- Canker is one of the most common and serious gardenia diseases. The fungus affects the plant's stem, which will look cracked, swollen and bright yellow rather than a healthy greenish white. Canker occurs near where the stem grows out of the soil. The disease is easily spread from plant to plant. Because of this, the University of Florida IFAS Extension recommends destroying all plants with canker.
- This type of leaf spot fungus infects gardenia leaves. Small (about 1/4 inch in diameter) circular-shapaed brown and tan spots appear on leaves. Rhizoctonia leaf spot first infects older leaves, but can spread upward into new growth on the plant. To prevent the fungus, avoid watering excessively or planting too closely together, the University of Rhode Island Landscape Horticultural program advises. To rid plants of the disease, destroy the leaves that are infected, and use soil that has not been exposed to the fungus.
- Another kind of fungus, simply called leaf spot, shows up as variously sized dark-brown spots on leaves. The spots may be encircled with a ring of yellow. The fungus may cause leaves to drop off the plants, but can be controlled with the use of a fungicide designed for use on leaves.
- Sooty mold fungus smudges the fronts of gardenia leaves with a black, soot-like substance. While the fungus itself doesn't kill the leaves, it blocks sunlight from penetrating the leaf so that photosynthesis is reduced. White flies, aphids, and mealybugs spread the fungus when they land on plants. Sooty mold can best be controlled by eliminating these insects in the garden, according to the University of Florida IFAS Extension.