Its always tricky to make a diagnosis without seeing the plant but it does sound like it could be a fungal disease. Fungal diseases thrive in damp humid conditions and greenhouses often provide the perfect climate for the spread of fungal spores which can affect a wide range of plants.
If it is still confined to just a few plants then destroy those affected by burning them - do not add them to the compost heap! It is still quite early in the season, so you could probably pick up some more plants at your local garden centre to replace them. The remaining plants can be sprayed with a fungicide on a cool, overcast day.
To prevent further outbreaks you should try to avoid overhead watering; always water directly into the soil. Improve ventilation in your greenhouse to reduce the humidity. Good plant hygiene is essential. Clean away any infected plant material that has dropped down the side of benches or is lying on the floor, remembering to burn it rather than compost it.
The spotting on your leaves sounds like a fungal disease. If it is only a few leaves then you can probably just pick them off and destroy them. Clear up any infected plant debris from the base of the plant too. It is worth knowing that Camellia leaf spots often colonise other small spots on the leaves that have been initially caused by overwatering or nutrient imbalances. So in many cases, this can be prevented by ensuring that your plant is growing in optimum conditions. If the problem persists then you may need to consider spraying with a fungicide.
It sounds as though they are suffering from a strain of fungal leaf spot. There are many different fungi that can cause this problem, which tends to occur during warm, wet weather, or where plants are watered from above the foliage. Plants that are fed with high nitrogen feeds and grown tightly together (thereby reducing ventilation) are also particularly susceptible.
The warm, wet conditions that we have had in recent weeks will have created ideal conditions for the spread of black spot fungal spores. Remove and destroy any affected foliage that you can and spray the new growth with a fungicide that is suitable for use on ornamental plants to try to restrict the spread of the problem.
It sounds as though the Buddleja may be suffering from fungal leaf spot which is caused by thousands of different fungi species and affects a wide range of plants. Fungal problems tend to be exacerbated by wet, humid conditions. The recent warm, wet weather that we have experienced have provided ideal conditions for the growth and spread of fungal spores which are dispersed by wind and rain splashes.
There are several measures that can be taken to reduce this problem. Firstly try to water the plant directly into the soil, from below the foliage, as this will help to keep the foliage drier and reduce the spread of spores. Cut back the infected areas and destroy the infected plant material by burning it or by disposing of it off site. Do not compost it. If the plant is in a container then move it to an open, well ventilated position. This will help to dry the foliage quicker following rainfall, and reduce the dispersal of spores. Any fallen plant debris should be raked up in autumn and destroyed to prevent spores from overwintering in the decaying foliage. This will help to prevent further infection in the spring.
There are also many fungicides available that are suitable for use against leaf spots on ornamental plants, and these can be used to help prevent further outbreaks.
A healthy plant is less likely to succumb to diseases so the avoidance of plant stresses such as drought will also help to control the problem.
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.