It was spring, 1987, when officials from Rodney County Council turned up at the door of DSIR Plant Protection’s laboratories in Mt Albert, Auckland, with a dead cab­bage tree trunk—and a problem.

Why, they asked, had this tree, and an increasing number of other seem­ingly healthy members of the spe­cies, suddenly dropped their leaves and died.

Five years later, “sudden decline,” as the syndrome has become known, is continuing to kill off thousands of cabbage trees across the top two-thirds of the North Island. Some sci­entists predict that at the present rate of loss, few cabbage trees will be left in Northland by the year 2000.

The original query from Rodney County, a mixed farming/lifestyle block district just north of urban  Auckland, had been sparked off by growing complaints from ratepayers that the spraying of herbicides along road edges by the council had been killing their cabbage trees.

“The council didn’t believe it was the spray, and wanted to know what was killing the trees,” recalls scien­tist Dr Ross Beever.

This was the first formal notifica­tion of the problem, though subse­quent detective work takes the out­break back to at least the early 1980s.

The first task for the DSIR plant disease scientists was to find mate­rial to work with.

“They’d only brought us the trunk, and in cases like this, where the leaves suddenly turn yellow, wilt and fall off, it is often a root disease that is involved,” said Dr Beaver. “The trunk wasn’t going to help our diagnosis.”

He’d noticed, however, that just outside his office window something identical seemed to be going on. For some weeks he’d been “sort of vaguely watching” the collapse of a cabbage tree in a suburban garden, on the slopes of nearby Mt Albert.

Across the road trooped the scien­tists, spades at the ready, and soon they were back with the first ex­hibit—roots and all—in what has be­come a long search for the cabbage tree killer.

For most New Zealanders, the cab­bage tree is as much a national sym­bol as the kiwi, and, like the flightless bird, this tree is a bit of an oddball. Although it is obviously a tree, it has more in common, botanically, with grasses and lilies than it does with typical trees. Its common name is misleading—a 19th century steal from the nikau palm, to which the description “cabbage tree” first be­longed. (See box, page 52.)

What’s more, unlike other trees, it is anchored to the ground not just by roots, but by a giant underground stem or rhizome. It is this sub-soil “branch” which gives the cabbage tree its legendary ability to defy death and live on—virtually for ever. It also provides the cabbage tree’s generic name: Cordyline is derived from the Greek kordyle, or club—a reference to the shape of the rhizome.

Just as, above ground, the cabbage tree is capable of producing a leafy shoot at the site of each leaf axil (and will do so in response to physical damage as slight as a cat’s scratch­ing), so underground on the rhizome there are thousands of buds, each with the potential to sprout forth as a new trunk.

Thus, by the time an old cabbage tree reaches 100 or so years, and the rot that lurks in all of them finally topples it, the plant doesn’t disap­pear. Instead, the underground rhi­zome will have produced several new trunks, creating the tell-tale circle of trees seen wherever the cabbage tree is part of the landscape.

Dr Philip Simpson, who is head­ing the Department of Conservation’s cabbage tree rescue project, says that at least some of the trees stricken by sudden decline appear to have re­tained this regenerative ability.

Ethnologist Elsdon Best collected several old Maori sayings which are based on the cabbage tree’s inde­structibility. “He uru a ki, he wana te ti, ka rito te ti” was one that warned of the pitfalls of passing on gossip. “The spoken word reappears, just as ti sprouts again.”

Another is a lament for a loved one. “Waiho kia tangi ahau ki taku tupapaku, kapa he uru ti e pihi ake” was translated by Best to mean “Let me mourn for my dead one; it is not as if he would spring up again as the ti does.”

Even felled cabbage trees can defy all odds and sprout again. Perhaps the most remarkable case on record is that of a Northland gumdigger who made a chimney out of hollow cab­bage tree trunks positioned side by side and nailed together. “For some months a fire was kept alight con­tinuously,” write Laing and Blackwell in Plants of New Zealand, “until the stems were burned through, and only parts of the outside bark left. The man then left the place, and within a short time, that which had been a blackened chimney, be­came a mass of living green.”

