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Abstract Although invertebrate herbivores commonly impact terrestrial plant diseases by facilitating transmission of plant pathogens and increasing host susceptibility to infection via wounding, less is known about the role of herbivores in marine plant disease dynamics. Importantly, transmission via herbivores may not be required in the ocean since saline ocean waters support pathogen survival and transmission. Through laboratory experiments with eelgrass (Zostera marina), we showed that isopods (Pentidotea wosnesenskii) and snails (Lacunaspp.) created grazing scars that increased disease severity and thus indirectly facilitated transmission ofLabyrinthula zosterae(Lz), a protist that causes seagrass wasting disease. Experiments also quantified different feeding preferences among herbivores: Amphipods (Ampithoe lacertosa) selectively consumed diseased eelgrass, while isopods and snails selectively grazed asymptomatic leaves, suggesting different herbivore taxa may have contrasting impacts on disease dynamics. Our experiments show no sign that herbivores directly vector Lz from diseased to asymptomatic eelgrass. However, we isolated live Lz from isopod, amphipod, and snail feces and detected Lz with quantitative polymerase chain reaction in amphipods and snails, suggesting that herbivores eating diseased eelgrass could pass the live pathogen. Finally, field surveys demonstrated a close association between seagrass wasting disease and invertebrate grazing scars; disease prevalence was 29 ± 4.7% (95% CI) higher on eelgrass leaves with herbivore scars. Collectively, these findings show that some herbivores can increase eelgrass disease risk by facilitating the spread of an important pathogen via wounding, but not via direct transmission. Thus, herbivores may play different roles in plant disease dynamics in terrestrial versus marine ecosystems depending on the pathogen's ability to survive and transmit without a vector. 

Abstract Host‐associated microbes influence host health and function and can be a first line of defence against infections. While research increasingly shows that terrestrial plant microbiomes contribute to bacterial, fungal, and oomycete disease resistance, no comparable experimental work has investigated marine plant microbiomes or more diverse disease agents. We test the hypothesis that the eelgrass (Zostera marina) leaf microbiome increases resistance to seagrass wasting disease. From field eelgrass with paired diseased and asymptomatic tissue,16S rRNAgene amplicon sequencing revealed that bacterial composition and richness varied markedly between diseased and asymptomatic tissue in one of the two years. This suggests that the influence of disease on eelgrass microbial communities may vary with environmental conditions. We next experimentally reduced the eelgrass microbiome with antibiotics and bleach, then inoculated plants withLabyrinthula zosterae, the causative agent of wasting disease. We detected significantly higher disease severity in eelgrass with a native microbiome than an experimentally reduced microbiome. Our results over multiple experiments do not support a protective role of the eelgrass microbiome againstL. zosterae. Further studies of these marine host–microbe–pathogen relationships may continue to show new relationships between plant microbiomes and diseases.