rhododendron bloom

Rhododendron in bloom

Updated: September 29, 2021

Key points

  • Rhododendrons and azaleas offer a wide range of flower colors in the spring and are one of the most popular landscape plants in Maryland. 
  • All rhododendrons and azaleas are members of the genus Rhododendron. There are evergreen and deciduous forms of both.
  • (PDF) Wildflower in Focus Pinxterbloom azalea, Rhododendron periclymenoides, a deciduous shrub, is one of several azaleas native to the Mid-Atlantic area.  
  • Other related landscape plants that require similar growing conditions include Japanese andromeda (Pieris japonica), mountain laurel, leucothoe,  fothergilla (flowers best in full sun), and blueberries.
  • Where growing conditions are unsuitable, azaleas and rhododendrons are susceptible to the disorders and diseases described below.

Growing azaleas and rhododendrons in Maryland

Site

Rhododendrons and azaleas prefer similar growing conditions: cool, moist, well-drained, acidic soil (pH between 4.5 and 5.5), partial sunlight (morning sun and afternoon shade), and protection from strong winds. Before planting, mix organic matter into the planting site. 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.

Spacing and mature size

Large variations depending on the genus and species planted.
Check the plant label for the mature height and width. 

Disease and insect resistant plants

Disease and insect-resistant plants prevent or help to reduce problems in your landscape.

Refer to the American Rhodendron Society good performer azaleas and rhododendrons web page for recommendations. 

Pruning

Prune and shape 6-weeks after they have finished flowering. Renewal pruning should be done in early spring before new growth appears, however there will no blooms.

Watering

If there is less than 1” of rainfall per week, water to maintain even soil moisture during dry periods. This is important for plants planted less than 2 years ago. Water at the base of the plants rather than overhead to minimize leaf wetness as much as possible.

Fertilizing

Fertilize azaleas and rhododendrons if a soil test indicates a nutrient deficiency and adjust the soil pH if recommended by the soil testing lab. Soil pH affects the availability of nutrients to plants.

Diagnostic table of azalea and rhododendron problems

Symptoms Details Possible Causes
Leaf Yellowing Towards interior of plant Fall Coloration (normal)
  Iron or magnesium chlorosis High soil pH and nutrient deficiency
  Leaves may also have sooty mold (black coating) Azalea Bark Scale
  Brightly colored, yellow-orange spots Azalea Leaf Rust
Leaf Stippling (tiny spots) Whitish spots on upper surface of leaves; black spots on the lower leaf surface Lace Bugs
Leaf Mottling (irregular marks or blotches) Margins of leaves cup, lower leaves covered with honeydew, then sooty mold; flat, oval transparent insects on the lower leaf surface Azalea Whitefly
Leaf Distortion Swollen green or white puffy areas on newly expanded leaves or flowers Exobasidium Leaf and Flower Gall
  Leaves roll and droop Drought and Water Stress
Leaves Eaten or Chewed Notching of leaf margins; interior leaves show the most damage Black Vine Weevil
  Notches chewed in leaf margins Two-banded Japanese Weevil
  Branches or entire plants may be defoliated Azalea Caterpillar
Blotches on Leaves Blotch mines on leaves of azalea in May Azalea Leafminer
Spots on Leaves Visible on upper leaf surface Various Fungal Leaf Spots
Leaf Scorch Margins of leaves turn brown in winter or spring Winter Injury
White Coating on Leaves May be on upper and lower leaf surfaces Powdery Mildew
Flowers Affected Small, water-soaked spots enlarge rapidly, cause flowers to collapse and feel slimy Ovulinia Petal Blight
  Swollen areas on buds, flowers and petals Exobasidium Leaf and Flower Gall
Failure to Flower Sudden temperature drop in fall, unusually cold winters, or spring freeze. Pruned after flower buds were formed Winter Injury, improper pruning
Branch Dieback Scattered dying branches Botryosphaeria Dieback
  Leaves on scattered branches wilt, roll and turn brown Phytophthora Dieback
  Wilting of leaves and twig dieback Rhododendron Borer
  Wilting terminals and dieback Rhododendron Stem Borer
Entire Plant Dies Early symptoms may include wilting, shoot dieback, Phytophthora Root Rot
  As above, result of larvae feeding on roots and crown Black Vine Weevil and Two-banded Japanese Weevil
  Plants recover after irrigation Drought and Water Stress

Abiotic problems and disorders of azaleas and rhododendrons

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. 

