Marine protists of the genus Labyrinthula cause the seagrass wasting disease, which is associated with regional die-offs of eelgrass Zostera marina and also infects turtlegrass Thalassia testudinum. The ability of seagrasses to resist pathogen attack is determined by multiple factors, which are poorly understood. One factor hypothesized to influence seagrass disease resistance is the presence of (poly)phenolic natural products such as caffeic acid, which inhibits the growth of L. zosterae in in vitro laboratory bioassays. This hypothesis has been supported by reports of pathogen-induced phenolic accumulations in eelgrass Z. marina. To test the response of T. testudinum to inoculation with Labyrinthula sp., we conducted a series of culture experiments wherein plants were inoculated with Labyrinthula sp. isolated from turtlegrass beds in Perdido Bay, Florida (USA). Concentrations of phenolic acids and condensed tannins were quantified in diseased leaves as well as those treated with 5 mM salicylic acid, a signaling molecule associated with pathogen-induced responses in plants.In infection experiments, increases in the concentrations of several phenolic acids, but not condensed tannins, were observed in tissues above, but not below, microbial lesions. Salicylic acid (SA) treatments did not induce any phenolic compound, either when applied alone or in concert with the pathogen. The induction of phenolic acids above, but not below, infection sites suggests that T. testudinum leaves did not respond to the pathogen specifically. Instead, the pattern is consistent with the predictions of the sink/source model of plant defense, which predicts increased phenolic contents in cases where wounds disrupt plant resource allocation and cause a local overabundance of carbonbased resources. Thus, we suggest that the emergence of Labyrinthula sp. lesions on turtlegrass blades causes a 'pseudo-induction' of specific phenolics as carbon resources over-accumulate in tissues located above wound sites.

Symptoms characteristic of wasting disease, thought to result from infection by protozoan pathogens (i.e. Labyrinthula spp.), are a common phenomenon affecting seagrass species worldwide. However relatively little is known about factors that control the survival and success of Labyrinthula in part due to the difficulty associated with quantifying the growth of this organism. Here we describe a simple and inexpensive method for measuring growth of Labyrinthula in liquid culture that takes into consideration both cell density and areal spread. The technique allows for examination of the effects of both abiotic and biotic factors on the growth of Labyrinthula apart from its seagrass host, separating the effects of environmental condition on the host from their effects on the pathogen.