Gardenias are flowering ornamental shrubs that grow in the southern United States. A popular choice for landscaping, they can grow two to 15 feet high, depending on the variety. Gardenia leaves are oval shaped, and the fragrant flowers are sweet smelling. Gardenias can be grown either indoors in a sunny spot, or outside in partial shade. To ensure healthy growth, address gardenia diseases at the first sign of any problem.
Canker is a common fungal disease that can be deadly to gardenia plants. The condition causes stunted growth and ultimately leads to a slow death. Infected gardening tools and implements spread canker. Identify canker by looking for swelling of the gardenia stem near or below the soil line, according to the University of Rhode Island Landscape Horticulture Program. Cankered gardenia bark has an appearance similar to cork, with numerous cracks. The only way to rid a gardenia of canker is to destroy the plant. Prevent future cases by planting any new plants in a different location.
Gardenia foliage that has turned black indicates the presence of sooty mold. Sucking insects that secrete a black fungus cause sooty mold. The fungus prevents sunlight from reaching the gardenia leaves, thereby stopping the photosynthesis process. Remove the sucking insects with an insecticide recommended by your local gardening center. Wash the gardenia leaves with soap and water after insect removal.
Rhizoctonia Leaf Spot
This fungus affects gardenia plants that receive excess water, or when overcrowding prevents proper air circulation. Avoid this disease by not wetting gardenia foliage when watering the plant. Circular, tan-to-brown spots indicate this disease. The spots begin on older leaves and multiply. Destroy all diseased leaves to prevent spreading.
Bacterial Leaf Spot
Infected gardenia plants have small spots on the leaves that enlarge over time. The center of the spots begins as a pale yellow color, then turns reddish brown with a yellow halo. Severe infections can cause defoliation. Avoid this gardenia disease by not over watering the plants. Do not start new plants with infected cuttings.
Bud drop is the abnormal dropping of buds from gardenia plants. This is usually caused by excessive hot weather and the plant's inability to rehydrate. Insect damage and root injury can also cause bud drop. Keep soil moist to prevent this condition from occurring.