Seagrass wasting disease: Nitrate enrichment and exposure to a herbicide (Diuron) increases susceptibility of Zostera marina to infection

Seagrass meadows suffered large-scale declines in the past century. The 'wasting disease', pathognomonically associated with Labyrinthula zosterae, reduced populations of Zostera marina on both sides of the North Atlantic in, and since, the 1930s, coinciding with intensive agricultural use of artificial fertilizers and herbicides. This study tests the long-standing hypothesis that nutrient enrichment and a herbicide increases vulnerability to pathogens. Z. marina shoots from the Thames Estuary grown in elevated nitrate concentrations had significantly higher rates of infection by L. zosterae than controls, but not by Aplanochytrium sp., another slime-mould like protist. Z. marina shoots grown in 2 mu g.l(-1) Diuron solutions and infected separately by L. zosterae and Aplanochytrium sp. had significantly higher wasting indices than controls. The results identified Aplanochytrium sp. as another opportunistic pathogen causing a seagrass wasting-type disease and support the hypothesis that pollution by herbicides and nitrate increases the susceptibility of Z. marina to infections.