Azalea diseases and pests of various kinds affect azaleas, and some are more common and more of an issue than others. Common diseases include powdery mildew, leaf spot disease and rust. While Phytophthora root rot and Galls/canker as less common, they can be much more serious.

Pests include the usual aphids, which can be treated with soapy water or pesticide bug killer if needed. Other pests also include scale insects, caterpillars, leafminers, roundworms and spider mites.

I discuss each of the common ones and give information on what to do about them below.

Azalea Diseases

Powdery mildew on azaleas

If you notice a white powdery substance on the leaves of your azalea, it’s probably powdery mildew, and it’s fairly common. This is a fungal disease that produces spores on the leaves that are then spread by wind currents to surrounding leaves. It’s not fatal to the plant, but it can detract from its beauty.

Powdery mildew is often found on young plants that are in a lot of shade. It’s common on many different types of plants, so it’s actually very common.

What to do

If possible, cut down surrounding plants so the azaleas receive enough sunlight; they preferred dappled shade or morning sun and then shade. Remove any dead or diseased leaves that have white powder on them, including those that have already fallen off. Fungicides can also control the problem.  Trying to remove the fungal spores from the leaves may just spread the disease further. I usually have this issue myself, so I always spray with a fungicide in early spring to try and help prevent it before it turns up.

Leaf spot disease

Leaf spot is caused by a fungal disease just like mildew. Irregular brown spots appear on both the upper and undersides of the leaves. These dead areas may have tiny black growths on them.

What to do

Fungicides are effective against leaf spot disease. Use one when the blooms have faded and continue at the recommended suggested intervals on the box throughout the growing season. Again because I start spraying my azaleas with a fungicide in spring, it helps prevent it in the first place.

Rust

Rust is another fungal disease that affects azaleas, among other plants and is also fairly common. You notice it through the red to orange spore masses on the leaves of the plant. There may also be galls (boil-like growths) on the bark of the plant as well. It’s not usually fatal but can sometimes kill badly infected plants.

What to do

Remove the infected leaves and stems as soon as you notice them. It’s best to burn them to avoid the rust from spreading through the compost pile if you throw them there. Again you can spray fungicides to help treat and even prevent rust.

Galls / Canker

Yet another fungal azalea disease and this one produces azalea galls. These are ugly growths that start to appear in spring on the leaves of the azalea. They look like boils and may be pale green, pink, white or brown in colour. The leaves become thick and start to curl. The galls may spread to other parts of the plant, including the branch tips and flowers.

Later in the growing season, this new fleshy plant tissue forms a white spore layer. This is one the disease becomes contagious. Finally, the galls turn brown, become hard and may drop to the ground.

What to do

Pay attention to the first sign of these galls in the spring. Hand pick them off the plant when you notice them, especially before the first white spores appear. A fungicide applied in the spring may help to control this problem.

You can help to prevent the gall problem in the first place by watering your azalea at the root and not through the leaves. Make sure the soil is well-aerated and that there’s good air circulation through the plant.

Phytophthora root rot

Root rot is a disease that can kill your azalea. It causes the leaves to turn yellow and wilt, the stems to dieback and the eventual death of the whole plant. Root rot starts in the root and occurs when the soil is thick, wet, and poorly drained.

What to do

Adjust the soil around the azalea so that it drains properly. Use grit or compost to lighten it up and make it finer-grained. Cut away the roots that have rotted to try to save the rest. However, if the fungus that causes root rot has made its home in the soil, then you have to recognise that you won’t be able to grow azaleas there without infecting them.

I recommend treating azaleas each spring with a fungicide

For most of these diseases, you will notice they can be treated with a fungicide and the good news is that most good fungicides will treat all these different diseases making it easy to treat your azalea in spring to help prevent them in the first place. Some people don’t like using fungicides and for good reasons but if you want to prevent them and your azaleas seem to be prone to disease, this is a good way to get on top of it before it gets out of hand.

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Azalea Pests

Azalea Bark Scales

These are tiny soft scale insects that are difficult to see. You know that your azalea has them by the white egg sacs left in the fork of the stems in late spring and early summer. In addition, these insects leave behind honeydew, a clear sticky, sweet substance that attracts ants, along with a black mould on leaves and stems. Another sign is that your leaves start to turn yellow, and the stems begin to die.

The scale insects sometimes look like a crusty covering on the plant.

What to do

Spray with a 2% solution of horticultural oil in the summer. Wait until the azalea blooms have faded so that you don’t kill the bees and other pollinators that hang around the flowers. Spray again later in the summer if the bark scales are still alive. To check this, scrape a few off the plant. If they’re dry and crusty, they’re dead. Otherwise, they’re still alive. scales can also be treated with pesticides but again, use with caution and not whilst in flower.

Azalea caterpillars

These pests are very distinctive. At several centimetres long when mature, it’s not difficult to spot their black bodies with white or pale yellow spots and red-brown legs. If you notice that your azalea branches are becoming leafless, check for these caterpillars. They appear in the late summer and the autumn.

What to do

You can shake your azalea plant to make the caterpillars fall to the ground or simply inspect and hand-pick them. You can dispose of them from there. Additionally, an insecticide spray can help with this problem. But remember that insecticides are non-specific and will kill whatever insects they come into contact with.

You can learn more about what might be eating your azalea leaves in my guide here

Leafminers

You know your azalea has an infestation of leafminers when you find brown blotches in the leaves in the late spring. Leafminers are caterpillars, and they feed on the leaves around the centre vein, between the upper and lower leaf surface. They weave webs to curl the leaves up so they can feed inside the curled part. You can see the webs on careful examination.

The adult moths are 1cm long and yellow-brown in colour. If you have a severe infestation, your azalea leaves turn brown and drop off the plant.

What to do

You need to prune the leaves or stems which are infected. Burn these diseased parts to avoid the disease spreading. Insecticides aren’t very effective as the insect protects itself by hiding inside the curled leaf.

Nematodes (Roundworms)

Nematodes are tiny roundworms which interfere with the uptake of nutrients and water by the azaleas’ roots. Symptoms of nematode problems are poor growth, especially stunted leaves, and yellow or brown foliage. The roots may also have galls on them. Nematodes travel naturally through the soil, but slowly.

What to do

Not all nematodes are bad. Anything you use to wipe out the bad ones ends up also killing the beneficial ones. There are no chemical solutions to help with this problem. The best thing to do is to admit that this plant has had it, dig it up and burn it to stop the nematodes from spreading elsewhere. Sterilise anything that has come into contact with the plant and wash your hands thoroughly.

Spider mites

Once again, these are tiny insects that hang around your plants. The leaves become flecked with yellow and stippled areas appear. If you look closely, you may notice delicate webs on the plant spun by the spider mites, and it’s usually the webs you notice first.

What to do

Spray the underside of the leaves with insecticide. Repeat this after a week to take care of any eggs that are due to hatch. You can usually buy pestcide sprays from garden centres marked as suitable for killing spider mites.

If your azalea has drooping leaves, check out this guide to learn about possible causes. If you feel your azalea is dying, I also have a helpful guide here.

Another common issue is the azalea leaves turning yellow. If your azalea is not flowering, check out this guide here

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