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Poisonous plants and fungi

by  Maggy Wassilieff

If you go down in the woods today, be sure you don’t eat a toxic mushroom. New Zealand’s poisonous plants and fungi can have nasty effects, although few people die. Māori knew how to make food from certain poisonous native plants – such as cakes from the orange karaka berry.


Poisonous native plants

Few people in New Zealand have died from plant and fungal poisoning, but each year about 75 people need hospital treatment. Reports of mushroom poisoning in New Zealand have been increasing since the early 1990s.

Why are some plants and fungi poisonous?

Plants and fungi make a wide array of chemicals to ward off bacteria, other fungi, insects and browsing mammals, and to protect their patch from competitors. Some of these chemicals are toxic to humans or animals.

In case of poisoning

If someone shows signs of poisoning, and you think they have eaten a poisonous plant, dial 111 for an ambulance. Keep any parts of the plant or fungus for an expert to identify.

Tree tutu and ongaonga or tree nettle are the only New Zealand native plants known to have killed humans by poisoning.

Tutu

The small tree tutu (Coriaria arborea) is found throughout the country, especially on bush margins and alongside streams. Except for its swollen petals, all parts of the plant are poisonous.

Around 1900, New Zealand chemists identified tutin as the poison. This acts on the central nervous system, causing convulsions and breathing problems that may lead to death. There have been few cases of human poisoning by tutu since 1900, although one man died in 1989.

Tree nettle (ongaonga)

Also known by its botanical name, Urtica ferox, this killer plant is well-documented. Like all nettles, it is covered in stinging hairs that put poison into the skin of a person or animal that brushes against it. The plant grows in coastal and regenerating shrublands up to 600 metres above sea level, where it may form dense thickets up to 2 metres tall.

On Boxing Day 1961 two young men hunting in the Ruahine Range stumbled through a patch of tree nettle and received a number of stings on their limbs. Within an hour one of them had difficulty in walking and breathing, and then lost his sight. He died five hours later in hospital. His friend had similar symptoms, but recovered. Although this is the only fatal incident on record, a number of people have been very ill for two to three days after being stung.

Other poisonous natives

Some other native plants are poisonous:

  • Kōwhai (Sophora species) – its yellow seeds are poisonous if chewed.
  • Tītoki (Alectryon excelsus) – contains cyanide-producing poisons.
  • Ngaio (Myoporum species) – has poisonous leaves.
  • Karaka (Corynocarpus laevigatus) – has a poisonous seed in its fleshy orange fruit.
  • Poroporo (Solanum aviculare, S. laciniatum) – their unripe green berries are poisonous. The early European settlers made jam from the ripe orange berries.
  • Tūrutu, blueberry lily (Dianella nigra) – their violet-coloured berries were implicated in the death of an infant in the late 1800s.

It is most unlikely that someone would eat enough of these plants to die or become seriously ill.


Poisonous plants used for food

Māori used some poisonous plants for food, processing them to make them safe. Usually the plant was cooked for a long time, and then soaked in water.

Karaka

Raw kernels of karaka fruit are poisonous and may cause paralysis. However, Māori gathered karaka berries as one of their staple foods. They would bake and soak them before making them into a floury cake that was safe to eat.

One of New Zealand’s early chemists, William Skey, isolated a bitter compound (karakin) from the kernels. He suspected this was the poison that caused convulsions and paralysis. It was later confirmed that karakin was toxic to birds, bees and mammals, but was destroyed when heated for an hour at 100°C.

Taro

The Polynesian ancestors of Māori brought taro (Colocasia esculenta) to New Zealand. All parts of the plant are poisonous when raw or partly cooked, and cause severe irritation of the mouth, tongue and throat. The leaves and tubers are safe after prolonged cooking.

Bracken fern

Bracken (Pteridium esculentum) is known to be carcinogenic (cancer-causing). This common fern causes cancers in animals, and is thought to cause throat and stomach cancers in Japanese people, who eat the young fronds.

The rhizomes (underground plant stems) of this widespread fern were an important food for Māori. They soaked then roasted the rhizomes, and beat the baked product to produce a starchy flour.

Tutu

Maori made an alcoholic drink (tutu beer or tutu wine) and a pudding from the fruit of the tutu tree. They were careful to strain out the toxic seeds. Not so knowledgeable were four French sailors, who died in the mid-1830s after eating tutu berry pudding.

