Azalea and Rhododendron Diseases: Identification and Control

Common Diseases of Azaleas and Rhododendrons

These popular landscape shrubs are susceptible to several fungal diseases. Proper cultural practices and targeted treatments can minimize damage.

Botryosphaeria Canker

Symptoms: Rough, sunken, dark-brown areas around wounds or natural bark openings; recently killed branch wood lighter brown than pith; dead bark falls off; leaves on affected branches wilt and die; chocolate-brown cankers enlarge along branches; tiny black fruiting structures on dead bark.

Pathogen: Botryosphaeria dothidea.

Management: Irrigate to prevent drought stress (predisposes plants). Prune infected branches back to where growth will resume; avoid large stubs. Disinfest pruning shears frequently. No chemicals adequately control this disease. Resistant hybrids: Boursault, Cunningham's White, English Roseum, Lebar's Red, Roseum Elegans, Roseum 2.

Botrytis Blight

Symptoms: Small water-soaked lesions on petals; gray fungal growth covers infected petals.

Pathogen: Botrytis cinerea.

Management: Space plants for good air circulation and avoid high humidity. Remove fading flowers and yellowing leaves. Apply fungicide to protect plants.

Cercospora Leaf Spot

Symptoms: Circular to irregular brown spots up to ½ inch on leaves (especially lower); spots may become tan-centered with yellow halo; dark-brown pimple-like fruiting structures in spots; infected leaves may fall.

Pathogen: Cercospora handelii.

Management: Inspect new plants; reject infected ones. Rake and destroy fallen leaves. Avoid overhead irrigation. Apply fungicide to protect leaves before spring infection.

Cylindrocladium Blight

Symptoms: Leaves turn brown to black and fall in 3–4 days; stems have brown spots with white spore masses; or roots die causing wilting without leaf spots; plants die rapidly.

Pathogen: Cylindrocladium scoparium.

Management: Rake and destroy fallen leaves. Use clean, disinfested tools. Remove severely infected plants. Apply fungicide to protect plants.

Leaf and Flower Gall

Symptoms: Young leaves and flowers become swollen, fleshy, pale green; turn white from spore formation; later become hard and brown. Infection occurs in spring; spores disperse but remain dormant until next spring.

Pathogen: Exobasidium vaccinii.

Management: Remove and destroy all galls before they turn white with spores. If galls were too numerous to pick in previous years, apply fungicide; cease applications when leaves reach full size.

Ovulinia Petal Blight

Symptoms: Pale-white to rust-colored spots on petals; spots enlarge rapidly; petals become slimy and fall apart.

Pathogen: Ovulinia azalea.

Management: Remove crop debris. Water to keep plant surfaces dry. Space plants for air circulation. Apply fungicide as blossoms open.

Phytophthora Root Rot and Top Dieback

Symptoms: Plants stunted and wilted; leaves yellow; entire plant dies; roots with few feeder roots die; stem wood at soil level shows red-brown discoloration. Top dieback phase: leaves have dark-brown spots; shoots die from tips back with dark-brown cankers; no obvious fruiting structures. Easily confused with Botryosphaeria canker.

Pathogen: Phytophthora species.

Management: Purchase disease-free plants; inspect southern-grown containerized material carefully. Use clean, disinfested tools. Avoid overhead watering. After confirmed diagnosis, remove infected plants. Fungicide applications to infected plants only delay death; apply to protect healthy plants.

Powdery Mildew

Symptoms: Faint yellow areas on expanded leaves; white fungal growth on yellow areas of some cultivars; small dead spots with no fungal growth on others.

Pathogen: Erysiphe polygoni or Microsphaera penicillata.

Management: Apply fungicide to protect plants.

Rhizoctonia Web Blight

Symptoms: Small tan to black spots on leaves expanding to engulf entire leaf; leaves fall; webbing may develop; small plants may die.

Pathogen: Rhizoctonia solani.

Management: Maintain good air circulation. Avoid late afternoon watering. Apply fungicide to protect plants.