Herbivores can benefit both plants and their pathogens through selective herbivory on diseased tissue

Infectious disease can be a key driver of community structure, particularly when pathogens affect foundation species. Seagrasses are foundation species that form meadows along coasts worldwide, controlling sediment deposition and biogeochemical cycling while supporting a diverse community of fish and invertebrates. These plants are affected by wasting diseases that cause necrotic tissue lesions. These lesions could alter seagrass value as food, habitat, and mediators of ecosystem processes.  We assess the role of a common eelgrass herbivore, Pentidotea resecata, in affecting prevalence and severity of eelgrass wasting disease lesions.  We found that while herbivores vector the eelgrass wasting disease pathogen (L. zosterae) among isolated leaves, on balance they reduce disease severity by more than 50% in field-realistic settings. This was likely because herbivores strongly prefer diseased rather than healthy tissue, consuming nearly twice as much lesion area in choice trials. This preference is caused by pathogen-driven changes in the host plant; lesioned tissue requires less force to penetrate than non-lesioned tissue. Additionally, as lesions increase in size, their phenolic acid concentrations drop, which further increases the magnitude of preference for lesioned tissue. Our results suggest that herbivores could help maintain disease in this system at a high prevalence (by vectoring) but low severity (due to preferential consumption), which is consistent with our field observations of nearly 100% prevalence and low severity in a natural bed where herbivore density is high.  Understanding these kinds of vector-pathogen-host interactions in marine systems will advance our predictions of future disease states beyond current understanding, which focuses primarily on the influence of environmental change on pathogen outbreaks.