The marine brown macroalga Saccharina latissima is an important primary producer in temperate to cold northern hemisphere shores and fundamental in the formation of marine habitats. Also known as sugar kelp, it is an economically relevant seaweed with high industrial potential. Morphological changes- such as dark spots, twisted stipes and deformation of the blades - have been observed in wild populations and seaweed farms. The putative cause for the occurring symptoms is the filamentous endophytic brown alga Laminarionema elsbetiae, which is highly prevalent in European Saccharina populations but has also been found in its secondary host, the brown alga Laminaria digitata. L. elsbetiae is known to invade stipes and fronds of its hosts, however nothing is known about the molecular mechanisms of the interaction. To get further insight into the host-endophyte relationship and possible defence mechanisms, a co-cultivation bioassay was developed in order to monitor the impact of the endophyte on growth of laboratory-raised individuals of both kelp species and to test the pathogenicity of the endophyte according to Koch's postulates. First results revealed that co-cultivation of L. elsbetiae with S. latissima juveniles does not induce changes in growth or morphology of its primary host. On the contrary, growth of L. digitata significantly decreased within less than a week when co-cultured with the endophyte. Ongoing experiments, including field surveys, aim to decipher these mechanisms and will help to better understand the bases of the biotic interaction between the endophyte and its hosts.