The leaves on both of my gardenia bushes are almost totally black. What do I need to do?
Gardenia Bush Disease
Certified GKH Gardening Expert
Sounds like you could have a problem with sooty mold, most often brought about by pests. Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-pests.htm
Neem oil is an effective treatment for both the pest and sooty mold.