Phytophthora root disease is one of the most damaging soil-borne pathogens affecting trees and shrubs in New Zealand gardens. While there is no outright cure, early detection combined with the right cultural and chemical treatments can slow the disease, stabilise affected trees, and in some cases allow them to recover. Acting quickly gives your tree the best chance.
What Is Phytophthora Root Disease?
Phytophthora (pronounced fy-TOFF-thora) is a water mould — not a true fungus — that thrives in wet, poorly drained soils. Several species affect NZ trees and shrubs, including Phytophthora cinnamomi, which is particularly destructive. It attacks fine feeder roots, cutting off the tree's ability to absorb water and nutrients.
Common host plants in NZ gardens include:
- Avocado, citrus, and other fruit trees
- Rhododendrons, camellias, and azaleas
- Kauri (Agathis australis) — a nationally significant concern
- Pohutukawa, rata, and other native species
Important note on kauri: Kauri dieback disease (Phytophthora agathidicida) is a nationally notifiable threat. If you suspect kauri dieback, do not attempt home treatment — contact the Department of Conservation or your regional council immediately, and follow all track hygiene protocols.
Recognising the Symptoms
Spot the warning signs early:
- Yellowing, wilting, or sparse foliage despite adequate watering
- Dieback starting at branch tips and progressing inward
- Bark at the base of the trunk that looks water-soaked, dark, or sunken
- Fine roots that are brown and rotted rather than white and firm
- Gummy or discoloured tissue just under the bark at the root crown
Because these symptoms mimic drought stress and other root problems, a definitive diagnosis often requires a soil or root sample tested by a plant pathology laboratory. Your local garden centre or regional council can point you toward diagnostic services.
Step 1 — Improve Drainage First
Phytophthora cannot spread or survive without moisture. Improving drainage is the single most important thing you can do:
- Avoid overwatering. Let the soil dry slightly between waterings.
- Raise the root crown by gently pulling mulch and soil away from the base of the trunk — never mound soil or mulch against bark.
- Install a French drain or raised garden bed if the area is chronically waterlogged.
- Improve soil structure by incorporating coarse grit or pumice around the drip line (avoid disturbing roots).
Step 2 — Remove Infected Material
- Carefully dig away and dispose of visibly rotted roots and infected soil from around the root crown.
- Do not compost infected material — bag it and put it in your general rubbish, or bury it well away from other plants.
- Sterilise any tools used with a 70% methylated spirits solution or a 10% bleach solution between cuts to avoid spreading the pathogen.
Step 3 — Apply a Phosphonate Treatment
Phosphonate (phosphorous acid) products are the most widely used and evidence-backed treatment for Phytophthora in NZ. They work by boosting the tree's own defence response rather than directly killing the pathogen.
How to apply:
- Soil drench: Mix phosphonate concentrate (available at garden centres and trade suppliers) to the label rate and water it into the root zone. This suits smaller trees and shrubs.
- Trunk injection or bark painting: For larger trees, a more concentrated phosphonate solution is painted onto freshly exposed bark or injected directly into the trunk. This method delivers the product more efficiently but requires care — follow label instructions precisely.
- Repeat applications are typically needed every 6–12 months; check the product label for guidance.
Always read and follow the product label. Wear gloves and eye protection when handling concentrates. Phosphonate products are not restricted-use chemicals for homeowners, but correct rates matter — too high a concentration can damage bark.
Step 4 — Support the Tree's Recovery
- Apply a balanced, slow-release fertiliser to support new root growth — avoid high-nitrogen products that push soft, vulnerable growth.
- Mulch the root zone (keeping mulch 100–150 mm away from the trunk) with coarse wood chip to moderate soil temperature and moisture.
- Reduce the tree's canopy load slightly by removing dead wood and a small proportion of live growth — this reduces the demand on a compromised root system. For significant pruning, engage a qualified arborist.
- Monitor the tree closely over the following growing season.
What Won't Work
- Standard fungicides are largely ineffective against Phytophthora because it is not a true fungus.
- Simply watering less without addressing drainage rarely solves the problem.
- Removing the tree and replanting the same species in the same spot without treating the soil will almost certainly result in re-infection.
When to Call a Professional
Contact a qualified arborist (look for NZ Arboricultural Association members or ISA Certified Arborists) if:
- The tree is large, leans over a structure, or poses a safety risk
- Symptoms are severe or the tree is declining rapidly
- You need trunk injection carried out correctly on a high-value tree
- You suspect kauri dieback or another notifiable disease
- You are unsure of the diagnosis
A professional arborist can assess whether the tree is worth saving or whether removal is the safer and more cost-effective option. You can find qualified local arborists on tradedirectory.co.nz.
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