Southern home gardeners often choose the aromatic gardenia as a groundcover, hedge or border. Best known for its fragrant creamy white flowers and thick glossy green leaves, these shrubs are often difficult to maintain. Powdery mildew, which is a common disease in a variety of landscape plants, often causes white fungal growth on gardenias.
Powdery mildew most often affects young succulent growth, causing damage to the health and appearance of infected gardenias. Powdery mildew is host specific, but causes similar symptoms in all infected plants. Moderate temperatures and humid climates are favorable for powdery mildew development. This disease is inactive during the hot summer months. Unlike most types of fungi, powdery mildew does not need water to infect plants.
Gardenia suffering from powdery mildew have a white coating of fungal growth on its stems, flowers and leaves. As the patches of fungi become larger, they cover more leaf tissue. Young gardenia shoots typically suffer the most damage from powdery mildew. Infected leaves may twist or curl. Leaves of infected gardenias often turn yellow then brown and flowers are disfigured. Although powdery mildew does not typically kill gardenias, it may cause premature leaf drop and fall dormancy. Gardenias infected with powdery mildew fungi, are often unsightly and unhealthy looking.
Since powdery mildew thrives in humid areas, avoid placing gardenia in heavy shade. Powdery mildew also occurs when plants grow close together, preventing proper air flow. Avoid wetting foliage during watering and water early in the day so gardenia leaves dry completely before nightfall. Using a soaker hose or drip irrigation helps reduce the wetting of foliage and prevents powdery mildew. Rake and destroy any infected leaves in your garden to prevent the spread of disease.
Fungicides are available for control of severe powdery mildew in gardenias. Choose products containing active ingredients such as triforine, propiconazole or triadimefon for powdery mildew control. Best results are achieved with fungicides when applied at the first sign of disease. Spray fungicides on gardenias on a regular schedule and spray more often when the weather is damp and cool. Cover both sides of plant leaves when applying fungicides to gardenias.