Tutu wine

Tutu wine packed a fair punch. In the early days of European settlement, two reverend gentlemen, Canon J. W. Stack and Bishop H. J. C. Harper, took a little drink of tutu wine. Stack later reported that they soon lost all feeling in their extremities and could hardly keep their balance. Their vision became impaired, and they felt they were being poisoned. But then the strange sensations passed, and feeling came back in their limbs.

Toxic honey

Honeydew is secreted from the rear end of tiny sap-sucking insects (passion-vine hoppers, Scolypopa australis) that feed on the poisonous tutu plant. Toxic honey is produced by bees that feed on this honeydew. The honey contains tutin and hyenanchin (a related toxin), and is extremely dangerous.

Outbreaks of honey poisoning have been documented since the early 1900s. The main areas are the Coromandel, eastern Bay of Plenty and Marlborough Sounds. Honey is most likely to be contaminated during hot dry summers, when vine hopper numbers are high. Beekeepers remove their hives from risk areas when toxic honeydew is abundant.

Rhubarb and potato

Parts of some common food plants are poisonous. Rhubarb (Rheum rhabarbarum) stems are edible when cooked, but the leaves are poisonous. Green and sprouting tubers of potato (Solanum tuberosum) contain toxic alkaloids and should not be eaten. Potatoes are normally stored in the dark to reduce their exposure to sunlight (which causes greening). In New Zealand one person has died after eating potato berries, the green fruit that grows on the potato stem after it flowers.


Poisonous garden plants

Dangers lurk in our own backyard. Most hospital admissions from plant poisoning are young children who have eaten or touched a plant in a local garden. Most of these poisonings are non-fatal.

Many poisonous plants in New Zealand have been introduced for horticultural reasons. Only nine native plants appear on Landcare Research’s list of 93 plants and plant groups that pose a danger to children. In a study of hospital admissions from 198 to 1998 for plant poisoning, 93% of the plants identified were introduced species.

The nightshades

Top of the list for hospital admissions are members of the potato and nightshade family (Solanaceae). Many admissions are for suspected poisoning by deadly nightshade (Atropa belladonna), but this plant is very uncommon in New Zealand, and the cause is usually black nightshade (Solanum nigrum). This very common weed is often mistakenly called deadly nightshade, although it is much less toxic.

Jerusalem cherries (Solanum diflorum and S. pseudocapsicum) are a more common danger. One child has died and several have become ill after eating the fruit. The small shrubs have attractive bright red berries and are often planted for winter colour. Both species have been spread by birds into nearby bush.

The arums

Plants of the arum family (Araceae) are the second largest group causing hospital admissions. Included in this family are:

  • calla lily (Zantedeschia aethiopica), known as arum lily in New Zealand
  • Italian arum (Arum italicum)
  • dumb cane (Dieffenbachia species)
  • taro (Colocasia esculenta)
  • elephant’s ear (Alocasia brisbanensis), also known as kape or ’ape by Pacific Islanders.

All parts of these plants are poisonous and cause severe irritation of the mouth and throat if swallowed. Most arum poisonings occur when children eat the bright fruit spikes of calla lily. This is widely grown in home gardens, and a common weed.

Datura

Datura stramonium (thorn apple) and the related angel’s trumpet (Brugmansia candida) are two of the most dangerous plants in New Zealand. These tall shrubs have hanging, trumpet-shaped flowers and are grown as garden specimens. Datura also grows as a weed. It is sometimes eaten by people wanting to experience hallucinations, caused by the plant’s powerful alkaloid chemicals. These have other side-effects that can be lethal: they over-stimulate the heart and act as powerful muscle relaxants. In New Zealand datura has been responsible for a number of admissions to intensive care, and has indirectly caused at least two deaths from drowning.

Pretty and poisonous

The most likely plants to cause severe poisoning in New Zealand are (in alphabetical order):

  • castor oil plant (Ricinus communis)
  • datura or thorn apple (Datura stramonium)
  • hemlock (Conium maculatum)
  • Jerusalem cherry (Solanum diflorum and S. pseudocapsicum)
  • laburnum (Laburnum anagyroides)
  • lily of the valley (Convallaria majalis)
  • yew (Taxus baccata).

Animals and poisonous plants

Native plants

Unfamiliar with the toxic effects of New Zealand’s plants, the early British settlers suffered major stock losses when their cattle, sheep and horses first browsed on tree tutu (Coriaria arborea) and ngaio (Myoporum laetum). Although both species were quickly identified as being poisonous to livestock, they continue to poison animals that stray into unfenced bush.