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.

Aluminum sulfate, which is a form of sulfur labeled to lower the pH for hydrangeas to turn the flowers blue, should not be used to lower the pH of the soil for azaleas or rhododdenrons. They can be sensitive to aluminum buildup in the soil. 

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.

Winter injury

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 have 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 azaleas and rhododendrons

Botryosphaeria dieback

botryosphaeria canker on rhododendron

Botryosphaeria canker is the most common disease of rhododendron in the landscape. A typical symptom of this fungal disease is scattered dying branches on an otherwise healthy plant. Leaves on infected stems droop and roll inward, then turn brown. These leaves often lay flat against the stem and will remain attached.

Phytophthora dieback

rhododendron phytophthora symptoms

Phytophthora dieback, although uncommon in the landscape, is a distinct phase of the Phytophthora disease syndrome on rhododendrons, azaleas, leucothoe, and Japanese andromeda (Pieris japonica). 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.

Ovulinia petal blight

petal blight

Flowers affected by petal blight turn brown and remain attached to the plant. This makes the shrub look unsightly. 

Exobasidium gall

exobasidium

Exobasidium leaf galls can be very noticeable after blooming but they do not cause a serious problem.  

Fungal leaf spots

brown leaf spots on a rhododendron leaf
Cercospora leaf spot on Rhododendron
Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.org 

There are many common fungal leaf spot pathogens of Rhododendron spp. but most do not cause serious harm and are cosmetic. 

Rust

rust on azalea leaves

Deciduous azaleas are the primary group of broadleaved shrubs damaged by rust.

Powdery mildew

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.

Insect pests of azaleas and rhododendrons

Azalea bark scale

 

Azalea Bark Scale. Photo: John Davidson, University of Maryland
Azalea Bark Scale
Photo: John Davidson, University of Maryland

Bark scales are a type of soft scale and an infestation is indicated by clear sticky honeydew with attendant sooty mold (black fungus) on leaves or stems, yellowing of leaves, and twig dieback. This scale is most obvious from May through June when white egg sacs may be found in twig forks. Heavy infestations over several seasons may kill plants.

Overwintering immature scales (nymphs) are less than ⅛-inch long, gray, and are usually found in twig forks. The females appear totally white when they produce their white, waxy egg sacs. This scale primarily attacks azalea and rhododendron but also has been found on Japanese andromeda (Pieris japonica), maple, arborvitae, willow, poplar, and hackberry. There are two generations each year in Maryland.

Management

  • Azaleas can tolerate low populations of this bark scale without significant damage. If there are no yellowing leaves, no treatment is necessary. Consider hand–removal by squishing the scales to prevent populations from increasing. 
  • Beneficial predators and parasites will usually provide adequate control of light bark scale infestations. Examine egg sacs for holes that will indicate control by parasitoids, and look for predators such as ladybird beetles. 
  • To control heavy infestations, spray dormant plants with a 4% rate of horticultural oil to kill developing nymphs on twigs. If necessary a 2% summer rate of horticultural oil may be applied in July after all of the eggs have hatched.
  • Systemic insecticides applied to the soil are also highly effective but follow cautionary warnings on the label to protect pollinators visiting azaleas and rhododendrons.

Azalea lace bugs

Lace bugs

Azalea lace bug damage. Photo: P.M. Shrewsbury, University of Maryland
Azalea lace bug damage
Photo: P.M. Shrewsbury, University of Maryland

Azalea lace bug (Stephanitis pyriodes) and
Rhododendron lace bug (Stephanitis takeyai)

Lace bug damage is indicated by stippling (very tiny spots) on leaves of plants growing on dry sites and those with low plant diversity. Damage usually begins in early May on old leaves and later appears on new growth. Black fecal spots will be present on the lower sides of stippled leaves. 

Heavy infestations of lace bug may cause leaves to turn yellow and then brown. Shrubs that are exposed to full sun and heavily infested may be killed. There are multiple generations of lace bug per year. Azalea, rhododendron, and Japanese andromeda (Pieris japonica) each have their own unique species of lace bug.

