How Do I Treat a Tree with Armillaria Root Rot in My Garden?

Armillaria root rot is one of the most serious fungal diseases a NZ gardener can face — and unfortunately, there is no chemical cure once a tree is infected. The best approach combines early identification, careful removal of infected material, and proactive management to protect surrounding trees and plants.

What Is Armillaria Root Rot?

Armillaria (commonly called honey fungus) is a group of soil-borne fungi that attack the roots and root collar of trees and woody shrubs. In New Zealand, several Armillaria species are present, and they are particularly common in areas where native bush has been cleared or where old tree stumps remain in the soil.

The fungus spreads underground via rhizomorphs — dark, boot-lace-like strands that grow through the soil and infect neighbouring root systems. It can persist in dead wood and old stumps for many years, making it a long-term garden management challenge.

How to Identify It

Look for these tell-tale signs:

• Yellowing, wilting, or dying foliage — often on one side of the tree first
• Honey-coloured toadstools appearing at the base of the tree or nearby in autumn
• White mycelial fans — flat, white fungal growth visible under the bark at the root collar when you peel it back
• Dark rhizomorphs (boot-lace strands) found in the soil around roots
• General decline in tree vigour despite adequate water and nutrition

If you're unsure, a qualified arborist can confirm the diagnosis. You can find local arborists on tradedirectory.co.nz.

Can Armillaria Root Rot Be Cured?

No — there is currently no registered fungicide in New Zealand that will cure an established Armillaria infection in a tree. Treatment is focused on:

1. Slowing the spread to other plants
2. Improving the infected tree's resilience where possible
3. Removing infected material safely

Management Steps You Can Take

1. Confirm the Diagnosis First

Before taking action, be confident in your identification. Misdiagnosis can lead to unnecessary tree removal. An arborist or plant pathologist can help confirm.

2. Remove Severely Infected Trees

If a tree is heavily infected and declining, removal is usually the recommended course of action. Leaving a dying tree in place allows the fungus to continue spreading through the root system to neighbouring plants.

• Have the tree professionally felled and the stump ground out as deep as possible
• Remove as much root material from the soil as you can — this is where the fungus lives and spreads
• Do not chip infected wood for use as garden mulch, as this can spread the disease

3. Expose the Root Collar on Mildly Affected Trees

For trees showing early symptoms, some arborists recommend carefully excavating soil away from the root collar (the base of the trunk) to expose it to air and dry conditions. Armillaria thrives in moist, anaerobic soil environments, so improving drainage and airflow can sometimes slow progression.

4. Improve Soil Drainage

Waterlogged soils favour Armillaria. Improving drainage around susceptible trees — through raised planting, drainage channels, or soil amendment — can help reduce stress on the tree and slow fungal activity.

5. Avoid Wounding Roots

Root damage from digging, cultivation, or construction creates entry points for the fungus. Be careful when working around the drip line of trees in your garden.

6. Choose Resistant Replacement Plants

Once you've removed infected material, wait before replanting in the same area. When you do replant, choose species with known resistance or tolerance to Armillaria. Generally, many New Zealand native species show reasonable tolerance, though no plant is completely immune. Seek advice from a nursery or arborist on suitable species for your region and soil type.

7. Dispose of Infected Material Carefully

• Do not compost infected roots, stumps, or wood
• Bag and dispose of material through your council green waste service, or arrange for it to be taken to a transfer station — check with your local council on accepted disposal methods
• Some councils allow burning on-site; check local bylaws first

What About Chemical Treatments?

There are no registered fungicides in New Zealand proven to eradicate Armillaria from established trees. Some overseas products have been trialled, but none are currently approved for this use under New Zealand's HSNO Act framework. Be cautious of unverified claims. For the latest information on approved products, check with the Environmental Protection Authority (EPA) or a registered plant pathologist.

Protecting Healthy Trees Nearby

When to Call a Professional

Armillaria management almost always benefits from professional input. You should contact a qualified arborist if:

• You are unsure of the diagnosis
• The infected tree is large, near structures, or poses a safety risk if it falls
• You want a full assessment of which surrounding trees may be at risk
• You need advice on safe removal and stump grinding

Tree removal and stump grinding near powerlines, buildings, or on steep sites can be dangerous and should always be carried out by a trained professional. Look for an arborist who is a member of the New Zealand Arboricultural Association (NZAA) or holds relevant qualifications. You can search for qualified local arborists on tradedirectory.co.nz.

Safety note: Never attempt to fell a large or leaning tree yourself. Serious injuries and fatalities occur every year from amateur tree felling. If in doubt, call a professional.

Sources & Further Reading

• WorkSafe NZ — Tree and chainsaw safety
• BRANZ — Building and plant health resources
• MBIE Building Performance — Working near trees and structures
• Standards NZ — Relevant arboricultural standards
• Consumer NZ — Hiring tradespeople