The invention relates to a method for the stimulation of the reactions of natural defence of agronomically useful and of decorative plants, characterized by the fact that there is applied on the plants to be treated an aqueous composition comprising an efficient concentration in sulfate of 1-3 glucan and more particularly a concentration of at least 1 mg/l, preferably of at least 5 mg/l, and still more preferably of at least 10 mg/l in laminarin sulfate, whose degree of sulfatation is higher than or equal to 1.9, preferably from 2 to 2.5, the said composition being applied to the culture or crop to be treated in an amount sufficient to provide per hectare an efficient amount of sulfate of 1-3 glucan, and in the case of laminarin sulfate in an amount sufficient to provide from 0.4 to 4 g of laminarin sulfate per hectare.