Azaleas and Rhododendrons: Common Diseases and Abiotic Problems

Rhododendron in bloom. Photo: Pixabay

Key Points

• Rhododendrons and azaleas offer a wide range of flower colors in the spring and have become popular landscape plants in Maryland. 

• All rhododendrons and azaleas are members of the genus Rhododendron. There are evergreen and deciduous forms of both.

• Rhododendrons and azaleas prefer similar growing conditions: cool, moist, well-drained, acidic soil (pH between 4.5 and 5.5), partial sunlight, and protection from strong winds. Azaleas will generally tolerate drier conditions than rhododendrons.

• Avoid planting these shrubs in poorly drained compacted soils, low areas that collect water runoff, and locations near downspouts. These sites will often cause root rot issues.

• Other related landscape plants that require similar growing conditions include andromeda, mountain laurel, leucothoe, and blueberries.
  

• Where growing conditions are unsuitable, these shrubs are susceptible to the disorders and
  diseases described below.
  
  
• See also Insect Pests of Azaleas and Rhododendrons.

Common Abiotic (Non-Pathogenic) Problems of Rhododendrons and Azaleas

Rhododendron chlorosis (yellowing), a symptom of iron deficiency. Photo: George Hudler, Cornell University, Bugwood.org

Leaf Chlorosis (Yellowing Leaves)

Chlorosis symptoms, caused by a deficiency of iron, appear as yellow leaves with prominent green veins. These symptoms are usually caused by high soil pH. 

A soil test can determine if the pH is too high. Iron is most readily available in acidic soils between pH 4.5-6.0. When the soil pH is above 6.5, iron may be present in adequate amounts, but is in an unusable form due to an excessive amount of calcium carbonate. This can occur when plants are placed too close to cement foundations or walkways.

Soil amendments that acidify the soil, such as iron sulfate or sulfur, are the best long term solution. Some fertilizers such as ammonium sulfate will also acidify the soil. Foliar sprays of iron sulfate or chelated iron can reduce symptoms.

Chlorosis caused by magnesium deficiency is initially the same as iron, but progresses to form reddish purple blotches and marginal leaf necrosis (browning of leaf edges). Epsom salts are a good source of supplemental magnesium. 

Other causes of chlorosis include poor root growth, root rot, root damage caused by over fertilization or excessive deep cultivation, soil nematodes, and poor drainage.


Frost and winter injury of rhododendrons and azaleas. Photo: Clemson University-USDA Cooperative Extension Slide Series, Bugwood.org

Winter Injury

Low temperatures can cause bark splits near the base of the stem, damaged flower buds and browning on the edges (marginal necrosis) of the leaves. Both bark splits and flower bud damage can be caused by a sudden temperature drop in the fall before new growth has hardened off, cold temperatures after dormancy has broken in the spring, or after a winter thaw. Winter hardiness will also influence the likelihood of winter damage of marginally hardy varieties. 

Marginal leaf browning that results from drying winter winds is generally called “winter burn”. Tissue necrosis is caused by the removal of water in the leaves faster than the plant can replace it through root uptake from frozen water in the soil. Winter burn can be reduced by placing plants in locations less exposed to winter winds. Physical barriers, placed about 18 inches from the plants on the windward side, made from materials such as burlap or plastic, can also lessen winter wind damage by reducing wind velocity.

Diseases of Rhododendrons and Azaleas

Botryosphaeria canker symptoms on Rhododendron. Photo: David L. Clement, University of Maryland

Botryosphaeria Dieback (Botryosphaeria dothidea)

This is the most common disease of older established rhododendrons in the landscape and is typically initiated by drought and heat stress. Typical symptoms of this fungal disease are scattered dying branches on an otherwise healthy plant. Leaves on infected stems turn brown, then droop and roll inward. These leaves often lay flat against the stem and will remain attached. 

