between pathogen loading and two of the four biomarkers (POX and EXOC), regardless of abiotic stress treatment. Finally, despite the widely variable response among individuals, regardless of treatment, we identify a potential trade-off mediated immune response in T. testudinum, when faced with pathogen invasion. The second investigation was a field study conducted in Florida Bay, a shallow, subtropical estuary characterized by many spatiotemporally unique basins, where T. testudinum dominates. Samples collected from 15 representative sites were analyzed using the methods developed in the first study as well as historical monitoring databases, in an effort to identify ecologically significant trends that existed in patterns between: 1) pathogen loading and immune status; 2) pathogen loading and geographic site; 3) pathogen loading and morphometric characteristics; 4) pathogen loading and water quality data; 5) immune status and geographic site; 6) immune status and morphometric characteristics; and 7) immune status and water quality data. The results revealed that both pathogen loading and immune status varied as a function of location in Florida Bay. Furthermore, based on the trends observed among and between sites with regards to pathogen loading, immune status, leaf morphology and water quality, a mechanism in which all four of these parameter sets interact is proposed as a potential explanation for the differences observed in Labyrinthula prevalence and severity within the bay. The results of both investigations address whether wasting disease susceptibility is driven primarily by variability in the environment or in the host species, and provide valuable insight regarding the extent to which seagrasses possess the capacity for resilience against marine pathogens.