Use of an extract or an extract fraction of agarophyte red algae as a plant defense elicitor/stimulator and application of said extract or said extract fraction

A method for stimulating natural defense in a terrestrial plant, consisting in applying to the whole of the plant or to a part thereof, comprising the vegetative apparatus, root system, fruit, grains and seeds of the plant, or on the soil or substrate for culturing the plant, an amount sufficiently effective to stimulate the natural defenses of the plant, of an extract or of an extract fraction of at least one agarophyte red alga, the extract or the extract fraction containing at least one oligosaccharide having 2 to 50 ose units covalently bound to one another, and polysaccharides having more than 50 ose units covalently bound to one another which can be contained without exceeding 0.01% by weight relative to the total oligosaccharides/polysaccharides.