Pinātoro or Strathmore weed (Pimelea prostrata), a ground-hugging, open-country shrub, is poisonous to horses and cattle but seems to have little effect on sheep.

Bracken poisoning of cattle often occurred in North Island hill country between 1950 and 1970, when cattle were used to trample bracken fronds on land being developed for pasture. After a few weeks of feeding largely on bracken, cattle begin haemorrhaging. Horses experience muscle and nerve disorders if they graze on bracken for long periods.

Tutu and elephants

On at least three occasions circus elephants have been poisoned by tutu. In the late 19th century there was no cure, and a stricken animal died a few hours after feeding on young shoots. In the 1960s two poisoned elephants recovered after being injected with barbiturates. Veterinarian David Marshall recalled that three elephants, poisoned (but not fatally) while travelling through the tutu-infested Buller Gorge, produced spectacular waist-high piles of vomit.

Introduced plants

British settlers brought many of their familiar plants, some of which are poisonous. The acorns of all oaks (Quercus) are poisonous, especially to cattle and sheep. Yew trees regularly kill browsing cattle and sheep. The toxins are soon absorbed and cause heart failure. Pregnant cows are likely to abort if they eat macrocarpa (Cupressus macrocarpa) leaves late in pregnancy.

Weeds also arrived with the new settlers. Ragwort (Senecio jacobaea) flourishes in pastures in high rainfall areas. It is highly toxic and causes liver damage in cattle and horses. High-country sheep are susceptible to poisoning from St John’s wort (Hypericum perforatum), a pretty, yellow-flowered weed. Sheep become sensitive to sunlight after eating the leaves: unpigmented parts of their body redden, swell and start oozing fluid.

Nitrates in plants

Nitrogen is an essential element for growth, but some plants growing in fertile soils accumulate so much nitrogen that animals grazing on them become poisoned. Green feed-crops such as turnips, and fast-growing pasture grasses are the main culprits. Cattle, sheep, horses and deer are at risk as they readily convert nitrate to nitrite, and in this form the nitrogen interferes with oxygen transport in the blood system. Affected animals may die a few hours after eating nitrogen-rich fodder.


Poisonous fungi

New Zealanders are not generally inclined to harvest the bodies of fungi (such as mushrooms or puff balls) from the wild, so there have been few cases of serious poisoning from eating toxic mushrooms. The majority of mushroom-forming fungi in New Zealand are not poisonous, but it is difficult to identify some species, and unknown fungi should not be eaten.

Death cap mushroom

Worldwide, most fatal fungal poisonings occur when people mistake death cap mushrooms (Amanita phalloides) for an edible species. Toxins in the mushroom act on the liver and kidneys. In June 2005 a recent Vietnamese immigrant became critically ill after eating some cooked death caps. Symptoms do not develop for some hours, by which time it is usually too late to save the victim’s liver.

Fly agaric and magic mushrooms

Two other types of toxic mushroom are fly agaric (Amanita muscaria), and several species of magic mushroom (Psilocybe species). Both types contain hallucinogenic chemicals, and some people deliberately risk the toxic effects to achieve a psychedelic high.

Fly agaric poison may cause severe stomach pain, vomiting and diarrhoea. Magic mushroom poisoning is unlikely to be fatal, but small, brown magic mushrooms are easily mistaken for other toxic species such as Clitocybe, Entoloma and Cortinarius.

Fungal poisoning of livestock

The most significant forms of fungal poisoning in New Zealand are those that cause facial eczema and grass staggers in livestock. Losses from facial eczema cost the New Zealand meat industry $80–$400 million annually.

The microscopic fungus Pithomyces chartarum grows on dead leaves at the base of pasture grasses. It produces toxic spores that cause liver damage in grazing animals, and facial eczema develops as a result.

The fungus that causes grass staggers (tremors, stumbling and muscle spasms) in stock animals is an endophyte – it grows within the grass without harming it. Perennial ryegrass (Lolium perenne) endophytes produce several toxins: one affects the nerves and muscles of grazing stock, and one causes heat stress. Tall fescue (Schedonorus phoenix) endophytes cause fescue foot (lameness) and ill-thrift (slow growth) in cattle and sheep.


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How to cite this page: Maggy Wassilieff, 'Poisonous plants and fungi', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/poisonous-plants-and-fungi/print (accessed 18 October 2017)

Story by Maggy Wassilieff, published 24 Sep 2007