If the leaves have a halo around them, it's a fungal infection, which I suspect. Treat with the systemic fungicide Tebuconazole, which also works topically. You can get this over the counter in Bayer Advanced Disease Control for roses, flowers & shrubs. Another possibility is that it's sooty mold growing on the excrement (honeydew) of sucking insects (scale/mealybug), in which case curing the infestation will rid the plant of the sooty mold. How is the air flow in the area they're being kept?

Al