Adult lace bugs are flat, about 1/8 inch long, with transparent lace-like wings. Immature lace bugs (nymphs) are black and covered with spines. Eggs overwinter in leaves and may begin to hatch as early as late April.

There are as many of four generations of azalea lace bug during the growing season and into the fall. 

The following azalea cultivars have shown resistance to azalea lace bug damage: ‘Autumn Amethyst’, ‘Autumn Twist’, ‘Autumn Royalty, ‘Autumn Sangria’, ‘Autumn Cheer’, ‘Indica alba’, ‘Flame Creeper,’ and ‘Delaware Valley White’, among others.

Azalea lace bug adult and fecal spots. Photo: P.M. Shrewsbury, University of Maryland
Azalea lace bug adult and fecal spots. Photo: P.M. Shrewsbury, University of Maryland

Management

  • Plant azaleas in partial sun (morning sun and afternoon shade). Mulch with chopped oak leaves, leaf mold, or pine needles and water deeply during drought periods. Stressed plants are more susceptible to lace bug feeding.
  • Natural enemies and predacious beetles feed on lace bugs. They can keep small populations of lace bugs under control. Do not spray if you find beneficial insects feeding on the lace bugs.
  • Look for nymphs and black fecal spots on lower leaf surfaces in early May to estimate potentially damaging populations. 
  • When lace bug populations are high, sprays of horticultural oil (at a 2% summer rate) or insecticidal soap will control lace bugs if the lower surfaces of the leaves where lace bugs are active are thoroughly covered.
  • A currently registered systemic insecticide may be necessary where coverage of the undersides of leaves is difficult.
  • Azaleas are visited by pollinators. Be sure to observe all cautionary statements on labels to protect pollinators.

Azalea caterpillars

Azalea caterpillar
Azalea caterpillar
​​​​​​Photo: David Held, Auburn University, Bugwood.org

Azalea caterpillars (Datana major) are black with rows of white or pale yellow spots, reddish-brown legs, head, and neck area, and are 2 1/2 inches when mature. Preferred host plants are azaleas, but they may also attack witch hazel, sumac, apple, red oak, and Japanese andromeda (Pieris japonica). The caterpillars feed in groups when young and disperse as they mature. Branches or entire plants may be defoliated. Damage occurs in late summer and fall.

Management

  • Look for caterpillars when chewing damage occurs. If only a few caterpillars are present, pick them off by hand. 
  • If hand removal is not feasible, spray shrubs with Bacillus thuringiensis (Bt), a microbial insecticide, that is specific for young caterpillars and is sold under various trade names. Insecticidal soap may also be used. Apply either spray when caterpillars are numerous and young (less than 3/4 of an inch long). Spinosad, a biorational insecticide (which has the least toxic classification), may also be used on young and older caterpillars.

Rhododendron borer

 

Frass (sawdust-like material) indicating the presence of rhododendron borer.
Frass (sawdust-like material) indicating the presence of rhododendron borer
Photo: University of Maryland Extension
Rhododendron Borer (adult)
Rhododendron Borer (adult)
Photo: John Davidson, University of Maryland

Rhododendron borer, Synanthedon rhododendri, ( is the larvae (caterpillar) of a clearwing moth. The borer causes wilting of leaves and twig dieback. Prune the suspected branches and split them open longitudinally to see if larvae are present. Mature larvae are about 1/2 inch long, white with brown heads. The adult moths, about ¼” long, resemble wasps, have mostly clear wings, and black bodies with 3 gold bands on the abdomen. Boring larvae may cause branches to crack. Heavy infestations cause wilting and eventual branch dieback. This borer prefers rhododendron but occasionally attacks deciduous azalea and mountain laurel. There is one generation a year and larvae overwinter in tunnels in branches.

Management 

  • There are no conventional insecticides that will kill borer larvae once they are inside the branches. The best control option for residents with only a few plants is to prune out (cut below active larvae) and destroy wilting branches in late summer or early spring before adults emerge. 
  • Beneficial nematodes, available under several trade names, are a control option. They may be injected into the active borer tunnels. See the package for specific instructions.

