Quick Guide: Common Azalea & Rhododendron Diseases (South Carolina)
Affected Plants:
- Azaleas and rhododendrons (Rhododendron spp.)
Primary Disease Types:
- Root & crown rot
- Petal blight
- Leaf gall
- Dieback
- Leaf spot diseases
Primary Causes:
- Fungal pathogens
- Poor drainage and excess moisture
- Environmental stress (drought, cold, wounding)
Most Important Prevention Strategy:
- Proper planting, drainage, and cultural practices
Chemical Control:
- Often ineffective once symptoms appear
- Fungicides may suppress (not cure) certain diseases when used correctly
Rhododendrons and azaleas are some of the most popular spring-flowering shrubs in the landscape. Healthy plants can provide years of enjoyment.
Many diseases and other problems can be prevented by following the recommended cultural practices for proper planting and care.
More information on successfully growing azaleas and rhododendrons is available in the fact sheets:
Root & Crown Rot on Azaleas and Rhododendrons
Cause of Root & Crown Rot
Phytophthora fungi cause one of the most common disease problems in the landscape for rhododendron and azalea. This fungus is a “water mold,” and thrives in poorly drained or wet conditions.
Symptoms of Root Rot in Azaleas and Rhododendrons
A wilted plant is usually the first sign of trouble. Rhododendron leaves will curl inward and droop. Drought can cause similar symptoms.
Roots of affected plants appear soggy or blackened, and the outer portion of the root easily pulls away from the inner portion.
Symptoms of Crown Rot
Crown rot causes a brown discoloration of the wood in the lower stem near the soil line. This disease is favored in poorly drained areas or when plants are set too deeply.
Plants may remain symptom-free until further stressed by drought or flooding.
Prevention & Treatment of Root and Crown Rot
Disease prevention is important because chemical controls are ineffective once symptoms appear in the landscape.
Planting Practices to Reduce Root Rot Risk in Azaleas and Rhododendrons
Plant Selection:
- Purchase disease-free plants from a reputable nursery
- Avoid plants that:
- Lack normal green color
- Appear wilted in the morning
- Have dark, discolored roots
- Select resistant varieties
Soil Preparation:
- Plant in well-drained and well-aerated soil
- Amend heavier clay soils with organic matter before planting
- Avoid planting where water collects around plant roots
- Plant at or slightly above the original soil level
- Plant in raised beds
- Firm the soil at the base of the planting hole to prevent plant settling
Pathogen Considerations:
- Avoid sites with a history of root rot
- Resistant plants can succumb
- Fungus survives in the soil; it cannot be eradicated once the soil is contaminated
Phytophthora root & crown rot-resistant azaleas & rhododendrons include the following:
Azaleas with Resistance to Phytophthora Root & Crown Rot
Resistant:
- R. sanctum
- R. simsii (Indian azalea)
- R. yedoense var. poukhanense (Korean azalea)
- ‘Corrine’
- ‘Fakir’
- ‘Fred Cochran’
- ‘Glacier’
- ‘Hampton Beauty’
- ‘Higasa’
- ‘Merlin’
- ‘Polar Sea’
- ‘Rose Greeley’
Moderately Resistant:
- ‘Alaska’
- ‘Chimes’
- ‘Eikan’
- ‘Jan Cochran’
- ‘Morning Glow’
- ‘New White’
- ‘Pink Gumpo’
- ‘Pink Supreme’
- ‘Rachel Cunningham’
- ‘Red Wing’
- ‘Shinkigen’
- ‘Sweetheart Supreme’
Rhododendrons with Resistance to Phytophthora Root & Crown Rot
Resistant:
- ‘Caroline’
- ‘Martha Isaacson’
- ‘Pink Trumpet’
- ‘Prof. Hugo de Vries’
- ‘Red Head’
- R. davidsonianum
- R. delavayi
- R. glomerulatum
- R. hyperythrum
- R. lapponicum
- R. occidentale
- R. poukhanense
R. pseudochrysanthum
- R. quinquefolium
- R. websterianum
Petal Blight on Azaleas and Rhododendrons
Cause of Petal Blight
This fungal disease, caused by Ovulinia azaleae, primarily affects azalea flowers, but mountain laurel and rhododendron flowers can also be infected. Indian and Kurume azaleas are especially susceptible.