In another instance felled cabbage trees lay on a beach for eight months “rolling up and down in the salt tide, or baking high and dry in the sun” before being returned to a paddock. There they promptly sprouted.

The genus Cordyline includes about 15 species, with representa­tives in India, South-east Asia, east­ern Australia and across the Pacific to South America. There are five New Zealand species, referred to by Maori collectively as ti rakau or simply ti:

• Cordyline australis, the common cabbage tree (ti kouka or ti whanake), is found throughout the three main islands in swamps, clearings, forest margins and suburban gardens, from sea level to around 600m. Growing to a height of 12-20m, older trees can reach a trunk diameter of 1.5m or more. The leaves are long and narrow (30-100cm by 3-6cm), and in spring the tree bears panicles of strongly scented white flowers.
• Cordyline indivisa, the broad-leafed or mountain cabbage tree (ti kapu or toii), is found from South Auckland to Fiordland, in wet areas from 450m to 1350m above sea level, usually in well lit forest clearings. It grows to about 8m in height, usually with a single trunk. The leaves, at 1­2m by 10-15cm, are longer, broader and thicker than those of australis. Dense flower panicles hang down beneath the tuft of sword-like leaves.
• Cordyline pumilio, the pygmy or dwarf cabbage tree (ti rauriki or ti koraha), is found in scrubland from the Waikato and Bay of Plenty north­ward. With little or no trunk, it is often mistaken for a broad-leafed (30- 100cm by 1-2cm) grass.
• Cordyline kaspar, found natu­rally only on the Three Kings and Poor Knights Islands, grows to about 10m and has a multi-branched trunk. It is similar in appearance to australis, but the leaves are broader (5-7cm) and less fibrous. The flowers are highly scented.
• Cordyline banksii, the forest cab­bage tree (ti ngahere or ti parae), is found from sea level to 1000m in wet forest margins, from Westland north. It is multi-trunked and grows to about 4m. Leaves are about 1-2m by 4-8cm, typically broader in the middle. Dis­tinctive flower panicles grow up to 2m long.

Of the two huge groups of flower­ing seed plants, the dicotyledons (which include most of the world’s woody shrubs and hardwood trees) and the monocotyledons (which are predominantly non-woody, and thus stemless, like grasses, orchids, rushes and sedges), the cabbage tree, along with the palms, belongs to the latter.

Traditionally, cordylines have been considered part of the lily fam­ily, and C. australis was regarded as the largest lily in the world.

However, a review of the whole monocotyledon group in the 1950s resulted in the cabbage tree and re­lated genera—those capable of form­ing trees, with stems mechanically able to support aerial branches—be­ing separated from the true lilies into the family Agavaceae.

They then found themselves in the company of 20 other genera totalling some 700 rhizomatous, woody plants found mainly in the drier, warmer parts of the globe. Family members included the New Zealand flax, the century plant (source of the Mexican drink mescal), the Joshua tree (a spe­cies of Yucca), Dracaena (from which comes a red resin called “dragon’s blood”) and sisal hemp, a source of cordage.

Over the past decade there have been further attempts to refine “lily” classification, and cabbage trees have been placed in the family Asphodelaceae (which includes the asphodels of Europe). Some taxono­mists believe even further splitting is warranted—with cabbage trees going into the Asteliaceae, a family with a southern hemisphere geographic fo­cus.

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New zealand’s original Poly­nesian settlers must have found the widespread distri­bution of cabbage tre

es a welcome sight. Throughout the Pacific, one species of ti (Cordyline fruticosa) had been cropped and cultivated for the strength of its leaves as cordage, for its medicinal properties and, espe­cially, for food.

Recent research has proved scien­tifically what the ancient Polynesians well knew: the young stem and the rhizome contain carbohydrates which exceed cane sugar in sweet­ness.

C.fruticosa (formerly called C.terminalis) is indigenous to South­east Asia and India, and is found throughout the Pacific Islands. Whether C. fruticosa is native to these islands or was introduced is a matter of some debate, but there is little argument that the early Polynesian voyagers to New Zealand brought this important crop with them, along with kumara, taro, hue (the gourd) and aute (paper mulberry).

