To obtain the subject controlling agent capable of manifesting remarkable controlling effects on algae and bryophytes growing in turf soil without exercising phytotoxicity onto turf by including a specific water-soluble polymer as an active ingredient therein.

This controlling agent is prepared by including a water-soluble polymer represented by the formula [R1 to R4 are each H, methyl, etc.; A is a 1-4C alkylene; (m) is 0 or 1; B is a 3-9C alkylene, etc.; X is a monovalent counterion, etc., for a cationic charge of the water-soluble polymer; (n) is 3-1,000]. The water-soluble polymer is obtained by carrying out a reaction of a compound selected from an epihalohydrin, an alkylene dihalide and a dihalogenoethyl ether with an amine or a ring opening polymerization, etc., of an alkyleneimine. The quantity of the controlling agent for treatment is preferably such that the agent containing 10-50,000 mg/L water-soluble polymer in a volume of 100-5,000 mL/m2 based on the turf area is used for the treatment.