This thesis presents an investigation into the responses to extracts of Ascophyllum nodosum (Maxicrop seaweed extracts - MSE) of two different plants species - wheat and strawberry, and their interactions with two soil-borne pathogens, Gaeumannomyces graminis and Phytophthora fragariae respectively, under various environmental conditions. The responses to MSE using hydroponic, glasshouse and field experiments showed that levels of Take-all infection in wheat were reduced by some of the treatments applied. Repeat experiments showed that consistency of results was poor but a positive trend for disease suppression followed MSE treatments. Studies of strawberry infection by Phytophthora fragariae revealed a significantly reduced level of disease severity in plants grown both in hydroponics and in the growth chamber in response to MSE. In vitro studies of the fungus demonstrated that the seaweed extract treatments severely altered mycelial growth, which drastically reduced formation of sporangia and release of zoo spores. Experiments using β-glucan, β-glucanase and laminarin showed that these could not reproduce the effects observed for MSE treatments suggesting that these components were not responsible for the MSE effect. Applications of potassium salts however, did reproduce the responses observed when applied at concentrations similar to the ones found in the seaweed extract. In these investigations, no significant benefits to non-inoculated strawberry plants could be identified as a response to MSE. Measurement of growth of disease infected plants, however clearly demonstrated that they benefited in terms of growth from the MSE amendments probably as a consequence of the disease suppression obtained.