Because of its tropical origins, ti pore (as C. fruticosa became known) proved hard to grow in New Zea­land’s harsher climate. Indeed, it seems only to have thrived in the Far North, and then only under culti­vated conditions.

The arrival of European traders, with their sweets and crystallised sugar, led to a rapid abandonment by northern Maori of their ti pore plan­tations, with the result that the plant, unable to seed in New Zealand, died out.

Naturalist Rev. Canon Walsh re­corded that in the late 1880s he had been shown examples of a, to him, hitherto unknown variety of cordyline while visiting Waimate North. They had been found “in a long-deserted native cultivation” and were “almost the last survivors of a variety that is practically extinct so far as New Zealand is concerned.”

A local settler confirmed that the plant, ti pore, had been common­place in Maori settlements 50 years before. “He was quite familiar with the cooked article, as the Maori nurses and other retainers of the mis­sion families used often to bring small quantities as presents to the chil­dren.”

Walsh describes it as having a “short, slender stem .. . surmounted by a handsome head of soft glossy leaves” from 45-60cm long by 8-10cm wide. In older plants the trunks forked into several heads at about one metre high.

“The root was by far the most im­portant part of the plant from the Maori point of view. It is a mass of greenish-white pulpy fibre, of such a consistency as to be easily cut through with a sharp spade.”

By “root” he is of course referring to the rhizome which, he records, is “out of all proportion to the rest of the plant. One that I transplanted was nearly 3ft long with a principal diam­eter at the upper third of from 3-4 inches.”

To propagate ti pore “the usual plan was to cut off and replant the stalk with a small portion of root attached, in the same manner as is done with taro.” Advantage was also taken of “offsets which often spring up at the foot of old stocks . . .”

To prepare it for eating, the rhi­zome was pounded with a wooden club until the fibre was broken up, and then steamed in a hangi for 12 to 24 hours.

“The substance then presented the appearance of a glutinous mass, and the taste is described as of a sugary sweetness far beyond that of the ti rauriki [the dwarf cabbage tree] but, like that root, with a slightly bitter after flavour.”

The cooked ti pore kept well, and was handy in times of war as emer­gency rations within the pa and as takeaways for war parties. However “it was most generally used merely as a sweetmeat.

“In fact, the Maori say that in the old times the chewing of a piece of prepared root when one had nothing else to do, gave the same satisfaction as is now afforded by a pipe and tobacco.”

One use for ti, seemingly undis­covered by the Maori, was its use as a base for alcohol—an application which T. F. Cheeseman, curator of the Auckland Museum, noted in 1900 had not escaped the attention of the Sandwich Islanders (Hawaiians). By bruising the rhizomes and leaving them in water to ferment, “an intoxi­cating drink” was produced. The Sa­moans were equally aware of this process.

It was left to a later wave of mi­grants to New Zealand to put cabbage trees on tap. Reverend Richard Taylor, who arrived in the Bay of Islands in 1839, was later to note, after observing that the cooked rhi­zomes had a “bitter sweet taste”, that “the early missionaries brewed ex­cellent beer from them.”

The indigenous varieties of ti were also adopted as an important food source by the Maori—espe­cially C. australis, C. pumilio and a mysterious plant which was culti­vated in Hawkes Bay, the Waikato,Taranaki and Whanganui. This latter     plant,known as ti para in Hawkes Bay, ti tawhiti  in Taranaki and ti kowhiti in
Whanganui, has long been consid­ered extinct.

Writing in 1880, explorer-botanist William Colenso noted that ti para was then so rare, the only specimens he knew were in his own garden “raised from a sin­gle plant I found in an old Maori cultivation . . . in 1845.” None had ever flowered.

“Thirty years ago, whenever some of the oldest chiefs here should happen to see the plant growing in my garden, they would invariably longingly beg for its stems to cook for a meal, saying how much they liked it.”

Recent   research by DSIR Botany Institute’s Peter Heenan and Warwick Harris indicates that this plant has not been lost, but has been sold for many years in the nursery trade under the name Cordyline kirkii. It is a distinc­tive dwarf plant that has not been known to flower, and its precise relationship to our other cabbage trees remains obscure.

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The proliferation of Maori names for ti—nine for the wide­spread C. australis alone—re­flects the status the cabbage tree had as a food source.

