A dangerous mix: Strain, dosage, and environment increase virulence of eelgrass wasting disease

Seagrass wasting disease, caused by the opportunistic marine pathogen Labyrinthula zosterae, has the potential to devastate important eelgrass habitats worldwide, yet little is known about the host-pathogen interaction and how the disease will be impacted by climate change. L. zosterae is part of a diverse taxon of opportunistic invertebrate and plant pathogens, which directly threaten fisheries and critical fisheries habitat. An area of particular concern is the role of virulence, the degree of host damage caused by a pathogen, often the product of its growth rate. Recent data suggests that temperature increases the virulence of Labyrinthulas, providing a mechanism for climate sensitivity. In this study we investigate the effect of L. zosterae strain, pathogen dosage, and temperature on the pathogen virulence. We tested L. zosterae virulence in Zostera marina by inoculating plant tissue with strains collected from a range of eelgrass populations. The 11 strains tested displayed qualitatively different virulence, with infection rates ranging from 0 to 100%. Pathogen virulence increased with dosage. Growth rates at a range of temperatures were tested for a subset of these strains. Results suggest the temperature increases growth rates, but the degree differs between strains. We conducted a controlled temperature experiment in which Z. marina adults and seedlings were allowed to acclimate to low (11° C), high (18° C) and fluctuating (between 11 and 18° C) water temperatures and then half these individuals were exposed to L. zosterae. Disease occurred more rapidly and with higher severity in seedlings and at high temperatures. Our results show that pathogen virulence is impacted by strain, dosage, and environment and suggests L zosterae will cause increased damage to eelgrass beds as water temperatures warm