High infectivity and waterborne transmission of seagrass wasting disease

Pathogen transmission pathways are fundamental to understanding the epidemiology of infectious diseases yet are challenging to estimate in nature, particularly in the ocean. Seagrass wasting disease (SWD), caused by <i>Labyrinthula zosterae</i>, impacts seagrass beds worldwide and is thought to be a contributing factor to declines; however, little is known about natural transmission of SWD. In this study, we used field and laboratory experiments to test SWD transmission pathways and temperature sensitivity. To test transmission modes in nature, we conducted three field experiments out-planting sentinel <i>Zostera marina</i> shoots within and adjacent to natural <i>Z. marina</i> beds (20 ± 5 and 110 ± 5 m from bed edge). Infection rates and severity did not differ among outplant locations, implicating waterborne transmission. The infectious dose of <i>L. zosterae</i> through waterborne exposure was assessed in a controlled laboratory experiment. The dose to 50% disease was 6 cells ml^-1 and did not differ with the temperatures tested (7.5°C and 15°C). Our results show <i>L. zosterae</i> is transmissible through water without direct contact with infected plants. Understanding the transmission dynamics of this disease in the context of changing ocean conditions will improve <i>Z. marina</i> protection and restoration in critical coastal habitats worldwide.