Protective effect of three brown seaweed extracts against fungal and bacterial diseases of tomato

The effect of extracts of the brown algae Cystoseira myriophylloides, Laminaria digitata, and Fucus spiralis against the tomato pathogens Verticillium dahliae and Agrobacterium tumefaciens was evaluated in vitro and in the greenhouse. A significant inhibition of growth was observed only with methanolic seaweed extracts (MSE). Disease resistance was assessed in the greenhouse against Verticillium wilt using spray application of aqueous seaweed extracts (ASE) on the whole plant or using seed imbibition. Both methods significantly reduced disease severity whatever the algal species, though protection observed after seed treatments was higher than that observed after spray treatment. Spray application of ASE from C. myriophylloides and F. spiralis also resulted in significant reduction of Crown gall disease caused by the bacterial pathogen A. tumefaciens. ASE-treated plants had significantly higher levels of activity of defense enzymes polyphenol oxidase and peroxidase compared to the control. ASE did not inhibit mycelium growth of V. dahliae or development of A. tumefaciens in vitro; it is therefore suggested that induced resistance is probably the main mechanism of protection afforded by ASE.