The pathogen can infect all ages of stem tissue through wounds, pruning cuts, and leaf scars. Heat, drought stress, and winter injury can increase disease incidence. Cankers on branches can gradually grow through the wood until the stem becomes girdled. 

Botryosphaeria canker symptoms close up. Photo: David L. Clement, University of Maryland

Diseased wood is reddish brown in appearance. Discolored wood viewed in longitudinal cross section often forms a wedge that points toward the center of the stem, and the pith may be darker brown than the surrounding wood.

Management: 

• Plants should be grown in partial shade with mulch and kept well-watered during dry periods. 

• All dying branches should be promptly pruned out in dry weather and all discolored wood should be removed at least 6 inches below the last symptoms of disease. Plants can be pruned back severely if needed since dormant lateral buds will initiate new growth after pruning.

• Plants should also be protected from rough treatment during maintenance activities to prevent unnecessary wounds.

Phytophthora root and crown rot symptoms. Ethel Dutky, University of Maryland, Bugwood.org

Phytophthora Root Rot and Dieback (Phytophthora spp.)

This soil pathogen, called a water mold, exists in low levels throughout Maryland and becomes a problem in wet sites. As roots are killed the leaves begin to turn a lighter green and eventually yellow. Infected plants initially appear wilted. As symptoms progress leaves roll inward towards the midrib and turn brown. 

Symptoms of disease usually occur within the first season of planting. Highly susceptible cultivars can die within two weeks, whereas more resistant plants may not die until many weeks after the plants have developed the initial wilt symptoms. The entire root system may become diseased or portions may escape infection and support the plant until other stress factors cause death. On older plants, symptoms of root rot may be present a season or more before death. In such cases plants often decline in vigor and suffer additional damage from other pathogens or insect pests.

Phytophthora stem dieback, although uncommon in the landscape, is a distinct phase of the Phytophthora disease syndrome on rhododendrons. It can be brought into the landscape on infected plants and can be severe on plants grown under overhead sprinkler irrigation. The disease occurs when the pathogen is splashed onto the foliage. Thus, infected plants may show symptoms on leaves and shoots, but may have healthy root systems.

Plants with dieback develop symptoms on the current season’s growth. Mature leaves are often resistant, however if they become infected, they usually fall prematurely. Infected leaves show chocolate brown lesions that often expand and cause dieback of the shoot tips. Infected leaves droop and curl towards the stem. Diseased leaves remain attached to the stem. Growth of the pathogen through the midrib tissue often produces a V-shaped lesion that extends along the leaf midrib into the stem.

Symptoms on older plants can be caused by introducing infected plant material into the same planting bed, changes in water drainage patterns, and low plant vigor.

Management: 

• Resistant varieties are available. However, if they are flooded for 48 hours or longer, or are drought-stressed to the point of wilting, resistance is temporarily lost and the pathogen can invade.

• The best disease prevention options are to avoid poorly drained compacted soils, low areas that collect water runoff, and locations near downspouts. Construction of raised beds or grade changes may be needed to ensure proper drainage. 

• Although chemical controls can be used in nursery production, these measures are often too expensive and impractical in a landscape. Several soil and spray applications are required throughout the summer season to control both of these diseases. In addition, no chemical treatments will cure plants that show symptoms. 

• All newly planted rhododendrons should be watched closely for symptoms and infected plants or prunings should be removed promptly.
  
  

Ovulinia petal blight of azalea. Photo: E. Dutky

Ovulinia Petal Blight (Ovulinia azaleae)

This disease only affects the flower petals. The first symptoms are small water soaked spots on the petals. These spots rapidly enlarge and cause the flower to collapse and feel slimy. This disease is most severe under warm moist conditions. Flowers blighted by other fungi such as Botrytis or those killed by frost will not feel slimy. Flowers affected by petal blight turn brown and remain attached to the plant. Small black resting structures called sclerotia will develop on the brown flower remains and will overwinter on the ground. Sclerotia can survive as long as two years in the soil and will start the infection cycle in the spring.

