FAQ

FAQ

What is the difference between Rhododendrons and Azaleas

All azaleas are rhododendrons. They belong to the subgenus Penthanthera (deciduous) and the subgenus Tsutusti (mostly evergreen) by the revised classification, or to the Azalea Series by the Balfourian System.

All azaleas never have scales.

All Azaleas have five lobes to the flower.

Most (not all) azaleas have only one stamen for each lobe of the flower, meaning they have five stamens, while most other rhododendrons have two stamens for each lobe, meaning that they have 10 or more stamens.

Azaleas tend to have hairs that grow parallel to the surface of the leaf. This is particularly true along the midrib of the undersurface of the leaf and is easily seen in the so called "evergreen" azaleas.

Azaleas have tubular funnel or funnel-shaped flowers.

Small irregular or semicircular notches on leaf edges

This is most likely because of the root weevil. Root weevils normally feed at night and their damage can be confirmed by checking plants at night with a flashlight. This does not normally hurt the plant but it can be unsightly. Visit your local garden center for chemicals to control these pests.

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Large irregular chewed leaf areas

Many types of caterpillars, cutworms and loopers can cause this type of damage. Many of these do their damage at night and can be spotted at night with a flashlight. If the damage is severe, the plant itself is threatened. Use either a registered pesticide or pick off the insects and destroy them.

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Leaves badly chewed, sometimes down to the midrib

This is caused by the sawfly larvae. These green caterpillar-like larvae are green and can blend in with the leaves. They are easily spotted during the day and can be controlled with pesticides when the larvae are present. They can also be picked off and destroyed if only a few plants are involved.

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Dark spots on leaves

This might be caused by stress to the plants. Some species are especially prone to it. The actual causes are not known. If the problem is persistent and severe, consider replacing the plant.

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Irregular brown spots and small black dots

Fungus leaf spot. The spots are irregular and some have red-brown borders with silvery gray center. The fungus usually enters through a wound on the leaf. Remove and destroy (do not compost) affected leaves. Spray with a registered fungicide after flowering and repeat at 10-14 day intervals until dry weather begins.

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Brown blisters, leaves are rolled/skeletonized, immature leaf drop

This is caused by the azalea leaf miner. Small yellowish caterpillars mine inside leaf tissues and later roll the leaves. Use chemicals if the infestation is severe. They breed in the leaf debris so rake and destroy (do not compost) dead leaf debris.

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Serpentine or straight mine across the midrib, leaf tip dying

This indicates a rhododendron leaf miner. Rarely causes severe damage. Remove and destroy infested leaves.

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Upper leaves brown or black from tips/edges to midrib

Marginal Leaf Necrosis. Several causes. Cold damage when temperature drops below hardiness level. Drought damage while plant is in active growth or on newly established plants. High amounts of salts in the soil. This is caused by excessive use of soluble fertilizers. Root damage due to poor drainage or injury to root system. Girdling due to weevil feeding or a nutrient deficiency.

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Yellowing of leaves, mainly between veins of new leaves

Iron or Manganese Deficiency. This may be caused by an overly alkaline soil, a lack of sufficient iron or manganese in the soil (this is uncommon), lack of sufficient air space and/or lack of good drainage. Excessive amounts of some herbicides may cause similar problems.

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Brown, indistinct blotches, mostly on top leaves

Heat damage. Symptoms are usually more severe on the south and southwest side of the plant. These can develop rapidly after a hot spell with intense sunlight. Plant a sun tolerant rhododendron or provide partial shade. Watering in the afternoon may help.

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Overall yellowing of leaves, mainly older/lower leaves

Nitrogen Deficiency. Lake of nitrogen or available nitrogen tied up in decomposing organic matter, such as sawdust in the soil around the plant. Wet feet. An early symptom of poor drainage is yellowing and wilting of newly emerging growth. If the plant is wilted and the soil is very wet, there is probably a root problem.

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Marginal yellowing of leaves, followed by browning

Root problems. These are caused by poor drainage, disease, soil compaction, heavy clay soils or being buried too deep. Roots need abundant air spaces to function.

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Yellow or red-brown rings, spots or blotches

Indicates a virus. This is not a common and often not severe but there is no cure. If severe, replace plant. Once infected, it remains infected.

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Flower buds turn brown and fail to open, covered with black bristles

Bud blight. The fungus Briosia azaleae causes bud blight. Remove and destroy (do not compost) affected buds. Fungicides are not usually necessary.

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Flower buds turn brown, fail to open or partially open, undeveloped

Cold injury. Freezing temperatures in late fall, winter or early spring. If this occurs regularly, replant with a more cold-resistant variety.

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New growth killed in spring

Frost injury. Late spring frost. New leaves and stems are not as hardy as older foliage. Varieties which start to grow early are more susceptible.

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Older leaves turn yellow/brown and fall

Leaf Senescence. Nothing lasts forever. The older leaves will fall off after a few years. This is normal.

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Bark turns brown

This is normal bark development.

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Yellowish speckling on top of leaf, tar spots underneath

Lacebug. Stephanitis rhododendri is the rhododendron lacebug. Most pesticides for root weevils will take care of this pest also.

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Variety of leaf discoloration not mentioned above

Herbicides and other chemicals can cause a variety of leaf discoloration. These can be caused by other chemicals not associated with plant treatment; moss killers, paint and paint thinners and roof preservatives.

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Leaves rolled, webbed and chewed

Leaf roller. There are several species of leafroller caterpillars. If the damage is significant, a pesticide may be used or destroy the insect. Some harmless spiders may roll leaves but they do not chew them.

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Drooping, rolled leaves in winter

This is a normal response in some plants to cold weather. If this occurs during the summer, it is a sign of root rot or extreme lack of water.

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AZALEAS: light green, bladder shaped galls on new leaves, turning brown, hard with white, powdery mildew

RHODIES: Leaves turning whitish and thickened. White fungal growth underneath leaves

Azalea leaf gall. Exobasidium vaccinii is the culprit here. Blossoms and seed pods may also be affected. Remove and destroy (do not compost) affected leaves. Spray with a fungicide just before bud break and repeat two or three weeks later.

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Leaves appear twisted, puckered or curled

Aphids. These small green pests can be found on the underside of the leaves. A sticky, shiny residue called honeydew is present. Ants are especially attracted to this substance. Late frost damage may also cause the leaves to appear this way except there is no evidence of insects.

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Small, diffuse spots, light green/yellowish. Underside has pustules producing orange, powdery spore

Rust. Chrysomysxa ledi is the most common cause of rust in the Pacific Northwest. Try to use rust-resistant varieties or the use of a preventative fungicide if it is an annual problem.

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Powdery mildew

Do not let decomposing leaves gather under the plant. If problem persists, spray with fungicide.

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Black sooty growth on leaf surfaces and bark, maybe white cottony masses

Sooty mold. This develops from the honeydew of sucking insects. Controlling the pests will control the mold. If there are small, white cotton-like masses present also, this may indicate azalea bark scale. Some insecticides will control these pests if applied when first hatching.

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Plant is unthrifty and thin. Tortoise-like bumps on bark

Lecanium Scale. If the plant is heavily infested, or is becoming stressed, chemicals will be necessary.

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Matted wooliness on surface of leaves and underneath

This is called Indumentum and is perfectly normal for certain species of plants. This is often a selling point for a plant and it also helps protect the leaves.

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Various colored fleshy/papery growth on bark

Lichens. Lichens cause no damage to plants but chemicals may be applied for appearance sake.

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Green, felt-like material on leaves

Algae. While this may be unsightly, is causes no damage to plants.

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