Azalea Leafminer

  

Azalea Leafminer (early instar) inside of a leaf.
Azalea Leafminer (early instar) inside of a leaf
Photo: John Davidson, University of Maryland
Azalea Leafminer (late instar)
Azalea Leafminer (late instar)
Photo: John Davidson, University of Maryland

Azalea leafminer (Caloptilia azaleella) damage is indicated by the presence of blotch mines in leaves of azalea beginning in May. Mines are initially formed near the midrib and caused by caterpillars feeding between the upper and lower leaf surface. As larvae mature they curl the tips of the leaves with silk and feed inside the curl. Large populations cause leaves to brown and drop prematurely. Curled leaf tips in June indicate the completion of the first generation. The second-generation mines begin in July. Adult moths are present in late June and August. The moths are 3/8 inch long and yellowish-brown. Mature larvae are 1/2 inch long and yellowish-brown. Azalea leafminer overwinters as pupae in the leaf mines.

Management

Rake and destroy fallen leaves in the fall to remove overwintering pupae.

Azalea whitefly

Azalea whitefly damage.
Azalea whitefly damage. Whiteflies would be found underneath the leaves.
Photo: Bruce Watt, University of Maine, Bugwood.org

This whitefly (Pealius azaleae) is usually limited to azalea varieties that have hairy leaves. Whitefly adults and nymphs feed on the underside of the foliage. Heavy infestations cause the margins of terminal leaves to cup. These infested leaves will eventually turn yellowish and appear wilted. The leaves become covered with honeydew, followed by sooty mold (a black coating). To check for the presence of whiteflies, shake the branches of symptomatic azaleas to flush out adult whiteflies that look like tiny white moths. Examine the lower surfaces of leaves for the presence of nymphs, which are flat, yellowish-green, and resemble scale insects. All stages occur on the undersides of leaves.

Management

  • If the infestation is light, little or no plant symptoms are evident, and if beneficial insects are present, spray the undersides of leaves with insecticidal soap or horticultural oil at the 2% summer rate. Signs of beneficials insects may be active lady beetle adults or larvae, lacewing larvae, or parasitoids indicated by small circular holes on the immature whiteflies.

Rhododendron stem borer

Rhododendron Stem Borer Damage
Rhododendron Stem Borer Damage
Photo: Jim Baker, North Carolina State University, Bugwood.org

Damage caused by the stem borer (Oberea myops), which includes wilting terminals and dieback, is similar to the Rhododendron borer. This borer, however, is a longhorned beetle. Adult beetles feed on the underside of leaves on the midvein, causing the leaves to curl. The larvae bore down the center of twigs causing individual branches to wilt, and eventually die back to the ground. The adult beetles are about 5/8 inch long, have long antennae, and are pale yellow with two black spots on the thorax and on the margins of the wing covers. Adults are present in June and July. The larvae are whitish and have no visible head. This beetle prefers rhododendron, but will also attack azaleas and mountain laurel. The larvae overwinter in branches the first year and in roots the second year.

Management

  • There are no conventional insecticides that will kill stem borer larvae once they are inside the branches. 
  • The best management option for residents with only a few plants is to prune out and destroy wilting branches when beetle larvae are in the branches in early spring or late summer. 
  • Beneficial nematodes are not effective on these borers

Black vine weevil

 

Black vine weevil feeding damage (leaf notching) to rhododendron.
Black vine weevil feeding damage (leaf notching) to rhododendron
Photo: Jim Baker, North Carolina State University, Bugwood.org

Black vine weevil (Otiorhynchus sulcatus) adult feeding damage (notching on leaf margins) appears on azalea and rhododendron in May and June. Interior leaves usually show the most damage. Small plants when heavily infested may be defoliated. Adult weevils are about 3/8 inch long and black with faint yellowish flecks. They feed in the evening and at night.

The larvae live in the soil and feed on roots and may also girdle the plant at the root crown, causing the plant to wilt and die. The larvae are “C-shaped”, legless, and white with brown heads.

Rhododendron varieties with resistance to weevils are available and include species such as Rhododendron cuneatumR. dauricumR. fastigiatum, and others.

