Gardenias are temperate climate shrubs and ornamentals, favoured for their cold weather resistance and white blossoms. Yellow leaves and brown spots on gardenias are a symptom of various fungal and bacterial diseases that threaten the vitality of the entire plant. Identification of the specific pathogen, prevention and early control are vital for treating these diseases.
Bacterial Leaf Spot
Bacterial leaf spot of gardenia is a foliage disease, caused by the bacterium Xanthomonas campestris. The disease affects foliage vitality, causing small brown spots on foliage and blossoms, leaf yellowing and eventual leaf drop. As the disease progresses, the spots enlarge and turn black. The disease favours warm temperatures ranging from 22.2 to 27.8 degrees Celsius for infection, reports Juanita Popenoe of the University of Florida IFAS Extension. Reduce free moisture on leaf surfaces by avoiding overhead irrigation and destroy infected plants to prevent disease spread.
Rhizoctonia Root Rot
Rhizoctonia root rot is a fungal disease of gardenia that causes extensive above ground symptoms. Symptoms of the disease consist of leaf yellowing, browning of leaves, leaf wilt and plant death. The disease favours wet soil, poor drainage and overwatering. Increase water drainage and avoid overwatering to prevent the fungus from infecting the roots of gardenia. Plant disease-resistant gardenia species in soil infected with Rhizoctonia to prevent the disease from spreading to susceptible gardenias. University-based and private labs can test for Rhizoctonia fungi in soil before planting susceptible gardenia.
Cercospora Leaf Spot
Cercospora leaf spot is a common foliage disease of many ornamentals, including gardenia, caused by Cercospora fungi. The fungi favour warm weather temperatures between 68 and 95F and free moisture on leaf surfaces for infection. Symptoms of the disease appear during the late summer and early fall seasons. Symptoms of infection consist of leaf yellowing, dark brown to black spots on leaf surfaces, yellow halo around leaf spots and leaf drop. Chemical fungicide applications can help prevent and control Cercospora leaf spot.
Phytophthora Root Rot
Phytophthora root rot is a disease of gardenia that threatens the vitality of the entire plant. Symptoms are similar to Rhizoctonia root rot, consisting of leaf yellowing, brown spots on leaves, reduced plant vigour, leaf drop, brown rotting roots and plant death. Phytophthora fungi favour wet soil conditions, poor soil drainage and deep roots for infection. Plant resistant gardenia species and destroy infected plants to reduce disease spread.