The waxy, dark green leaves of a gardenia plant frame its flowers, making the shrub look even more attractive. If the leaves begin to turn brown, however, the bush can become unappealing and gardeners should take note: brown leaves are often a sign of an unhealthy growing environment, disease or pest problem.
Gardenias require high humidity for successful growth; warm, dry heat will turn the leaves brown quickly. Aphids and other pest insects feed on gardenias, turning the leaves brown. Some diseases that affect gardenias and cause the leaves to become brown include powdery mildew and a number of leaf spot diseases caused by fungi. Fungal leaf spot diseases cause the leaves to turn brown at the tips and across the surface of the leaves.
Gardenias should be watered at least once a week to keep the moisture level of the soil high. However, the soil should be able to drain water to prevent fungi from growing in the standing water; this can lead to leaf spot diseases and root diseases that affect the health of the gardenia. Water gardenias in the morning so water is not left standing on the leaves during the day, which can encourage powdery mildew.
Spraying gardenia bushes with a pesticide in the early spring helps destroy aphid populations and keeps them from feeding on the leaves of the gardenia. Aphids are a serious problem, as they frequently spread fungal diseases to gardenias or leave wounds in the plants that diseases enter. Aphids not only damage gardenias themselves, they make the gardenia more susceptible to fungal leaf spot problems that turn the leaves brown. Apply pesticides according to the directions for the best results.
Fungicides and Cultural Control
Remove leaves affected with fungal spots to stop the disease from spreading to other parts of the gardenia shrub and doing further damage, and pick up any fallen leaves that fungi could live on. Prune the gardenia annually and remove any weak or previously diseased sections of the bush to prevent them from contracting or spreading fungal leaf spot diseases further. Apply a fungicide to the gardenia to help kill off existing spores and spore spots.