The invention relates to a method for preventing or treating fungoid growth of the specie Rhizoctonia, Pythium and/or Phytophthora in a plant, in particular for treating chestnut disease, wherein the method comprises making a hole down to the root zone of the plant and putting into this hole a fungicidal composition that comprises an insect repellent age and a fungicide, where the insect repellent agent contains at least garlic and/or an extra thereof. The combination ensures that symbiotic fungoid growth and the biologic equilibrium of the soil are maintained and the vitality of the plant improves. As a result this, recovery is encouraged and recurrence of the disease syndrome after completion treatment is prevented to a large extent The method of administration according to the invention ensures homogeneous distribution by means of take up of the composition via the roots.