My Gardenia Bush Is Covered in Black Mildew

Powdery Mildew Signs

• Your gardenia may have the fungus that causes powdery mildew if you notice rounded patches of fungal growth that are brown or black. The patches usually start off as white or gray powdery growth appearing on the top of the leaves. Powdery mildew typically appears in the summer and reaches it peak in late summer, advises Clemson Cooperative Extension. The fungus attacks young leaves and shoots and can cause leaves to be deformed, turn yellow or fall off the shrub.

Powdery Mildew Treatment

• The fungus that causes powdery mildew thrives on humidity. To ensure proper air circulation around your gardenia shrubs, make sure the plants are not overcrowded. Prune away diseased portions of your gardenias, and destroy any leaves that fall off the plants. If these steps don’t get rid of powdery mildew, try applying a fungicide to prevent it. Clemson Cooperative Extension recommends products containing myclobutanil, thiophanate-methyl, chlorothalonil, triadimefon, propiconazole, triforine and horticultural oil with baking soda. Read the product packaging and follow the instructions completely when applying fungicide.

Sooty Mold Signs

• Sooty mold can cause a black coating to appear on the leaves of your gardenia bush. This fungus feeds on honeydew excreted by sap-sucking insects. To determine if your plant has sooty mold, try wiping it off with a wet paper towel. If this procedure removes the black coating, sooty mold is the problem. While sooty mold is not a disease, it may affect a plant by blocking the sunlight it needs for food production. The insects causing the problem can also damage the gardenia by feeding on it.

Sooty Mold Treatment

• To combat sooty mold, you must focus on the insects causing it. These insects include whiteflies and aphids, which can be controlled with insecticidal soap. “It is important to note that aphids are very difficult to control because they multiply so rapidly,” advises the Clemson Cooperative Extension. “Leaving even one aphid alive can quickly result in a population explosion.” You can combat insects on gardenia bushes by scraping them off or pruning off and destroying infested branches. Other treatment options include suffocating them with horticultural oil or applying an insecticide to the soil to control insects throughout the growing season. Read and follow the package instructions before applying insecticides to shrubs.