Symptoms of Petal Blight
The disease starts on the flower petals as tiny, irregularly-shaped spots, giving a “freckled” appearance. On colored flowers, the spots are white; on white flowers, they are brown.
The spots quickly enlarge and become soft and watery. Flowers rot and stick to the leaves.
Disease Spread and Survival
Infection is easily spread from flower to flower by wind, rain and insects. The fungus survives the winter in the soil.
Prevention & Treatment of Petal Blight
Petal Blight Prevention:
- Pick and destroy infected flowers
- Avoid overhead watering
- Remove ground litter and replace with fresh mulch
- This fungus survives in the soil
Fungicides:
- Captan
- Chlorothalonil
- Mancozeb
Note: Apply all chemicals according to label directions.
See Table 1 for product examples.
Leaf and flower gall (Exobasidium vaccinii) on deciduous native azalea.
Joey Williamson, ©2013 HGIC, Clemson Extension
Leaf Gall on Azaleas and Rhododendrons
Cause of Leaf Gall
Leaf gall (Exobasidium vaccinii) is a very common fungal disease in the early spring on azaleas and occasionally on rhododendrons. Some of the native rhododendron species (azaleas) are more susceptible than hybrid rhododendrons.
Symptoms of Leaf and Flower Gall
In April and May, leaves and buds of infected plants develop distorted growth. Leaves and possibly stems become thickened, curled, fleshy, and turn pale green to white.
In the later stages of the disease, the galls become covered with a white powdery substance.
As the galls age, they turn brown and hard.
Prevention & Treatment of Leaf Gall
Exobasium flower gall on Catawba rhododendron (Rhododendron catawbiense).
Joey Williamson, ©2015 HGIC, Clemson Extension
Prevention:
- This disease rarely does enough damage to require chemical control.
Treatment:
For small infections:
- Pick and destroy galls.
Fungicides:
- Mancozeb
- Chlorothalonil
Note: Apply all chemicals according to label directions.
See Table 1 for product examples.
Dieback on Azaleas and Rhododendrons
Cause of Dieback
Botryosphaeria rot and canker (Botryosphaeria spp.)
Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org
Dieback is an important disease of hybrid rhododendrons in the landscape, caused by the fungus Botryosphaeria dothidea. Azaleas with similar symptoms are more likely to be infected with Phomopsis species fungi.
Symptoms of Dieback
Typically, dying branches (stem dieback) begin to appear on an otherwise healthy plant. The leaves die and can remain attached to the plant until late summer.
Usually, a single branch on an established plant is affected.
Scraping away the bark with a knife reveals a reddish-brown discoloration under the bark on dying branches of rhododendron.
On azaleas, the discolored wood under the bark appears chocolate brown.
Azalea and Rhododendron Varieties with Resistance to Dieback
Azalea varieties that are least susceptible:
- ‘Delaware Valley White’
- ‘Hershey Red’
- ‘Pink Gumpo’
- ‘Snow’
Rhododendron varieties considered resistant:
- ‘Boursalt’
- ‘Chionoides White’
- ‘Cunningham’s White’
- ‘English Roseum’
- ‘Le Barr’s Red’
- ‘Roseum Two’
- ‘Wissahickon’
Prevention & Treatment of Dieback
Dieback is difficult to control on rhododendrons and azaleas in the landscape.
Prevention Tips for Dieback
- Reduce plant stress by planting in partial shade and watering during dry periods.
- Drought stress and freeze injury predispose azaleas to infection.
- Avoid wounding the plant.
Dieback Treatment
- Prune infected branches well below all discolored wood and dispose of dead plant material.
- Clean pruning tools between cuts with a dilute solution of household bleach (1 part bleach to 9 parts water) or 70% rubbing alcohol.
Fungicides:
For azaleas:
- Thiophanate-methyl
- Mancozeb
For rhododendrons:
- Copper-based fungicide
- Chlorothalonil
Note: Apply all chemicals according to label directions.