In pre-European times, the cultivation and preparation of the stem and rhizome for food was a major industry—one that was both time-consuming and labour-intensive. Yet within a few years of the first settlers ar­riving, it had been abandoned.

This was partly the result of the settler-introduced market economy, which totally altered Maori life. Trad­ing fish, potatoes, grain and flax for bags of sugar was a much simpler way of gaining sweet carbohydrates than farming ti trees through a three-or four-year cropping cycle.

Also, often the land containing the ti plantations passed out of Maori hands. One detailed account by an old South Island Maori of how they used to prepare kauru—the food from the ti—ends bluntly:

“Such was the work on the kauru­producing lands every year from very ancient times right down to three years after Mantell came, when the Maori stopped preparing the food because the Pakeha had settled here and burnt the kauru-producing plains, and they were all destroyed.”

(Walter Mantell was sent to Can­terbury and Otago in 1848 as land commissioner, with the express pur­pose of “extinguishing native claims in the Middle Island.”)

Cultivation methods varied from the intensive farming of C. fruticosa in sheltered spots in the north to the more informal cropping of natural and planted plantations of the common C. australis.

Ownership rights to planta­tions, even those many kilome­tres from a tribe’s home base, were jealously protected—and respected. The same South Is­land account notes that in late September, when harvesting the ti begins, “each hapu goes straight to its section on its work­ing territory. A hapu never works on the territory of any other hapu without permission—the Maori have the strictest customs with regard to a person or hapu trespassing.”

Both the stems of young (three- to four-year-old) trees and the rhizomes were cooked, and sometimes the young leaves, though the latter ap­pear to be something of an acquired taste.

Explorer William Colenso, lost and “hunger-impelled,” in the Hawkes Bay bush in 1847, broke off some cabbage tree leaves “and roasted the stalks of the young leaves, which, though both tough and bitter, served to allay our pangs.” Wrong recipe, it seems.

Chief Hone Tare Tikao from Rapaki (near present-day Lyttelton) recalled to Elsdon Best that “the rito [the crown of young undeveloped leaves] was plucked off and cooked and,when saturated with the fat of eels or birds cooked with it, was good eat­ing.

“In like manner was used the tap root of the ti. It was dug up and roasted at a fire, buried in the hot ashes until cooked.

“It was then pounded to soften it, spread out on flax mats, sprinkled with the sweet water (nectar) of flax blossoms and, when so saturated the edible matter was shaken into bowls . . . this process was adopted during the month of November.

“At other times it might be cooked in a steam oven.”

Steaming in a hangi for up to 48 hours, especially where the young stems are concerned, seems to have been the main method of cooking.

In South Canterbury and Otago, the remains of many huge cabbage tree cooking ovens, umu ti, have been discovered. Most are circular, with diameters up to 7 metres and oven stones weighing up to 16kg.

The South Island report quoted above describes a pit of two chains (40 metres) in length, and nearly two metres in depth, that took 20 men two days to dig and stack with fire wood. Much ritual attended the proc­ess.

West Coast explorer Thomas Brunner noted in January 1847 that “this morning opened our oven which smelled like a sugar-boiling establishment. Found the ti excel­lent, but rather too sweet for a diet; however, this and the fish make a fine meal.”

University of Otago chemist Donald Brasch and anthropologist Barry Fankhauser have pinpointed scientifically what Brunner and the ancient Polynesians discovered by experimentation. The common cab­bage tree is a rich source of carbohy­drate which could be used for the preparation of fructose syrup, an ex­cellent sweetening agent now widely used in the food processing industry. Being twice as sweet as sucrose, it is popular for diet foods because fewer calories are required to achieve the same level of sweetness.

The cabbage tree “contains more usable carbohydrate on a fresh weight basis than either sugar cane or sugar beet.”

As well as its potential as a com­mercial sweetener, the researchers suggest that cabbage trees could be a useful feedstock for a fuel ethanol industry.

The cabbage tree’s underground sugar factory is just one of the fea­tures that fascinate Department of Conservation scientist Dr Philip Simpson about this remarkable tree.

