The invention concerns a method for potentiating and stimulating the natural control system of agriculturally useful plants, using a phytosanitary composition containing one or several oligo beta 1-3 glucans consisting of 3 to 250, preferably, 3 to 50 and more preferably 3 to 30 carbohydrate units, the oligo beta 1-3 glucan concentration characteristics and the use of this composition being selected such that it brings per cultivated hectare an amount of oligo beta 1-3 glucans less than the amount which directly induces the natural control reactions, said amount ranging in practice from 1 to 200 g, preferably 4 to 80 g per hectare in the case of cereals and in particular of wheat.