Tree fungus is a common problem in New Zealand's warm, humid climate, and many homeowners want to tackle it themselves before calling in a professional. While some homemade remedies can help manage minor fungal issues on trees, it's important to understand their limits — the wrong approach can stress a tree further or mask a serious underlying problem.
Understanding Tree Fungus First
Before reaching for a spray bottle, it helps to know what you're dealing with. Tree fungus covers a wide range of conditions, from surface moulds and sooty mould on leaves, to powdery mildew, bracket fungi (conks) on trunks, and serious internal wood-rot pathogens like Armillaria (honey fungus). The treatment that works for a mild leaf mildew won't touch a deep-seated root rot.
NZ's damp winters and humid summers — particularly in Auckland, Northland, and the West Coast — create ideal conditions for fungal spread. Poor airflow, overwatering, compacted soil, and wounds from pruning or storms are common entry points for fungal pathogens.
Homemade Treatments That Have Some Evidence Behind Them
The following remedies are widely used by gardeners and have reasonable support in horticultural practice. None are silver bullets, and results vary depending on the fungus type, tree species, and severity of infection.
1. Baking Soda Spray (Sodium Bicarbonate)
Best for: Powdery mildew on leaves and young shoots.
Mix 1 teaspoon of baking soda with 1 litre of water and a few drops of mild liquid soap (to help it stick). Spray onto affected foliage, covering both sides of leaves. Repeat every 7–10 days. Baking soda raises the surface pH, making it harder for fungal spores to germinate. Avoid applying in full sun as it can cause leaf burn.
2. Neem Oil Solution
Best for: Sooty mould, powdery mildew, and some leaf spot diseases.
Neem oil is a plant-derived oil with antifungal and insecticidal properties. Mix 2–3 teaspoons of cold-pressed neem oil with 1 litre of warm water and a small amount of dish soap as an emulsifier. Spray thoroughly on affected areas in the early morning or evening. Neem oil is available at most garden centres and hardware stores including Bunnings and Mitre 10.
3. Copper-Based Sprays (Bordeaux Mixture)
Best for: Leaf curl, downy mildew, and some bacterial/fungal cankers.
While not strictly "homemade," Bordeaux mixture — a combination of copper sulphate and hydrated lime — is a traditional remedy that can be mixed at home. It is widely used in organic horticulture. Pre-mixed copper sprays are also available commercially. Use with care: copper can accumulate in soil and harm earthworms and beneficial organisms if overused.
4. Diluted Apple Cider Vinegar
Best for: Very mild surface moulds only.
A weak solution of 1 part apple cider vinegar to 10 parts water is sometimes used as a surface spray. Evidence for this is largely anecdotal, and it can damage foliage if applied too strongly. Use sparingly and test on a small area first.
What Homemade Treatments Cannot Fix
It's critical to be honest about the limits of DIY treatment. Homemade sprays will not effectively treat:
- Bracket fungi or conks on trunks — these are the fruiting bodies of fungi that may have been rotting the tree internally for years.
- Root rot caused by Phytophthora or Armillaria — these require professional diagnosis and, in many cases, the tree may need to be removed.
- Cankers that have girdled major branches or the trunk.
- Systemic fungal infections that have spread through the vascular tissue.
If you notice mushrooms growing at the base of a tree, large shelf-like brackets on the trunk, significant dieback of branches, or the tree leaning unexpectedly, stop DIY treatment and call a qualified arborist immediately. A structurally compromised tree is a serious safety hazard.
Good Cultural Practices to Support Treatment
Homemade sprays work best alongside good tree care habits:
- Prune out infected material cleanly with sterilised tools (wipe blades with methylated spirits between cuts).
- Remove fallen leaves and debris from around the base — fungal spores overwinter in leaf litter.
- Improve airflow by thinning dense canopies.
- Avoid overwatering and ensure good drainage around the root zone.
- Mulch around the base (keeping mulch away from the trunk) to regulate soil moisture and temperature.
For guidance on tree health and best practice, BRANZ and your local council's arboricultural guidelines are useful starting points.
When to Call a Professional
Call a qualified arborist if:
- The fungus is on the trunk, major limbs, or root zone.
- You can see visible decay, soft wood, or hollow sections.
- The tree is near a structure, power lines, or public area.
- DIY treatments haven't improved the situation after a few weeks.
- You're unsure what type of fungus you're dealing with.
A certified arborist can properly diagnose the problem, recommend registered fungicide treatments if needed, and assess whether the tree is structurally safe. You can find qualified local arborists on tradedirectory.co.nz.
For trees protected by a Notable Tree schedule or within a district plan overlay, check with your local council before carrying out any treatment or pruning — consent may be required under the NZ Building Code and Resource Management Act framework.
Safety note: When mixing and applying any spray, wear gloves and eye protection. Keep sprays away from waterways, as copper-based products in particular can be harmful to aquatic life. Check the WorkSafe NZ guidelines for safe handling of garden chemicals.
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