A disease control device has light emitting diodes (D1 to Dn) emitting green light and a controller (2) controlling a drive circuit (1) for turning on the light emitting diodes (D1 to Dn). The drive circuit (1) is controlled by the controller (2) to cause the light emitting diodes (D1 to Dn) to emit green light to irradiate plants with the light. The irradiation with the green light enhances resistance of the plants against diseases.