Rhododendrons and azaleas are some of the most popular spring-flowering shrubs in the landscape, and healthy plants can give years of pleasure. 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 HGIC 1059, Azalea Care; HGIC 1058, Azalea Planting; and HGIC 1073, Rhododendron.
Azaleas are gorgeous blooming shrubs that add a pop of color to gardens. However, they are prone to fungal diseases like root rot, which can quickly kill the plant if left unchecked. In this article, we’ll explore the causes of azalea root rot and provide effective solutions to prevent and treat it.
What Causes Root Rot in Azaleas
Several factors can contribute to root rot in azaleas
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Overwatering – Excess moisture around the roots encourages fungal growth leading to rot.
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Poor drainage – Waterlogged soil deprives roots of oxygen, causing them to die.
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Compacted soil – Lack of air pockets in dense soil also leads to root asphyxiation.
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Poor nutrition – Weakened azaleas are more susceptible to root infections.
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Wounded roots – Damage to roots provides an entry point for fungus.
Two common fungal pathogens responsible for azalea root rot are Phytophthora and Pythium. They thrive in moist conditions.
Symptoms of Root Rot in Azaleas
Watch for these signs of root rot in azaleas:
- Wilting and drooping leaves
- Yellowing and browning of leaves
- Premature leaf drop
- Discolored and mushy roots
- Stunted growth
- Branch dieback
- Plant collapse
Act promptly when you notice these symptoms to save the plant.
Preventing Root Rot in Azaleas
Prevention is key to avoiding root rot in azaleas:
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Plant in well-draining soil or use containers with drainage holes.
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Water only when the top inch of soil is dry.
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Use drip irrigation and avoid excessive splashing.
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Mulch around the plant to retain soil moisture.
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Prune lower branches for airflow around the base.
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Space plants adequately to allow air circulation.
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Fertilize azaleas properly and avoid over-fertilization.
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Avoid wounding roots during cultivation or transplanting.
Treating Root Rot in Azaleas
If you detect root rot early, you may be able to rescue the plant:
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Improve drainage immediately if soil is compacted or soggy.
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Remove diseased roots carefully using sanitized shears.
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Treat remaining roots and soil with copper-based fungicides.
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Replant in fresh soil amended with compost.
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Prune affected branches but avoid excessive pruning.
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Water moderately until the plant re-establishes.
Severely infected azaleas with substantial root loss often can’t be cured. Remove and destroy such plants to avoid spread.
Choosing Disease-Resistant Azalea Varieties
Some azalea varieties have better genetic resistance to root rot than others. Choose these robust types if you have trouble with rot:
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Kurume hybrids – Excellent rot resistance. Popular varieties include ‘Coral Bells’, ‘Hinodegiri’ and ‘Sherwood Red’.
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Satsuki hybrids – Bred specifically for disease resistance. ‘Gumpo White’ and ‘Osakazuki’ are good choices.
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Evergreen azaleas – More rot-tolerant than deciduous types. Southern Indica hybrids like ‘George L. Tabor’ work well.
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Gable hybrids – Hardier with good rot resistance. ‘Rose Greeley’ and ‘Tradition’ are recommended.
Talk to your local nursery to pick disease-resistant azalea varieties suitable for your climate. With proper care, you can enjoy healthy, thriving azaleas in your garden.
Frequently Asked Questions About Azalea Root Rot
Here are answers to some common questions about root rot in azaleas:
How do I know if my azalea has root rot?
Look for symptoms like wilting, yellowing leaves, stunted growth, branch dieback and mushroom-like growth on roots. Dark brown, mushy roots confirm root rot.
What causes root rot in container azaleas?
Overwatering, poor drainage from blocked holes, dense potting mix, and contaminated containers can cause rot in potted azaleas.
Is root rot in azaleas contagious to other plants?
Yes, the fungal pathogens can spread from infected azaleas to other plants through water, infected soil or garden tools.
Should I prune azaleas with root rot?
Prune only the minimum – remove dead branches and just the obviously infected parts of roots. Excessive pruning stresses the plant further.
Can azaleas recover from root rot?
Early treatment can help azaleas recover from mild to moderate root rot. But severe infections often kill the plant, making recovery impossible.
By learning about the causes, symptoms and treatment of azalea root rot, you can take prompt action to protect your plants. With vigilance and proper care, you can maintain vibrant, flowering azaleas in your garden for years.
Root & Crown Rot
The fungus Phytophthora species causes 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. 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.
Crown rot causes the lower portions of the stem to have a brown discoloration of the wood near the soil line. This disease is favored in poorly drained areas or when plants are set too deeply. Plants may remain without symptoms until further stressed from drought or flooding.
Prevention & Treatment: Prevention of disease is important, because chemical controls are ineffective once symptoms appear in the landscape. Begin by purchasing disease-free plants from a reputable nursery. Avoid plants that lack normal green color, appear wilted in the morning, or have dark, discolored roots. Select resistant varieties for planting from the Table below.
Plant azaleas and rhododendrons in a well-drained and well-aerated soil. Heavier clay soils should be amended with organic matter before planting. Avoid planting in areas where water can collect around plant roots.
The following azaleas & rhododendrons have some 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’
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
Do not set new plants any deeper than the original soil level. Planting in raised beds is suggested. Firm the soil slightly at the base of the planting hole to prevent the plant from settling into the bed. Do not plant azalea and rhododendron plants into sites where plants have previously died from root rot. Even resistant plants may succumb under these conditions. The fungus survives in the soil and cannot be eradicated once an area is infected.
Chemicals that are available will only suppress disease and not cure an infected plant. Fungicides available for use on azaleas and rhododendrons include metalaxyl and mefenoxam. Apply all chemicals according to directions on the label.
