The gardenia (Gardenia augusta) is an evergreen shrub that produces a highly fragrant white flower in the summer months. Gardenias adapt well to most soil conditions but prefer a well-drained, acidic soil. Plant gardenias near windows or as borders alongside a patio so that you may enjoy their sweet fragrance. Use taller varieties as hedges or screens. The gardenia is susceptible to a variety of common plant diseases but, with proper care, your gardenia will thrive.
Gardenias are prone to bud drop, a problem that is signaled by buds dropping prematurely or by the plant's failure to bloom. According to the University of Florida, bud drop happens during periods of extreme heat and dryness. The plant is unable to absorb water fast enough to replace moisture lost through the leaves, which causes the buds to wither and drop. Reduce the chance of bud drop by keeping soil moist, but not overly wet, during the blooming period.
Sooty mold appears on the tops of gardenia leaves as a thin, black coating. It's relatively harmless, but it does keep leaves from absorbing sunlight, which reduces photosynthesis and makes the production of food difficult for the plant. Insects including the white fly, mealy bugs and aphids cause sooty mold. They leave honeydew, a waste product, that attracts mold spores to the plant. Rid gardenias of sooty mold by washing the leaves; prevent its re-occurrence by taking proper measures to control pests.
The dusty white or gray coating that appears on the leaf surface of gardenias is powdery mildew. Powdery mildew is a fungus that causes stunting and the disfigurement of leaves and buds. Plant leaves yellow and drop prematurely. Signs of powdery mildew most often appear when there is poor air circulation and excessive moisture on the leaves. Fungicides, used in the proper manner, are effective in stopping the spread of powdery mildew. Prevent mildew by improving the airflow around gardenias to reduce excessive moisture.
Bacterial Leaf Spot
Normally bacterial leaf spot first appears on the younger leaves of the gardenia. Tiny, round spots start out with a pale-yellow center. As they enlarge, the center turns reddish-brown and is encircled by a yellow halo. Severe cases of bacterial leaf spot cause plant defoliation. Bacterial leaf spot appears during high temperatures and is spread by contaminated garden tools. The University of Rhode Island suggests sterilizing all pots and tools that have come in contact with infected plants. Keep overhead watering to a minimum and remove infected plants from the vicinity of healthy gardenias.