Choosing planting

Lilac diseases and parasitic pests

Identification, prevention and natural remedies

Contents

Modified the 3 February 2026 by Angélique 4 min. Difficulty: Débutant

Lilac, also known as Syringa, is a bush much loved in gardens. It produces large clusters of fragrant flowers, from April to June, adding touches of colour in white, pink, mauve or purple. There are around twenty species, the most widespread being Syringa vulgaris or common lilac. Very hardy, lilac can however sometimes be affected by a disease or parasitic pests. Discover diseases and parasites that can affect lilac, how to identify them, prevent them and treat them naturally.

Difficulty

Powdery mildew

The lilac can be affected by a fungal disease called powdery mildew. It covers the leaves with a characteristic white powdery coating, usually in late summer when the weather is warm and humid.

Symptoms

Powdery mildew appears as a white, powdery felt appearing on the leaves, stems and sometimes the flowers. The leaves may become distorted and drop prematurely, weakening the bush.

Prevention

To prevent powdery mildew, it’s important to properly space lilac bushes to promote air circulation. Avoid overwatering and favour watering at the base of the plants. Regular pruning of the branches to remove the affected parts is also recommended. You can also spray horsetail decoction.

Treatment

Start by cutting the affected branches with a sterilised pruning shear with alcohol. Then spray a sulfur-based fungicide, nettle manure or treat with Bordeaux mixture.

For further reading, see our fact sheet: “Powdery mildew or white disease.”

Powdery mildew on lilac leaves

Root rot

When lilac is planted in very damp or waterlogged soil, it may develop Phytophthora root rot, a cryptogamic disease causing the bush’s roots to rot.

Symptoms

Phytophthora root rot manifests as root rot, bark peeling and sometimes the appearance of white patches on the trunk.

Prevention

To prevent root rot, ensure good drainage of the soil in which your lilac is planted.

Treatment

There is no treatment; only prevention of the disease can help keep lilac healthy.

Fire blight

Fire blight is prevalent in some regions and is a serious disease that can quickly kill lilac within a few weeks. The cause is the bacterium Erwinia amylovora, which spreads in warm, humid weather, at temperatures between 25 and 30°C.

Symptoms

The bush appears to have been burnt. The disease appears in spring as the flowers wither. Subsequently, the branches appear burnt and cankers may form.

Prevention

Remove all affected parts and dispose of them at the waste disposal site. In spring, you can spray as a preventive measure Bordeaux mixture or a horsetail decoction. If the bush is affected entirely, dig it up and take it to the waste disposal site.

Treatment

There is no curative treatment; prevention is key to preventing its spread. In the event of a fire blight outbreak, you must inform your local council or the Regional Plant Health Service.

For further reading, see our advicesheet: Fire blight – identifying and combating this disease

Symptoms of fire blight

Mealybugs

Mealybugs are small insects that cannot be seen with the naked eye. They suck the sap from the plant and thereby weaken it. They thrive in a confined environment, in warm and humid conditions.

Symptoms

Mealybugs signal their presence through small white, cottony clusters on the leaves and the branches.

Prevention

Ensure the bush is not confined and that it is well ventilated. You can also introduce natural enemies into your garden such as ladybirds, lacewings or blue tits.

Treatment

Take a cotton wool ball or cloth soaked in 90% alcohol and use it to remove the mealybugs. If they are too numerous, try to get rid of them by blasting a strong jet of water on the affected branches and leaves.
You can also make sprays based on liquid black soap and rapeseed oil.
In 1 litre of water, dilute:
– 1 teaspoon of liquid black soap
– 1 teaspoon of denatured alcohol
– 1 teaspoon of rapeseed oil
Repeat the operation every 8 days until all mealybugs have disappeared.

For more information, see our advice sheet: Mealybug: identification and treatment

Mealybugs

Vine weevils

The vine weevils are small beetles that nibble the edges of lilac leaves. They resemble small weevils and come out at night. Their presence is favoured by hot, dry summers.

Symptoms

The leaf edges are punctured or dentate. Small beetles can sometimes be observed on the leaves, but they drop to the ground and are difficult to catch.