Canker (fungus – Phomopsis gardeniae): One of the most common gardenia diseases. The main stem is swollen near or below the soil line. The bark becomes corky and contains numerous longitudinal cracks in the cankered area. The stem above the canker is bright yellow in contrast to normal greenish white. If the humidity is high, a yellowish exudate may be seen on the surface. Affected plants are stunted and die slowly. Destroy all diseased plants to prevent spread of the disease. Place new plants in a different location. Disease is easily spread on propagating knives.
Bacterial Leaf Spot (bacteria – Pseudomonas gardeniae, Xanthomonas campestris cv. maculifolium-gardeniae): Small, rounded ovoid spots on young tender leaves. As the spots enlarge, the center is at first pale yellow and later become reddish-brown surrounded by a yellow halo. Margins of the lesions are thickened and water-soaked in appearance. Spots may coalesce to form large, irregular-shaped spots. Severe infection may cause defoliation. Avoid overhead watering. The disease is spread by taking cuttings from infected plants. Use sterilized soil and pots.
Rhizoctonia Leaf Spot (fungus – Rhizoctonia spp.): Leaves have tan to brown spots up to one-fourth inch in diameter. Spots are circular and zoned. The disease begins on the older leaves and spreads upward when the plants are watered excessively or when air circulates poorly because of overcrowding. Diseased leaves should be destroyed and sterilized soil should be used. Disease-free plants should be used for propagation. Avoid wetting foliage when watering.
Leaf Spot (fungi – Cercospora spp., Phyllosticta spp.): Spots of various sizes on leaves throughout the year. Spots may be small, dark-brown necrotic areas surrounded by a yellow halo. In severe cases, premature leaf drop may occur. control is obtained by spraying with a foliar fungicide at regular intervals.
Sooty Mold (fungus – Capnodium spp.): Black, thin layers of the fungus form over the upper surface of leaves. Sooty mold is caused by a fungus that grows on sugary exudate from white flies. Control white fly.
Bud Drop: Abnormal dropping of buds. Occurs during period of high night temperatures or during periods of low light intensity. Some bud drop is a natural condition. Every effort should be made to keep the soil uniformly moist, but not wet, during flowering.
Powdery Mildew (fungus – Erysiphe polygoni): White powdery spots on leaves. Use preventive fungicide.
Other Diseases: See the sections on Cotton Root Rot, Crown Gall, Mushroom Root Rot, and Root Knot Nematodes.