Q.Gardenia Leaves Have Brown Encrustation On Larger Leaves At Base
Black flies were attacking New growth. Plant are kept inside with high humidity.
Certified GKH Gardening Expert
You can remove the leaves if there aren't too many. It's likely a fungal disease.
Hard to diagnose without more information and pictures. It may be a pest (scale, etc.). Review the information on this link and post again if necessary: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/growing-gardenias-indoors.htm