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.
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:
- R. sanctum
- R. simsii (Indian azalea)
- R. yedoense var. poukhanense (Korean azalea)
- ‘Fred Cochran’
- ‘Hampton Beauty’
- ‘Polar Sea ’
- ‘Rose Greeley’
- ‘Jan Cochran’
- ‘Morning Glow’
- ‘New White’
- ‘Pink Gumpo’
- ‘Pink Supreme’
- ‘Rachel Cunningham’
- ‘Red Wing’
- ‘Sweetheart Supreme’
- ‘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, chlorothalonil, maneb, or triforine. 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|
|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.