Black Vine Weevil (adult). Photo: M. Raupp, University of Maryland
Black Vine Weevil (adult)
Photo: P Shrewsbury, University of Maryland
Black Vine Weevil (adult and larva). Photo: John Davidson, University of Maryland
Black Vine Weevil (adult and larva)
Photo: John Davidson, University of Maryland

Management

  • Inspect for adult weevils at night using a flashlight. Weevils can be hand-picked and dropped into a container of soapy water for disposal. 
  • Beneficial nematodes may be effective in controlling larvae in the soil. Nematode species recommended for control are Heterorhabditis bacteriophora and Heterorhabditis megidis. Pull mulch back prior to application. Follow instructions on the label for timing and soil temperatures.
  • A registered foliar insecticide (e.g., azadirachtin) may be applied beginning in mid-June.
  • A registered systemic insecticide (e.g., Imidacloprid) may be applied by a certified pesticide applicator only (as per the Maryland Pollinator Protection Act). Insecticides applied to the soil can harm ground beetles, which are beneficial natural predators of a variety of pests.

Two-banded Japanese beetle

Two-banded Japanese weevil (actual size is 3/16 inch). Photo: M. Raupp, University of Maryland
Two-banded Japanese weevil (actual size is 3/16 inch)
Photo: M. Raupp, University of Maryland

Two-banded Japanese beetle (Pseudocneorhinus bifasciatus) adult feeding damage is very similar to that of black vine weevil but notches on leaf margins tend to be smaller. Adult feeding damage begins on lower leaves in mid-June. Small shrubs may be defoliated in heavy infestations. Larvae also feed on the roots and crowns of hosts. Heavy larval populations may cause stunting, wilting, and may kill small shrubs and tree seedlings.

The adult weevils are about 3/16 inch long, brown, broadly oval, thick-bodied, and brown to gray with two darker bands across the wing covers. Mature larvae are about 1/4 inch long, “C-shaped”, legless, and white with brown heads.

This weevil prefers azalea, rhododendron, privet, mountain laurel, forsythia, spirea, deutzia, lilac, and euonymus. There is one generation a year and all stages overwinter in the soil.

Management

  • Adult beetles feed for two to three weeks before laying eggs. To check for adults, place a tray under damaged shrubs and shake the plants. Adult Japanese weevils feed during the day and will drop from the plants when disturbed. 
  • If control is necessary, use a registered insecticide when damage begins in mid-June.
  • As with black vine weevil, soil drenches of entomopathogenic nematodes may reduce populations of grubs in the soil. Follow instructions on labels for timing and soil temperatures.

Rhododendron gall midge

Gall Midge damage. Photo: Photo: M. Raupp, University of Maryland
Gall Midge damage
Photo: Photo: M. Raupp, University of Maryland

This tiny fly (Clinodiplosis rhododendri) attacks the new growth of rhododendrons. Gall midge larvae feed on the developing leaf tissue causing leaves to become distorted with puckered and curled margins. This deformation will last as long as leaves remain on the plant. 

Rhododendron gall midge immatures spend the winter and early spring in the soil where they complete development and pupate. In spring, adults emerge from the soil and fly to the tips of branches where they lay eggs in new developing leaf tissue. Eggs hatch into tiny white maggots less than 1/16 inch in length. Their feeding causes leaf margins to curl and the maggots complete their development within the curled leaf tissue. Damage to new foliage will be evident in May and July. Fully developed larvae drop to the ground where they will pupate. Studies in Connecticut indicate as many as three to five generations annually depending on climatic conditions. Susceptible cultivars include those of Rhododendron catawbiensis and R. maximum

Management

  • Inspect plants for signs of early damage and remove curled leaves promptly. This will interrupt the life cycle and reduce subsequent populations. Diligent, yearly removal of infested leaves may dramatically reduce damage in a few years.  

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. 

Authors:

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

Insect pests: Originally written by Mary K. Malinoski, University of Maryland Extension Specialist (retired). Revised and edited by Dr. Michael J. Raupp (retired) and Dr. Paula M. Shrewsbury, University of Maryland, April 2020.

Complied by: Christa Carignan, Coordinator, Digital Horticulture Education, HGIC and Debra Ricigliano, Lead Horticulturist, HGIC

Rev. 2021

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