PROBLEM TO BE SOLVED: To obtain a controlling agent to be inoculated to a seed potato or a culture soil to improve the controlling effect on common scab of potato and increase the yield by using a specific true fungi as an active component. SOLUTION: A true fungi belonging to the genus Gliocladium is used as an active component of the objective agent. The true fungi is e.g. Gliocladium virens, Gliocladium roseum, etc. A cultured product obtained by culturing the microbial cell in a medium can be used as the true fungi acting as an active component. The cultured product may be used in a form supported on a carrier. The carrier is preferably a porous material such as attapulgite and montmorillonite from the viewpoint of water-retaining potency, fertilizer-holding potency and convenience for use. The controlling agent is preferably applied at a rate of about 0-5-40kg per 10 are in the case of an agent containing 1×10<8> CFu in terms of the colony-forming unit of the Gliocladium cells per 1g of the controlling agent.
