Abstract: The phytotoxic action of too high a concentration of peroxygen compounds such as peroxycarboxylic acids and hydrogen peroxide in the combating of phytopathogenic microorganisms, such as fusaria, in the water systems of plant irrigation systems, especially greenhouses, can be eliminated. The water is treated with a peroxycarboxylic acid solution which is brought in contact with a solid decomposition catalyst for active-oxygen compounds before the water is used for irrigation, in order to reduce the content of peroxycarboxylic acid and hydrogen peroxide. The water contacts and flows through a fixed bed of decomposition catalyst, by trickling over it or flowing through the conduits of a monolithic block coated with a decomposition catalyst such as platinum, palladium or rhodium.