If you want your Gardenia to be a feast for the eyes, especially in summer and autumn, the plant needs proper care throughout its life cycle. Gardenia Diseases and Pests: Common Family Problems Pests: spider mites, aphids, whitefly Diseases, caused by fungi: Canker, Powdery Mildew, Sooty Mold, Leaf Spot, etc. Root-related problems: microscopic worm-like organisms nematodes Yellowing leaves Gardenia and Vinegar