A disease control device has light emitting diodes (Dl to Dn) emitting green light and a controller (2) for controlling a drive circuit (1) for lighting the light emitting diodes (Dl to Dn). The drive circuit (1) is controlled by the controller (2) to cause the light emitting diodes (Dl to Dn) to emit the green light to irradiate plants with the light. The irradiation increases resistance of the plants against diseases.