Phytophthora is one of the most destructive diseases affecting trees and shrubs in New Zealand gardens. Often called root rot or collar rot, it is caused by water mould-like organisms that thrive in wet, poorly drained soils — a common challenge in many parts of Aotearoa. Catching it early gives your tree the best chance of survival.
What Is Phytophthora?
Phytophthora (pronounced fy-TOFF-thora) is not a true fungus but a water mould (oomycete) from the genus Phytophthora. Several species affect NZ garden trees, including P. cinnamomi, P. cactorum, and P. cambivora. These pathogens attack roots, root crowns, and the lower trunk (collar), cutting off the tree's ability to absorb water and nutrients.
New Zealand's warm, wet winters and heavy clay soils in many regions create ideal conditions for phytophthora to spread. Plant health researchers note that the disease is widespread across the country and affects a wide range of species.
Common Signs and Symptoms
Phytophthora can be tricky to diagnose because many of its above-ground symptoms look like drought stress or nutrient deficiency. Look for a combination of these signs:
Above-Ground Symptoms
- Yellowing or browning leaves that wilt even when the soil is moist
- Premature leaf drop or sparse canopy
- Dieback starting at branch tips and progressing inward
- Stunted or poor growth over one or more seasons
- General decline with no obvious pest or mechanical cause
Below-Ground and Collar Symptoms
- Dark, water-soaked roots that are soft and rotten rather than firm and white
- A dark brown or reddish-brown discolouration just beneath the bark at the base of the trunk (collar rot) — scrape the bark gently with a knife to check
- A distinct boundary line between healthy cream-coloured wood and diseased dark tissue
- Foul or musty smell from rotted roots
Species commonly affected in NZ include avocado, pohutukawa, kauri (Agathis australis), rhododendrons, fruit trees, and many conifers. Kauri dieback (P. agathidicida) is a nationally significant biosecurity issue — if you suspect it, contact MPI immediately (note: MPI is not in our standard link list, so check with your local council or a registered arborist for guidance).
How to Confirm the Diagnosis
Visual inspection is a good starting point, but phytophthora can only be confirmed through laboratory testing. If you are unsure, collect a small sample of affected root or collar tissue in a sealed bag and send it to a plant diagnostic laboratory. Your local arborist or garden centre may be able to advise on the nearest testing service.
Treatment Options
There is no guaranteed cure for phytophthora, but a combination of cultural and chemical approaches can slow or halt the disease — especially if caught early.
Improve Drainage
This is the single most important step. Phytophthora cannot spread without free water in the soil.
- Avoid overwatering and fix any irrigation that pools near the trunk
- Install subsoil drainage or raise garden beds to lift roots above saturated zones
- Keep the root crown clear — remove mulch, soil, or lawn that has built up against the trunk base
Phosphonate (Phosphite) Treatments
Phosphonate-based fungicides (e.g., products containing potassium phosphonate or fosetyl-aluminium) are the most widely used chemical treatment for phytophthora in NZ. They work by boosting the tree's own defences rather than directly killing the pathogen. They can be applied as:
- Foliar sprays on leaves
- Trunk injections (more effective for large trees — best done by a professional)
- Soil drenches around the root zone
Always follow label directions carefully and check that the product is registered for use in NZ. Your local garden centre or a qualified arborist can recommend suitable products.
Sanitation and Hygiene
- Sterilise tools between trees using an appropriate registered disinfectant (such as a bleach solution or a product recommended for phytophthora hygiene) or a registered disinfectant
- Do not move soil from affected areas to other parts of the garden
- Remove and dispose of infected plant material — do not compost it
- Clean footwear before entering unaffected garden areas
Soil Amendments
- Adding coarse grit or pumice to heavy clay soils can improve drainage over time
- Some research suggests that compost with good biological activity may help suppress phytophthora, though results vary
Severely Affected Trees
If a tree is more than 50% affected, recovery is unlikely. Removal and replacement with a resistant species suited to your site conditions is often the most practical option. Seek advice from a qualified arborist before making this decision.
Preventing Phytophthora Spreading
- Choose resistant or tolerant species when replanting — ask your local nursery for advice
- Never plant new trees into soil where phytophthora has been confirmed without treating the site first
- Avoid working in wet garden areas and spreading soil on tools, boots, or machinery
- Keep water away from trunk bases with good garden design
When to Call a Professional
You should contact a qualified arborist if:
- You are unsure of the diagnosis and need expert assessment
- The affected tree is large, near a structure, or poses a safety risk if it declines further
- You want trunk injection treatments, which require specialist equipment
- The tree is a native species, particularly kauri, where strict biosecurity protocols apply
- Multiple trees are affected, suggesting widespread soil contamination
A qualified arborist will have the training, tools, and product access to give your trees the best chance. You can find experienced local arborists on tradedirectory.co.nz.
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