I'm not much on non-edible plants but I would say it's one of two things. Either overwatering or a fungus. I think I can see little black spots on the leaves but I can't see if they have a little yellow halo around them or not. What I would do is spray the plant with Neem Oil late in the afternoon or early in the morning. Then I would make a batch of cornmeal tea and spray that about 3 days from now. I would also sprinkle the dry whole ground cornmeal around the base and on the leaves. The fungus, if that's what it is, looks like the beginning of Black Spot Fungus. I think I also see blackened stems in the background, another sign of a fungal disease.