Abstract text: The fungal disease Ramularia Leaf Spot leads to reduced yield, and grain quality, in cereal crops, and in particular barley. Increased prevalence of the causative agent, Ramularia collo-cygni, as a result of conducive climatic conditions associated with global change, as well as more stringent legislation reducing the availability of effective fungicides, necessitate identification of effective, sustainable and novel controls. Seaweeds have a long history of use in agriculture and have been shown to contain a range of bioactivities including antifungal. Seaweeds are a diverse group of Eukaryotes, belonging to several lineages, many of which diverged from each other prior to the origin of photosynthetic eukaryotes. Resultingly seaweeds are a rich source of a wide range of diverse and complex cell wall components, with potential benefits for plant health. We extracted cell wall polysaccharides and glycoproteins from a range of seaweeds native to Ireland, and screened them for their ability to either directly impact the growth of Ramularia, or to stimulate the barley immune system, thereby offering protection against Ramularia and improving overall plant health. We determined that the activities of several native Irish seaweeds support their future application as plant protection products.