Experts and BHG readers answer.
Black spots on dianthus
Thanks for writing. It sounds as though your dianthus have a fungal disease, but without being able to examine the plants, I can't be sure of what the disease is.
You can try spraying them with a fungicide (either traditional or organic) and see if that stops the fungus from spreading any farther.
Also: Be sure there's good airflow around your geraniums, let the potting mix dry between waterings, and don't water your plants after about 2.00 p.m.
Do the plants seem as though they could be rootbound? If so, that may also be encouraging the fungal disease.
---Justin, Senior Garden Editor, BHG.com