Ovulinia petal blight on Azalea. Small black sclerotia (fungal resting structures) develop on brown petals. Photo: Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.org

Management: 

• Remove old garden debris and old mulch below plants to reduce the sclerotia (fungal structures) that will cause new spring infections. Add a new layer of mulch.

Exobasidium gall. Photo: Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.org

Exobasidium Gall (Exobasidium spp.)

While very noticeable, these galls will not threaten the health of the plant. This problem is more common during cool and wet spring weather. The first symptoms are swollen or puffy portions on newly expanding leaves, shoots, buds or flowers. The galls range in color from green to pink or red, depending on the part of the plant infected. 

As these galls age they develop a white surface growth which is a layer of reproductive spores. Eventually the infected tissue will turn brown and shrivel up into hard galls.

Management: 

• Fungicide sprays are not effective for the control of this disease. 

• Prevention strategies involve hand picking the galls off before they develop the white surface growth to reduce the incidence of disease next season.

Powdery Mildew
Erysiphe azaleae, Erysiphe vaccinii 

This disease is usually more common on deciduous azaleas compared to evergreen plants. Young plants grown in heavy shade are the most seriously affected by this disease. Infected plants appear to be covered with a powdery white substance on the leaves. The disease is more severe during periods of cool, moist weather. These fungi produce spores on the surface of the infected leaves which are spread by wind currents to surrounding leaf tissue. These fungi overwinter in the bud scales for initiation of infection next season.

Management: 

• Maintain proper plant spacing to ensure good air circulation. 

• Check the label registration on horticultural oil products and registered fungicides for powdery mildew control listings.

Cercospora leaf spot on Rhododendron. Photo: Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.or

Leaf Spots
Various Fungal Species

In general, most fungal leaf spots are not threatening to the health of the plant, although under severe conditions some defoliation can occur. The symptoms usually include discrete spots with tan to brown centers surrounded by a darker border. The most common fungal leaf spot pathogens of Rhododendron spp. are Botrytis cinerea, Pestalotia sydowiana, P. rhododendri, Septoria azaleae , Colletotrichum azaleae (Glomerella cingulata), Cercospora handelii, Phyllosticta cunninghamii, and P. rhododendri.

Management: 

• Strategies for disease prevention include pruning and removal of infected leaves, proper plant spacing to allow good air circulation, minimizing water on the foliage from overhead irrigation, and removal of fallen leaves followed by applications of fresh mulch.. 

• Although fungicides are registered for disease control, they are not necessary in most situations.
  
  

Rust (Puccininastrum vaccinii)

Of the six rusts in North America, the hemlock-blueberry rust, is the one most common in Maryland. This rust is only sporadically severe and typically infects deciduous azaleas. Symptoms are easily recognized by the brightly colored yellow-brown spores present in disease pustules on the lower or upper surfaces of leaves. Infected plants lose their leaves prematurely and show reduced growth. Other fungi may then attack weakened plants causing further injury. This rust also causes yellowing and leaf or needle drop on blueberry and hemlock.

Management: 

• Prevention includes planting resistant varieties (see resistant plant list), removal of diseased leaves, proper plant spacing to maximize air circulation.

By Dr. David L. Clement, University of Maryland Extension Specialist, Home and Garden Information Center. Revised 4/2020.

References:

Johnson, W.T. and H.H. Lyon. 1991. Insects That Feed on Trees and Shrubs, 2nd Ed. Ithaca, NY: Comstock Publishing Associates, Cornell University Press. 560 pp.

Pirone, P.P. 1978. Diseases & Pests of Ornamental Plants, 5th Ed. New York: John Wiley & Sons. 566 pp.

Sinclair, W.A.. H.H. Lyon, and W.T. Johnson. 1987. Diseases of Trees and Shrubs. Ithaca, N.Y.: Comstock Publishing Associates, Cornell University Press. 574 pp.