TECHNOLOGY FOCUS - AGRICULTURE - Preferred System: The supplying unit is a spraying or sprinkling unit configured to spray or sprinkle the aqueous dispersion (I) to the plant. The system (S1) further comprises an ultraviolet irradiation unit and a conveying unit, in which the monitoring unit is configured to monitor the hue of the plant that is sequentially conveyed by the conveying unit. The monitoring unit is configured to detect the lesions of the plant depending on the hue difference between the hue of the plant that has been preliminarily stored in a computer and the hue of the plant to be monitored.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Agent: The plant epidemic agent (A1) is an aqueous dispersion (I) containing the photocatalytic powder in a dispersed state in water. The photocatalytic powder has a volume average particle diameter of 50 nm to 5 microns. The solid content of the aqueous dispersion is 0.001-30 (preferably 0.001-1) mass%. The photocatalytic powder is an apatite (preferably calcium hydroxy apatite Ca10(PO4)6(OH)2) comprising a metal atom required to have photocatalytic activity, and the metal atom is titanium (Ti). ACTIVITY - Plant protectant. The plant epidemic prevention agent containing photocatalytic titanium apatite (test) was sprayed to the Phalaenopsis plants using spray at a ratio of 11.25 ml/plant. The culture medium was incubated with the pathogenic bacterium and diluted with sterilized water to prepare a bacterium liquid, which was sprayed onto and inoculated to stems and leaves of the plants. The plants were marked on the basis of three sites in leaf surface to slightly wound them. Twenty-one days later the wounded sites were observed. The pathogenic bacterium (comparative) was inoculated into plants of Phalaenopsis orchid in the same manner as in test, except that no plant epidemic prevention agent was supplied to plants. The disease onset level of plants was observed, the result showed inoculation after twenty-one days for test/comparative was 17.3/4.8 respectively.    MECHANISM OF ACTION - None given. No biological data is given.    ADMINISTRATION - The aqueous dispersion with photocatalytic powder is sprayed or sprinkled onto plant in an amount less than or equal to30 (preferably less than or equal to1) mass%.    EXAMPLE - No suitable example is given.