Azaleas and Rhododendrons: Identify and Manage Problems

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, 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.

Flowers affected by Ovulinia petal blight turn brown and remain attached to the plant. This makes the shrub look unsightly. This condition is caused by a fungus, Ovulinia azaleae.

• This disease only affects the flower petals of azaleas and rhododendrons.
• The first symptoms are small water-soaked spots on the petals.
• The 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 overwinter on the ground. These sclerotia can survive as long as two years in the soil and will start the infection cycle in the spring.

To manage this disease:

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

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

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

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

• 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.

• 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.

Lace bugs

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.

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 (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, 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 (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.

• 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 whitefly life 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.

• 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 (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 cuneatum, R. dauricum, R. fastigiatum, and others.

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 weevil (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 and 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 weevils 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.

• 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.