Not only is it a rarity amongst the monocotyledons, being one of only about 100 species (out of something like 60,000) that have developed a tree form, but its vertical rhizome system is shared with only one other species, its American cousin the yucca.

Simpson explains the process by which the rhizome forms: “When the seed germinates into a tiny seedling, a bud in the axil of one of the first leaves grows out and straight down.”

What we get is not a continuation of the main stem but “a side branch which has come out of one of the earliest leaves and grown straight down into the ground.”

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Underground, it adapts to its spe­cial roles: to act as the plant’s water and food storage chamber, to anchor the tree and to provide a continuous new surface for the roots that radiate out in great numbers. The roots, most plentiful near the surface, are 4-5mm thick and grow up to 10m in length. Simpson says that on a mature tree there are thousands of these strong, robust roots spreading out from the upper portion of the rhizome.

As well as trying to unravel the cabbage tree’s morphological secrets, Wellington-based Simpson heads a task force set up to prepare a conser­vation plan for this, our most distinc­tive native tree. While the outbreak of sudden decline sparked off the project, the reality is that the cabbage tree, though still common through­out the country, has been declining in numbers since the introduction of pastoral farming last century.

Simpson says that while general observation puts the cabbage tree’s natural habitat as lowland wetlands and forest margins, human interven­tion by both Maori and pakeha over several centuries makes it difficult to pin down its precise distribution.

He suggests populations could have actually increased in pre-Euro­pean times, with Maori developing huge plantations in South Canter­bury, for instance, and in Hawkes Bay. With the arrival of pakeha, how­ever, the plantations were aban­doned, and much of the land was turned into farms.

The newly cleared land initially proved excellent for cabbage tree colonisation, but once livestock was introduced, regeneration ceased. The problem was simple: the stock loved the tender shoots of the new seed­lings and the sprouts emerging un­derground from the rhizome.

So, over vast areas of farmland, all that are now left are the ancient trees dating from about the time the area was cleared.

The picture then, is of a plant population in some stress before the emergence of sudden decline.

Explains Simpson: “It’s a wetland plant, so that’s where it is at its best, but there are very few nice healthy wetlands left in New Zea­land. Most of the cabbage trees are out in the hills on farms, where they’re geriatric, or in ur­ban centres where they have to put up with other stresses such as soil and water problems and the urban population explosions of insect pests. Just have a look at the cabbage trees around the city—many are in terrible condi­tion because of insect attack.”

Gordon Hosking, forest ento­mologist with the Forest Re­search Institute, Rotorua, has travelled the country investigating the distribution of the cabbage tree as part of the over-all study. He con­firms Simpson’s view “that in many areas, the population dates back to the original breaking in of the land. As you go down through the Wairarapa, all the hills are dotted with widely spaced old cabbage trees. But because of the browsing animals, none are regenerating.

“So most of the young trees are along roadsides, railways or places where cattle can’t get at them, and in the forests.”

An early DSIR survey showed that sudden decline was confined to an area north of Taumarunui, and both Hosking and Simpson agree that the disease shows a distinct geographical pattern. Hosking says it’s hard to find evi­dence of it south of a line drawn across from Wairoa to Awakino. It also tends to be worse to the north and in the east.

Simpson takes the line further south, and remembers driving up from Wellington and reaching Norsewood on the edge of the  Hawkes Bay plains, “and whammo, it suddenly becomes a major feature of the landscape—most of the cabbage trees are dead.”

To both men, the localised nature of the problem suggests that some environmental or climatic factor is playing at least a part in sudden de­cline.

Recent mild winters could be hav­ing an effect, suggests Simpson, by allowing disease organisms to persist in the trees year-round. Other observ­ers have suggested that sudden de­cline may be connected with the ap­parent over-flowering of cabbage trees in recent years. Flowering is an ex­hausting process for the plant, which, instead of building up its reserves and strength, has to direct its ener­gies into flowering.

Simpson rejects the theory that cabbage trees are flowering them­selves to death. Rather, he suggests, it could be the natural response of a distressed tree to put all its resources into a mass flowering in a last desper­ate endeavour to reproduce before death.

