PURPOSE:To obtain an environment suitable for inoculating a spawn of a mycorrhizal fungus by previously preparing a population of rootlets of a parasitized tree. CONSTITUTION:A water-permeable and light shielding material (5) constructed of an inorganic substance with hardly any various germs is placed on horizontal roots (2) of a parasitized tree with a water-permeable membrane (6) placed therebetween and allowed to pass for 1-2 years. The light shielding material (5) and water-permeable membrane (6) are then removed to inoculate a mycorrhizal fungus into a population (7) of rootlets grown under the water-permeable membrane (6) according to a conventional method. Since the light on the ground is shielded, the horizontal roots (2) of the parasitized tree start rooting toward the upper light shielding material (5) due to physiological properties of the horizontal roots for the light. Since the light shielding material (5) is constructed of the inorganic substance which any various germs hardly exist, the developed rootlets are prevented from elongating in the upward direction with the water-permeable membrane (6) and congested and the various germs are hardly grown. The population (7) of the rootlets is formed on the under side of the water-permeable membrane (6). Thereby, an environment suitable for inoculation of a spawn of the mycorrhizal fungus is prepared to enhance the efficiency of infection with the mycorrhizal fungus.
