Pathogenicity and phylogeny of Labyrinthula spp. isolated in Washington and Oregon, USA

The class Labyrinthulomycetes constitutes a multitude of species found ubiquitously in the environment, and includes pathogens of corals, hard clams, turfgrasses, and seagrasses. Labyrinthula zosterae, the causative agent of seagrass wasting disease, has been associated with declines in seagrass coverage since the 1930s. However, pathogenic and nonpathogenic Labyrinthula spp. have been isolated from seagrass tissue. These isolates are difficult to distinguish morphologically, and the diversity of isolates where seagrass wasting disease is present is often unknown. This study aimed to increase knowledge on the pathogenicity and phylogeny of Labyrinthula spp. in Washington and Oregon, USA where a high prevalence of seagrass wasting disease has been associated with eelgrass, Zostera marina, declines. We tested the pathogenicity of 14 Labyrinthula isolates and compared partial 18S rRNA gene sequences of 12 isolates to sequences from around the world through the NCBI database. We found that pathogenic isolates could be identified as Labyrinthula zosterae, while nonpathogenic isolates did not form a clade with any previously identified SSU ribotypes. These results add to the growing data on Labyrinthula and seagrass wasting disease and can improve our understanding of pathogen evolution and spread in the future.