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Azalea and Rhododendron Diseases

Informational table showing disease name, symptoms, pathogen/cause, and management of Azalea and Rhododendron diseases.
Disease Symptoms Pathogen/Cause Management
Botryosphaeria canker Rough, sunken, dark-brown areas form around wounds or natural openings in the bark. The wood of a recently killed branch is lighter brown than the pith. Dead bark falls off the cankered area. Leaves on affected branches wilt as affected branches die. Chocolate-brown cankers enlarge along the branch more quickly than around its circumference. Tiny black fungal fruiting structures that pepper the dead bark are most easily seen on the light tan bark. Botryosphaeria dothidea Irrigate plants to prevent drought stress, which predisposes the plants to this canker. Prune infected branches, cutting back to where growth will resume. Do not leave large stubs of non-growing tissue. Disinfest the pruning shears frequently. No chemicals adequately control this disease. Rhododendron hybrids considered to have some resistance include Boursault, Cunningham's White, English Roseum, Lebar's Red, Roseum Elegans, and Roseum 2.
Botrytis blight Small, water-soaked lesions develop on petals. Gray fungal growth covers infected petals. Botrytis cinerea Space plants to ensure good air circulation and to avoid excessively high humidity. Remove fading flowers and yellowing leaves. Apply a fungicide to protect plants.
Cercospora leaf spot) Circular to irregular brown spots up to 1/2 inch in diameter form on leaves, especially lower leaves. Spots may become tan in the center and may have a yellow halo. Dark-brown, pimple-like fungal fruiting structures form within the spots. Infected leaves may fall. Cercospora handelii Inspect new plants and do not use if found to be infected. Rake and destroy fallen leaves. Avoid overhead irrigation. Apply a fungicide to protect leaves before infection in the spring.
Cylindrocladium blight Leaves turn brown to black and fall in 3 to 4 days. Stems have brown spots on which white masses of spores later form. Or roots die and plants wilt without having leaf spots form. Plants die rapidly. Cylindrocladium scoparium Rake and destroy fallen leaves. Use clean, disinfested tools. Remove severely infected plants. Apply a fungicide to protect plants.
Leaf and flower gall Young leaves and flowers become swollen, fleshy, and pale green. These become white due to the formation of spores by the fungus on the surface. Later, the galls become hard and brown. Infection occurs in the spring. The new spores formed on the surface of the galls are dispersed but do not cause more galls to form during that same season. They remain dormant until the following spring. Exobasidium vaccinii Remove and destroy all galls before they become white with new spores. If many plants had the disease in previous years and galls were too numerous to pick, apply a fungicide. Applications can cease when the leaves reach their full size.
Ovulinia petal blight Pale-white to rust-colored spots form on petals. Spots enlarge rapidly. Petals become slimy and fall apart easily. Ovulinia azalea Remove crop debris. Water in a manner that keeps plant surfaces dry. Space plants to ensure good air circulation. Apply a fungicidel as blossoms open.
Phytophthora root rot and top dieback Plants are stunted and wilted. Leaves yellow, and the entire plant eventually dies. Roots with few feeder roots die. Stem wood at the soil level has red-brown discoloration. In the top dieback phase, leaves have dark-brown spots. Shoots die from the tips back as dark-brown cankers form. No obvious fungal fruiting structures are formed. Compare these symptoms to those of Botryosphaeria described above because these diseases are readily confused. Phytophthora spp. Purchase disease-free plants. Especially inspect southern-grown, containerized material before planting. Use clean, disinfested tools. Avoid overhead watering. Following a diagnosis confirming that the plant is infected, remove infected plants. Apply a fungicide to protect plants. Fungicide applications to infected plants merely delay eventual death.
Powdery mildew Faint yellow areas form on expanded leaves. White fungal growth forms on the yellow areas of some cultivars while only small dead spots with no fungal growth occur on other cultivars. Erysiphe polygoni or Microsphaera penicilata Apply a fungicide to protect plants.
Rhizoctonia web blight Small tan to black spots form on leaves and expand to engulf the entire leaf. Leaves fall. Webbing may develop. Small plants may die. Rhizoctonia solani Maintain good air circulation. Avoid late afternoon watering. Apply a fungicide to protect plants.

Leaf and flower gall
Leaf and flower gall

Ovulinia petal blight
Ovulinia petal blight

Botryosphaeria symptoms and signs
Botryosphaeria symptoms and signs

Botryosphaeria symptoms and signs
Botryosphaeria symptoms and signs

Botryosphaeria symptoms and signs
Botryosphaeria symptoms and signs

Botryosphaeria symptoms and signs
Botryosphaeria symptoms and signs

Cercospora leaf spot
Cercospora leaf spot

Phytophthora top dieback
Phytophthora top dieback

Stem browning caused by Phytophthora
Stem browning caused by Phytophthora

Prepared by Gary W. Moorman, Professor of Plant Pathology

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Article Details

Title

Azalea and Rhododendron Diseases

Code

XL0007

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