It sounds as though they are suffering from a fungal disease. Fungal leaf spot infections tend to develop in humid, moist conditions and the spores are easily spread by wind and splashes of water. Start by picking off any heavily infected leaves and dispose of them by burning or removing them from the site. Do not compost them as the spores may remain in the compost to re-infect plants later on. Spray the plants with a fungicide. There are lots available from the garden centre but make sure that you choose one that is suitable for use on ornamentals.
There are also a couple of things that you can do in order to reduce the spread of the spores. Ensure that the plants receive adequate ventilation particularly after watering. If they are in the greenhouse then I would recommend that you water before midday and open windows to improve air circulation during the daytime and dry out any splashed water. This will ensure that the foliage of the plants remains dry and that the humidity is reduced. The windows can then be closed late in the afternoon. I would also strongly suggest that plants are watered from beneath their foliage as overhead watering is the main cause of fungal diseases spreading.
If plants succumb to the disease then they should be removed immediately to prevent them infecting other plants.