PURPOSE: To prevent the damage by fungi with a highly safe substance and cultivate a mushroom, by using an extract of an inner bark of a tree consisting of a flavanol of specific composition and physical properties and a water-soluble saccharide with hot water as a controlling agent against fungi of the genus Trichoderma, etc.

CONSTITUTION: An extract of an inner bark of a tree having the following composition and physical properties: Composition: 87.2% organic component (43.0% flavanol and 44.2% water-soluble saccharide therein), 3.2% inorganic component and 9.6% moisture. Elementaty analysis of the organic component: C, 51.9%; H, 5.5%; O, 42.5%; N, 0.1%. Molecular weight distribution of the flavanol: 300W3,000 (1W10 expressed in terms of polymerization degree). Average molecular weight of the flavanol: 830. Specific 13C-NMR spectrum and IR spectrum with hot water is used as a controlling agent against fungi of the genus Trichoderma and/or Hypocrea to cultivate a mushroom, e.g. a SHIITAKE mushroom.