This fungal disease, caused by Ovulinia azaleae, primarily affects the flowers of azalea, but mountain laurel and rhododendron flowers can also be infected. Indian and kurume azaleas are especially susceptible. The disease starts on the flower petals as tiny, irregularly-shaped spots, giving a “freckled” appearance. On colored flowers the spots are white, and on white flowers the spots are brown. The spots quickly enlarge and become soft and watery. Flowers rot and stick to the leaves. Infection is easily spread from flower to flower by wind, rain and insects. The fungus survives the winter in the soil.
Prevention & Treatment: The most important things that you can do to control this disease in the home landscape are to pick and destroy infected flowers and avoid overhead watering. This fungus survives in the soil, so it is important to replace the ground litter with uncontaminated mulches. Fungicides are available for cases of severe infection on azaleas. Select a product that contains captan or chlorothalonil. See Table 1 for examples of products. Apply all chemicals according to directions on the label.
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.
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: This disease rarely does enough damage to require chemical control. If only a few plants are affected, pick and destroy galls. If chemical control is necessary on azaleas, mancozeb, or chlorothalonil fungicide sprays can be used according to label directions. See Table 1 for examples of products.
Dieback is an important disease of hybrid rhododendrons in the landscape and is caused by the fungus Botryosphaeria dothidea. Azaleas with similar symptoms are more likely to be infected by the fungus Phomopsis species. 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.
Prevention & Treatment: Dieback is difficult to control on rhododendrons and azaleas in the landscape. The azalea varieties that are the least susceptible include: ‘Delaware Valley White,’ ‘Hershey Red,’ ‘Pink Gumpo’ and ‘Snow.’ The following rhododendron varieties are considered resistant: ‘Boursalt,’ ‘Chionoides White,’ ‘Cunningham’s White,’ ‘English Roseum,’ ‘Le Barr’s Red,’ ‘Roseum Two’ and ‘Wissahickon.’
Reduce stress to the plants by planting in partial shade and watering during dry periods. Drought stress and freeze injury may predispose azaleas to infection. Avoid wounding the plant. 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. For azaleas, fungicide sprays containing either thiophanate-methyl or mancozeb can be used. For rhododendrons apply a product containing a copper-based fungicide or chlorothalonil. See Table 1 for examples of products. Apply all chemicals according to directions on the label.
Throughout the year, fungal spots (Cercospora species, Septoria species, Phyllosticta species and Colletotrichum species) of various colors appear on azalea and rhododendron leaves.
The diseases caused are usually minor, only affecting the aesthetic value of the plant. Cases of severe infection may result in early leaf drop, reducing the general health of the plant.
Prevention & Treatment: Remove fallen leaves. Keep leaves dry when watering plants. Fungicide sprays during periods of high humidity will prevent serious foliage damage. Fungicide sprays recommended for azaleas include copper hydroxide, copper-based fungicides, thiophanate-methyl or chlorothalonil. For Cercospora leaf spot on rhododendron use propiconazole, thiophanate-methyl, chlorothalonil or mancozeb. See Table 1 for examples of products. Apply these fungicides according to directions on the label.
Leaf Curl: Rhododendron leaves begin to cup and curl at the edges when temperatures drop to below 35 ºF. At 25 ºF, the leaves will be curled very tight 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.
Table 1. Fungicide Products for Azalea and Rhododendron Disease Control in the Home Landscape.
| Active Ingredient | Examples of Products |
| Captan | Southern Ag Captan Fungicide WP Bonide Captan 50% WP Drexel Captan 50W Arysta Captan 50% WP Hi-yield Captan 50 W Fungicide |
| Chlorothalonil | Bonide Fung-onil Concentrate; & RTU1 Ferti-lome Broad Spectrum Landscape & Garden Fungicide Concentrate GardenTech Daconil Fungicide Concentrate Hi-Yield Vegetable, Flower, Fruit & Ornamental Fungicide Concentrate Ortho MAX Garden Disease Control Concentrate Southern Ag Liquid Ornamental & Vegetable Fungicide Concentrate Tiger Brand Daconil Concentrate |
| Copper-based Fungicides | Bonide Copper Fungicide Spray or Dust Bonide Liquid Copper Concentrate; & RTU1 Camelot Fungicide/ Bactericide Concentrate Monterey Liqui-Cop Fungicide Concentrate Natural Guard Copper Soap Liquid Fungicide Concentrate Southern Ag Liquid Copper Fungicide |
| Mancozeb | Bonide Mancozeb Flowable with Zinc Concentrate Southern Ag Dithane M-45 |
| Propiconazole | Banner Maxx Fungicide Bonide Infuse Systemic Disease Control Concentrate; & RTS2 Ferti-lome Liquid Systemic Fungicide II Concentrate |
| Thiophanate Methyl | Cleary’s 3336-WP Turf & Ornamental Fungicide Southern Ag Thiomyl Systemic Fungicide |
| 1 RTU = a pre-mixed spray bottle. 2 RTS = a hose-end spray bottle. | |
Para obtener la versión en español de esta hoja informativa, consulte HGIC 2050S, Enfermedades de Azalea & Rhododendron.
Pesticides are updated annually. Last updates were done on July 15, 2021 by Joey Williamson.
If this document didn’t answer your questions, please contact HGIC at [email protected] or 1-888-656-9988.
Nancy Doubrava, Former HGIC Horticulture Information Specialist, Clemson UniversityJames H. Blake, EdD, Extension Associate/Adjunct Professor, Dept. of Plant and Environmental Sciences, Clemson University
This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.
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