Those black leaves are due to sooty mold growing on honeydew, a substance secreted from insect pests. Common insects of gardenias that might be present and producing honeydew include aphids, scale, whiteflies and mealybugs. I suggest you take a sample of a few leaves in a ziplock bag to your County Extension Office and see if they can identify the specific insects and if necessary prescribe an appropriate remedy.
Thanks for the question. Please stop in again soon!
Q&A Library Searching Tips