Campanula produces healthy leaves in spring and as the stems grow taller the lower leaves start to turn brown and now I think whatever this disease is seems to be spreading to other plants and my small garden is looking very sad. What can I do about it?
If you see it spreading from plant to plant you may have either an insect or fungus problem.
Don't rule out watering issues. Check to make sure the soil is adequately moist, but not soggy.
Inspect the plants to see if you can identify anything, but certainly treat with Neem Oil.
This works as both as both an insecticide and fungicide. It's safe for people, pets and bees.
Here are a few links with more information.