Diseases of Gardenia

There are more than 200 species of gardenias, one of the most popular shrubs in the United States. Their pungent, tropical scent and brilliant white blooms stir up memories of hidden Southern gardens. Gardenias can be a bit finicky to grow, but with proper care you can avoid most diseases.

Growing Gardenias

Gardenias prefer partial to full sun, depending on your hardiness zone. Gardenias being grown in tropical south Florida, for instance, will need to be shaded from the strong afternoon sun. They grow best in USDA zones 7B through 11. Severe winters in colder areas can kill the shrubs, but they often will regenerate.

Plant gardenias in well-drained, acidic soil. Keep the soil slightly moist, but not wet, and mulch with pine needles or pine bark. Consider drip irrigation to avoid getting water on the leaves or flowers.

Fertilize your shrubs each month from April through September with a fertilizer designed for acid-loving plants, such as azaleas and rhododendrons.

Bud Drop

Bud drop is the most common gardenia complaint. Flower buds will drop off right before opening. Bud drop can be caused by overwatering, underwatering, temperature fluctuations, low humidity, very high temperatures or moving the plant. Insects also can damage the buds and keep them from opening.

Canker

Stem canker is a fungus that causes a main stem near or below the soil line to become swollen. The bark will become spongy and have long cracks. Look for bright yellow stems right above the canker. If your shrub gets stem canker, it will slowly die. There is no chemical or fungicide that can save it.

Remove the plant and destroy it. Do not compost any part. If you purchase new gardenias, plant them in a different location. Because the disease can be spread via clippers or knives, wash your tools with a mild bleach solution and rinse thoroughly.

Leaf Spot

Leaf spot fungi cause spots of various sizes and colors. The spots range from dark brown to yellow to red. If the plant is heavily infected, its leaves will turn yellow and drop. Avoid watering overhead. If the case is severe, try two or three applications of a foliar fungicide once per week.

Bacterial leaf spot shows up as small, round spots on young gardenia leaves. The spots will start off with pale yellow centers, then become reddish brown with a yellow halo. If the plant becomes severely infected, it may lose all its leaves. Using drip irrigation and sterilized soil and pots will help you avoid bacterial leaf spot.

Rhizoctonia leaf spot produces circular brown spots, starting on older leaves and spreading upward. To avoid it, make sure your plants have adequate air circulation, and use drip irrigation.

Sooty Mold and Powdery Mildew

Sooty mold causes black layers of fungus on the top of leaves. It occurs when a plant is infested with white flies and a fungus grows on the honeydew secretions from the insects. Although the mold does not actually infect the plant, it coats the leaves so heavily that sunlight cannot penetrate, reducing or stopping photosynthesis. The mold can be removed with a solution of mild soap and warm water.

Powdery mildew is a dusty, white coating on leaves. It occurs most frequently during particularly humid summers or when plants are placed too close together. Use preventive fungicide, and destroy (don't compost) any fallen leaves.

References

• Gardenia Care
• Growing Gardenias
• Gardenia Care Fact Sheet

Resources

• USDA Hardiness Zones