Beschreibung:
<jats:title>Abstract</jats:title><jats:p>The protist <jats:italic>Labyrinthula zosterae</jats:italic> (Phylum Bigyra, sensu <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#b37">Tsui <jats:italic>et al.</jats:italic> 2009</jats:ext-link>) has been identified as a causative agent of wasting disease in eelgrass (<jats:italic>Zostera marina</jats:italic>), of which the most intense outbreak led to the destruction of 90% of eelgrass beds in eastern North America and western Europe in the 1930s. Outbreaks still occur today, albeit at a smaller scale. Traditionally, <jats:italic>L. zosterae</jats:italic> has been quantified by measuring the necrotic area of <jats:italic>Z. marina</jats:italic> leaf tissue. This indirect method can however only lead to a very rough estimate of pathogen load. Here, we present a quantitative real‐time polymerase chain reaction (qPCR) approach to directly detect and quantify <jats:italic>L. zosterae</jats:italic> in eelgrass tissue. Based on the internal transcribed spacer (ITS) sequences of rRNA genes, species‐specific primers were designed. Using our qPCR, we were able to quantify accurately and specifically <jats:italic>L. zosterae</jats:italic> load both from culture and eelgrass leaves using material from Europe and North America. Our detection limit was less than one <jats:italic>L. zosterae</jats:italic> cell. Our results demonstrate the potential of this qPCR assay to provide rapid, accurate and sensitive molecular identification and quantification of <jats:italic>L. zosterae.</jats:italic> In view of declining seagrass populations worldwide, this method will provide a valuable tool for seagrass ecologists and conservation projects.</jats:p>