Gardenias grow in the southern part of the United States, where they are not subject to harsh winters. When they do not receive proper amounts of sunlight, humidity, water and fertilizer, gardenias become susceptible to disease. Knowing how to identify these diseases will help you treat the plant and keep it blooming attractively.
This is the most serious disease of the gardenia plant. Stem canker occurs when fungi or bacteria spread by wind, rain, insects or infected pruning tools enters a wound on the gardenia. This disease manifests with a swollen stem at the soil line or right below the soil line. The bark may become corky and cracked, and the area above the canker will be bright yellow. As the tissue of the stem dies, the plant will become stunted and will start to die slowly. Infected plants cannot be saved; take them out of the ground and destroy them to stop the spread of disease to other plants.
Gardenia can contract bacterial, Rhizoctonia and Phyllosticta leaf spot. It spreads through infected stem cuttings or overhead watering. Small spots of tan, brown or reddish-brown appear on the leaves. In some cases, these spots coalesce to form irregular shapes. In severe cases the leaves may drop off the plant. To control leaf spot, gather and dispose of infected leaves and branches in the fall. Also, avoid wetting the leaves when watering and provide good air circulation to prevent leaf spot.
Powdery mildew occurs as a result of the fungus Erysiphe polygoni. Powdery white circles that look like dust develop on the leaves of the gardenia. As the disease progresses, the circles grow and coalesce. The fungus invades the tissue, and the leaves may turn yellow and drop off the plant. To control this disease, gather infected leaves and branches in the fall and dispose of them. Treat the plant with a fungicide containing immunox if the disease is serious.
Oedema occurs when the gardenia takes up water faster than it can use it. As a result, water pressure builds up in the leaves and flowers and small blisters form. As this continues, the leaves may turn yellow and drop off the plant. This disease is most likely to occur in cool, cloudy, moist weather when the plant receives little light and lots of water. To cure this disease, expose the gardenia to more light and decrease watering frequency.
Sooty mold occurs when white flies leave a sugary substance on the leaves of the gardenia. The fungus Capnodium clings to this substance, causing a black film to coat the leaves. This disease does not infect the plant but grows only on the surface. As a result, the plant cannot receive sunlight and cannot perform photosynthesis. To control the fungus you must control the white flies that leave the honeydew on the plant, which can be tricky. Apply an insecticide containing diazinon or acephate every five to seven days until the fly population disappears. Follow the directions on the package for application instructions.