How Do I Identify If My Tree Has Root Rot?

Root rot is one of the most serious and deceptive conditions a tree can suffer from — by the time obvious symptoms appear above ground, significant damage may already have occurred below the surface. The good news is that knowing what to look for can help you catch it early and potentially save your tree, or at least protect your property and family from a structurally compromised one.

What Causes Root Rot?

Root rot is most commonly caused by soil-borne fungal pathogens. In New Zealand, the most widespread culprits include Phytophthora species (sometimes called water mould), Armillaria (honey fungus), and Fusarium species. These pathogens thrive in:

• Waterlogged or poorly draining soil — common in heavy clay soils found across many NZ regions
• Compacted ground around the root zone (from foot traffic, vehicles, or construction)
• Overwatering of ornamental or garden trees
• Soil disturbance during building or landscaping work that damages roots and allows pathogens to enter

NZ's wet winters and humid conditions in regions like Northland, Waikato, and the West Coast can make trees particularly vulnerable.

Above-Ground Warning Signs

Because roots are hidden, you'll often notice symptoms in the canopy and trunk first. Look out for:

• Yellowing, wilting, or undersized leaves — especially when it isn't drought season
• Early leaf drop or sparse foliage compared to previous years
• Dead branches (dieback) starting at the tips and working inward — known as crown dieback
• Stunted growth over one or more seasons
• Leaning — a tree that has recently started to lean may have lost structural root support
• Mushrooms or fungal brackets at the base of the trunk or on surface roots, which can indicate Armillaria or other wood-decay fungi
• Bark discolouration or cankers near the base of the trunk

Importantly, these symptoms can also be caused by other issues (drought stress, pests, nutrient deficiency), so above-ground signs alone aren't conclusive.

Below-Ground Signs to Check

If you suspect root rot, a closer inspection around the root zone can reveal more:

• Carefully scrape away soil near the base of the trunk (within the first 30–50 cm) and examine the surface roots. Healthy roots are firm and white or light tan in colour.
• Rotting roots will appear dark brown or black, feel soft or mushy, and may have an unpleasant smell.
• Examine the root collar (where the trunk meets the soil). Discolouration, soft tissue, or fungal growth here is a strong indicator of root rot.
• Look for white mycelial fans (flat, white fungal growth) under the bark near the base — a classic sign of Armillaria root rot.

Avoid digging extensively yourself, as this can further damage the root system. A qualified arborist has the tools and expertise to assess root health safely and thoroughly.

Assessing Structural Risk

Root rot doesn't just threaten the health of your tree — it threatens its structural integrity. A tree with severely compromised roots can fall without warning, posing serious risk to people, buildings, and neighbouring properties. This is especially important to consider if your tree:

• Is large or overhangs a structure, driveway, or play area
• Has recently started leaning or shifted position
• Shows significant crown dieback alongside basal symptoms

WorkSafe NZ provides guidance on managing hazards from trees in workplaces, and similar risk principles apply to residential properties.

Can Root Rot Be Treated?

Treatment depends on the pathogen, the extent of infection, and the tree species. Options may include:

• Improving drainage around the root zone to reduce waterlogging
• Removing affected soil and replacing with well-draining material
• Fungicide treatments — some Phytophthora infections can be managed with registered phosphonate-based products, but this is best handled by a professional
• Pruning to reduce the canopy load on a weakened root system
• Removal — in advanced cases, removal is often the safest and only practical option

Note that Armillaria has no reliable chemical cure once established; management focuses on slowing spread and removing infected material.

When to Call a Professional

You should contact a qualified arborist if:

• You notice multiple symptoms described above, particularly in combination
• The tree is large, near a structure, or poses a potential falling hazard
• You're unsure whether symptoms are root rot or another condition
• You want a professional risk assessment before deciding on treatment or removal

Tree removal and major pruning work on large trees can be complex and dangerous — it is not a DIY job. A certified arborist (look for NZ Arboricultural Association members or ISA-certified practitioners) can carry out a proper diagnosis, recommend treatment, and safely manage removal if needed. You can find qualified local arborists on tradedirectory.co.nz.

If your tree is protected under a Notable Trees schedule or your local council's district plan, you may need resource consent before removing or significantly pruning it — always check with your local council first.

For general guidance on tree health and building near trees, BRANZ offers useful technical resources.

Sources & Further Reading

• WorkSafe NZ – Managing tree hazards
• BRANZ – Building and landscaping guidance
• MBIE Building Performance – Building Code and site considerations
• Standards NZ – NZS 3604 and related standards
• Consumer NZ – Hiring tradespeople