After visiting more than 700 sites, Hosking, while concerned by evi­dence of sudden decline, also ex­presses alarm “that pastoral farming is really eliminating cabbage trees from a large part of the country. Year by year, the residual population of trees that may be 150 years old con­tinues to disappear. And on flat land the trees are still actively cut down—if you’re making hay you don’t want a tree in the middle of your pad­dock.”

Because of such concerns, DOC has taken the key sites Hosking’s sur­vey work has revealed and moved in to try and protect them. Many farm­ers, when made aware of the general attrition of the cabbage tree popula­tion, have agreed to help, often by fencing their best stands to allow natural regeneration. Wetland areas are increasingly being recognised as important repositories of biological diversity, and are being afforded high conservation priority.

Where areas are low in cabbage trees, DOC is encouraging the propa­gation and replanting of local off­spring—plants which, by virtue of adaptation, should have the best chance of survival in each particular location.

For the North Island’s cabbage trees, though, survival is a matter not just of replanting, but of halting the disease that is afflicting this special New Zealand tree.

At this stage, it is premature to predict what control measures might be successful, but the DSIR has iden­tified two broad approaches. First, to reduce the population of the insect that is spreading the disease by using biological control. Second, to iden­tify resistant forms of cabbage trees.

Unfortunately, there are few pros­pects for a quick fix, and in the mean time we are going to see a lot more cabbage trees dying.

A cabbage by any other name

When Captain james Cook came upon “a cabbage tree which we cut down for the sake of the cabbage” at Tolaga Bay in October 1769, he was referring to a nikau palm, not the ti or Cordyline australis which now bears this common name.

So was his companion, Sir Joseph Banks, when he wrote that the “cabbage” of the cabbage tree “. . . made us one delightful meal.”

The commonly held belief, still referred to by some histori­ans, that explorers and settlers tucked into meals of boiled ti leaves as a cabbage substitute is based on confusion surrounding the name “cabbage tree.”

Amateur botanist James Beever has delved back through the library shelves to uncover how the misnomer arose. It seems that during the 17th century the common name “cabbage tree” was used to describe several palms, first in the West Indies, then elsewhere.

The name referred to the tender, and usually very tasty, terminal bud of the palm which affords, in the words of one Jamaican explorer in 1756, “a tender green or cabbage.”

This was the usage adopted by Cook and Banks in their journals. On Cook’s second voyage, botanist J.R. Forster records in Dusky Sound that “we met with a great many Cabbage-palm trees as they were commonly called by our sailors, but on examination we found them to be a kind of Dragon tree.”

On arrival in New Zealand, sailors, and later whalers and settlers, christened the palm-like cordyline with their usual colloquial name for such trees.

Missionaries and explorers, on the other hand, used the Maori word ti. With the common manuka universally known as tea-tree, the opportunities for ambiguity were obvious.

For a couple of decades, scientists persevered with “cabbage tree” when describing the nikau palm, while in com­mon parlance cabbage tree became the name for ti. By 1863, it seems the latter usage had spread to polite society too, with author Samuel Butler, in a letter back to England, using “cabbage tree” and “ti palm” interchange­ably. A year later William Colenso did the same. The formal stamp of acceptance came in 1867 when eminent scientist Ferdinand Hochstetter published a book which included an engraving of a ti tree entitled “cabbage tree.”

By 1927, when the first popular book on New Zealand native plants was written by Laing and Blackwell, the origins of the common name had been forgotten. Instead, the authors say it comes from “early settlers who used the young and tender heads in place of cabbage.”

Beever says a few settlers might have done that, as a last resort, or because they were misled by the name itself. But in general, the young shoots were not as highly regarded by Euro­peans as the name would sug­gest.

With Maori, on the other hand, cooked ti crowns were and are relished as a green, particu­larly to accompany fatty meats such as pork and eel.

In his book Native Edible Plants of New Zealand, Andrew Crowe presents ti recipes pub­lished by Maori Women’s Wel­fare League in 1977, including the cooking of cabbage tree centres with puwha and corned beef or pork, the preparation of ti salad (“slice the ti core very finely and garnish with French dressing”) and the making of Te Whanake Sauce: “clean and slice finely one cabbage tree heart, boil with three cups of water and simmer till tender. Use sparingly either hot or cold with baked eels, canned beef or any meat.”

