Plant biostimulants are products that have a growth-promoting and/or stress-reducing effect on agricultural crops by improving the condition or resistance of the plant. This allows e.g. better absorption of nutrients, reduced susceptibility to abiotic stresses such as drought, and better plant defences against biotic stresses such as fungal or bacterial attack. Extracts from various seaweed species may have such a biostimulating effect. In this project, published literature on research into the biostimulative effects of various seaweed species on crops and various (probable) candidate substances in seaweed that cause this effect was reviewed. Greenhouse and field experiments were then carried out to examine the effects of seaweed products produced by the French company Olmix on tomatoes, onions and potatoes affected by the fungus Phytophthora infestans (the cause of e.g. potato blight). The biostimulant effect of a self-produced extract of the seaweed species Saccharina latissima was tested in a small additional trial. Saccharina is a seaweed species that occurs naturally in the North Sea and is currently one of the most commonly used species in commercial cultivation of seaweed in European waters. Olmix's commercial seaweed product (Seamel) is produced from green and red seaweeds harvested in the wild off the Atlantic coast of France. In addition to seaweed, full-formulation Seamel, which is designed for use in crop production, contains various micro- and macro-elements added to enhance the effect of the seaweed extract. Field and greenhouse experiments on potatoes and tomatoes affected by P. infestans confirmed the biostimulating effect of the seaweed product, with crops sprayed with Seamel suffering significantly less late blight infection. A dose effect in f.e. the pot trial in tomato in 2018 was also observed, with a higher Seamel dose giving a stronger reduction or delay in infection in field experiments on potatoes in 2019 and 2021. In field experiments on onions in 2018 and 2019, no effect on yield or ...