Most trees that get sick give you warning signs long before they fail. The trick is knowing what you're looking at. Here are the diseases and pests we deal with most often across Waikato, what they do to a tree, and when it's time to get someone out to look.
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Cypress canker (macrocarpa and Leyland hedges)
If you have a macrocarpa shelterbelt or a Leyland cypress hedge in Waikato, cypress canker is the problem you're most likely to meet. It's a fungal disease (Seiridium) that gets into the bark and slowly kills branches from the inside.
The tell-tale signs are random patches of foliage turning rusty brown while the rest of the tree looks fine, sunken cankers on the branches, and resin bleeding down the bark. It usually takes hold when a tree is already stressed by drought or poor drainage.
There's no spray that cures it. Management means cutting infected wood out well below the canker and cleaning tools between cuts to avoid spreading it. Badly affected hedges are often better removed and replaced.
Myrtle rust (pohutukawa, manuka, feijoa, eucalyptus)
Myrtle rust is a wind-blown fungus that attacks plants in the myrtle family: pohutukawa, rata, manuka, ramarama, feijoa, lilly pilly and eucalyptus. It reached New Zealand in 2017 and is now established across the upper North Island.
Look for bright yellow powdery spots on soft new growth, leaves and shoot tips. Badly infected growth becomes distorted and dies back.
This one matters beyond your own garden. If you think you've found it, don't touch or prune the plant, and report it to the Ministry for Primary Industries on 0800 80 99 66. Pruning an infected plant just scatters the spores further.
Honey fungus (Armillaria root rot)
Armillaria spreads through the soil from old stumps and buried wood, which makes it common on recently cleared or developed sections. It attacks the roots and lower trunk, so by the time you see symptoms the damage is usually well advanced.
The classic signs are clumps of honey-coloured toadstools at the base of the tree in autumn, and a layer of white fungal growth under the bark at ground level that smells of mushrooms. Affected trees often decline for a season or two, then fail suddenly.
Because it lives in the roots and soil, a tree with established honey fungus is usually a removal job. Grinding out old stumps nearby helps slow it down.
Phytophthora root and collar rot
Phytophthora is a group of soil-borne water moulds that rot roots and the base of the trunk, especially in heavy, wet or poorly drained ground. It's the same family of pathogen behind kauri dieback, and it affects a wide range of ornamental and fruit trees too.
Above ground you'll see a thinning, yellowing canopy that doesn't recover, and sometimes dark weeping lesions at the base of the trunk. It is easy to mistake for simple drought stress.
Improving drainage and keeping machinery off the root zone can help, but a tree with collar rot has usually lost the structural wood at its base and becomes a stability risk.
Sap-suckers and sooty mould
Scale insects, aphids and psyllids feed on sap and excrete a sticky honeydew, which then grows a black coating of sooty mould. You see it a lot on citrus, pittosporum and eucalyptus.
The mould itself doesn't infect the tree, it just blocks light from the leaves and looks unsightly. The fix is dealing with the insects producing the honeydew, not scrubbing the leaves.
When it's a disease and when it's just stress
Plenty of trees that look sick are simply stressed by a dry Waikato summer, compacted soil, or roots damaged during building work. Those trees often bounce back once the cause is sorted. A genuine disease tends to keep getting worse and spread.
If a tree is declining and you can't work out why, or you've spotted something on this list, get a qualified arborist to assess it before you decide anything. Call us on 022 167 1268 for an onsite look. We'll tell you whether it can be managed, monitored, or needs to come out.