See Table 1 for product examples.
Leaf Spots on Azaleas and Rhododendrons
Cercospora leaf spot (Cercospora handelii)
Florida Division of Plant Industry Archive, Florida Department of Agriculture and Consumer Services, Bugwood.org
Cause of Leaf Spot Diseases
Throughout the year, fungal spots (Cercospora species, Septoria species, Phyllosticta species and Colletotrichum species) of various colors appear on azalea and rhododendron leaves.
Symptoms of Leaf Spots
The diseases that occur are usually minor, affecting only the plant’s aesthetic value.
Severe infections may lead to early leaf drop, reducing the plant’s overall health.
Prevention & Treatment of Leaf Spot Diseases
Leaf Spot Prevention:
- Remove fallen leaves.
- Keep leaves dry when watering plants.
Leafspot Treatments
- Spraying fungicide during periods of high humidity to prevent serious foliage damage.
Fungicides
For azaleas:
- Copper hydroxide
- Copper-based fungicides
- Thiophanate-methyl
- Chlorothalonil
For Cercospora leaf spot on rhododendron:
- Propiconazole
- Thiophanate-methyl
- Chlorothalonil
- Mancozeb
Note: Apply all chemicals according to label directions.
See Table 1 for product examples.
Other Problems Affecting Azaleas and Rhododendrons
Leaf Curl on Rhododendrons
Rhododendron leaves begin to cup and curl at the edges when temperatures drop to below 35°F. At 25ºF, the leaves will be tightly curled and begin to droop.
This problem is not caused by insects or disease but is a way the plant reduces water loss from its leaves during cold, dry, windy weather.
Plants should recover when the weather warms again.
Fungicide Products for Azalea and Rhododendron Disease Control (Home Landscape)
Examples of fungicide products labeled for azalea and rhododendron disease management in the home landscape.
Table 1. Fungicide Products for Azalea and Rhododendron Disease Control in the Home Landscape.
| Active Ingredient | Examples of Products |
| Captan | Arysta LifeScience Captan 50 Wettable Powder Fungicide Drexel Captan 50W Fungicide Southern Ag Captan Fungicide |
| hlorothalonil | Bonide Fung-onil Multi-purpose Fungicide Concentrate Bonide Fung-onil Multi-purpose Fungicide Ready to Use Ferti-lome Broad Spectrum Landscape & Garden Fungicide GardenTech Daconil Fungicide Concentrate GardenTech Daconil Fungicide Ready-to-Use Hi-Yield Vegetable, Flower, Fruit And Ornamental Fungicide Ortho MAX Garden Disease Control Concentrate Southern Ag Liquid Ornamental and Vegetable Flowable Fungicide Tiger Brand Daconil |
| Copper-based Fungicides | Bonide Captain Jack’s Liquid Copper Fungicide Bonide Copper Fungicide Spray or Dust Ready to Use Camelot O Fungicide/Bactericide Espoma Organic Copper Soap Ready-to-Use Fungicide Monterey Copper Fungicide Garden Spray Liqui-Cop Natural Guard by ferti-lome Copper Soap Fungicide Concentrate Natural Guard by ferti-lome Copper Soap Fungicide Ready to Use Southern Ag Liquid Copper Fungicide |
| Mancozeb | Bonide Mancozeb Flowable with Zinc Concentrate Southern Ag Dithane M-45 |
| Propiconazole | Banner Maxx II Fungicide Bonide Infuse Systemic Disease Control Bonide Infuse Systemic Disease Control Lawn & Landscape Ready to Spray Ferti-lome Liquid Systemic Fungicide II Quali-Pro Propiconazole 14.3 Fungicide |
| Thiophanate Methyl | Southern Ag Thiomyl Systemic Fungicide |
| 1 RTU (Ready to Use) = a pre-mixed spray bottle. 2 RTS (Ready to Spray) = a hose-end spray bottle. |
|
Language Availability
Para obtener la versión en español de esta hoja informativa, consulte HGIC 2050S, Enfermedades de Azalea & Rhododendron.
Pesticides are updated annually. Last updated 2/2026 by N. Jordan Franklin.
Originally published 05/99
