How to Treat Sooty Mould on Tree Leaves

Sooty mould is one of the most common complaints NZ homeowners notice on their trees and shrubs — those dark, dusty-looking coatings on leaves that look almost like soot from a fire. The good news is that sooty mould itself is not directly attacking your tree; it's a secondary fungus growing on the sticky honeydew secreted by sap-sucking insects. Tackle the insects, and the mould will gradually disappear on its own.

What Causes Sooty Mould?

Sooty mould is caused by several species of fungi that colonise honeydew — a sugary, sticky liquid excreted by sap-sucking insects. The most common culprits in New Zealand gardens include:

• Scale insects (including black scale, soft scale, and armoured scale)
• Aphids
• Mealybugs
• Whitefly
• Psyllids (particularly on lilly pillies and pittosporums)

The fungi themselves don't infect the plant tissue, but a heavy coating of sooty mould blocks sunlight from reaching the leaf surface, which can reduce photosynthesis and weaken the tree over time if left untreated.

How to Identify the Problem

Before you treat anything, confirm what you're dealing with:

1. Look for insects — Check the undersides of leaves, stems, and branch junctions for scale, aphids, or mealybugs. Scale insects often look like small brown or white bumps attached to bark or leaves.
2. Check for sticky residue — Run your finger along a leaf. If it feels tacky, honeydew is present.
3. Look for ants — Ants actively farm sap-sucking insects for their honeydew. A trail of ants up your tree trunk is a strong indicator of an insect infestation.
4. Assess the mould coverage — Light mould on a few leaves is minor. Heavy black coating across most of the canopy suggests a significant infestation that needs prompt attention.

Step-by-Step Treatment

Step 1: Control the Insects First

This is the most important step. Without removing the honeydew source, the mould will keep returning.

• Horticultural oil sprays (such as Conqueror Oil or similar products available at Bunnings or Mitre 10) smother scale insects and mealybugs effectively. Follow label directions carefully — some plants are sensitive to oil sprays, especially in hot weather.
• Insecticidal soap sprays work well against soft-bodied insects like aphids and whitefly. Again, follow the label.
• Systemic insecticides (applied as a soil drench or trunk injection) can be effective for severe or persistent infestations, but these should be used with caution around edible plants, waterways, and flowering plants where bees may be present. Always read the label and follow the instructions.
• Ant control — Use a sticky tree-banding product around the trunk to prevent ants from climbing up and protecting the insects from natural predators.

Step 2: Encourage Natural Predators

Ladybirds, lacewings, and parasitic wasps are natural enemies of aphids and scale. Avoid broad-spectrum insecticides where possible, as these kill beneficial insects too. Planting a diverse garden helps attract these predators.

Step 3: Remove the Mould

Once the insect population is under control, the sooty mould will often weather off naturally over several weeks. To speed things up:

• Gently wash leaves with a soft cloth or sponge dampened with a mild soapy water solution. This works well for smaller trees and shrubs.
• For larger trees, a gentle spray with a garden hose can help dislodge loose mould.
• Avoid scrubbing hard, as this can damage leaf surfaces.

Step 4: Improve Tree Health

A stressed tree is more susceptible to insect attack. Consider:

• Mulching around the base (keeping mulch away from the trunk) to retain moisture and improve soil health.
• Watering during dry spells, particularly for young or recently planted trees.
• Avoiding over-fertilising with nitrogen, which promotes the soft new growth that sap-sucking insects love.

NZ-Specific Considerations

New Zealand's warm, humid climate — particularly in the upper North Island — creates ideal conditions for both sap-sucking insects and sooty mould. Certain tree species commonly planted in NZ gardens, such as camellias, gardenias, citrus, and lilly pillies, are particularly prone to scale and sooty mould. If you have citrus trees, the BRANZ and local council extension services sometimes publish region-specific pest management guidance worth checking.

Be aware that some insecticide products are regulated in NZ. Always purchase registered products and follow the label — it is a legal requirement under the HSNO Act.

When to Call a Professional

Most sooty mould treatments are well within a capable homeowner's reach for small to medium trees. However, consider calling a qualified arborist if:

• The tree is large and you cannot safely reach the canopy without climbing or working at height — WorkSafe NZ guidelines apply to working at height, and falls are a serious risk.
• The infestation is severe and widespread across a large tree.
• The tree appears to be in serious decline despite treatment.
• You're unsure what pest is causing the problem.
• You're considering trunk-injection treatments, which require specialist equipment and knowledge.

A qualified arborist can also assess whether the tree has any underlying health issues making it vulnerable to attack. You can find experienced local arborists on tradedirectory.co.nz.

Sources & Further Reading

• WorkSafe NZ — Working at Height
• BRANZ — Building and Home Research
• Bunnings NZ — Garden Pest Control Products
• Mitre 10 NZ — Garden Care
• Consumer NZ — Pesticide Safety