Bon appetit!

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The search for the killer

In Late 1991 a research team I led by Dr Ross Beever and Dr Richard Forster at DSIR Plant Protection, Auckland, made the long hoped for breakthrough in the search for the cabbage tree killer.

Their discovery of traces of the DNA of a minute disease-causing organism in samples from diseased trees brought to a climax a lengthy and at times heated public debate about the possible cause of sudden decline in cabbage trees.

Everything from the hole in the ozone layer to an excess of wasp urine had been put forward as an explanation, but the DSIR scientists and Department of Conservation project leader Dr Philip Simpson had believed all along that a disease organism would eventually be found.

Explains Dr Simpson: “When you see a perfectly healthy cabbage tree succumb through wilting, and do so in a matter of a few weeks, there’s something radically wrong. And when you open up that tree and see all the blocked vascular tissues it seems pretty clear that some toxic or physical effect is involved.

“Besides which,” he adds, “blocked conducting tissues are characteristic of many plant diseases.”

The problem facing the DSIR team was to identify the organism responsible—a challenge on a par with looking for an invisible needle in a very complicated haystack.

The culprit could have been a virus or viroid, mycoplasma or bacterium, fungus or protozoan, or even something more obscure. From the outset, though, DSIR scientists suspected a mycoplasma-like organism (MLO), as these microscopic entities, which fall somewhere between viruses and bacteria in the natural order, had already been identified in two related diseases: yellow leaf disease of New Zealand flax and lethal yellowing of coconut palms.

During the 1980s an MLO spread through Florida and coastal Mexico, wiping out large numbers of palms, particularly a cultivar known as Jamaican Tall which was valued both for its scenic beauty and its commercial impor­tance.

Lethal yellowing, as the disease came to be known, was found to be spread by tiny sap-sucking insects—leaf hoppers—which inject the MLO into the tree’s food-carrying “veins” in much the same way as a mosquito injects malarial parasites into a human victim.

Flax yellow leaf gives rise to similar symptoms to lethal yel­lowing in palms: abnormal yel­lowing of leaves and stunting of growth. In some cases the whole plant collapses.

In the 1940s and ’50s DSIR scientists established that this disease, which is endemic within New Zealand flax, was caused by a minute organism which was in turn transmitted by a small native leaf hopper, Oliarus atkinsoni. At that time they concluded that the organism was a virus, but following recognition of MLOs as disease-causing agents of plants in 1967, the culprit has proved to be an MLO.

Using advanced molecular biology techniques, DSIR scien­tists began their search for the cabbage tree culprit. After months of analysis of diseased cabbage tree tissue they discov­ered a DNA signature which indicated the presence of a mycoplasma-like organism.

Although the final proof—a photograph of MLOs inside cabbage tree tissues—still eludes them, the DSIR team are confi­dent that they have found the cause of sudden decline.

As yet, they do not know which MLO is involved. It could be identical to the flax organ­ism—tests so far indicate that both belong to the same general MLO grouping. But if they are the same, why has the cabbage tree disease only become appar­ent recently, while flax yellowing has been with us for hundreds, if not thousands, of years?

Dr Beever offers two sugges­tions: the cabbage tree MLO may have arisen by mutation from the flax MLO, or it may be a totally new immigrant which has evaded quarantine controls and slipped across our borders.

Irrespective of its origin, there still remains the question of how the organism is being spread. An insect is almost certainly the carrier, but the flax leaf hopper is an unlikely candidate, as it is not known north of Taupo and is largely confined to flax.

The prime suspect is the Australian passion vine hopper Scolypopa australis, a common nuisance in New Zealand gar­dens since its arrival a century ago, and an insect which regu­larly feeds on cabbage trees.

Proving that Scolypopa is the villain will not be easy. Unlike many plant disease agents, MLOs cannot yet be grown in artificial culture and inoculated into healthy plants. To carry out transmission experiments, plant pathologists will have to rely on the insects picking up the MLOs themselves—a much more hit-and-miss process.

Nevertheless, the DSIR has already begun a series of experi­ments in which newly-hatched MLO-free insects will be fed on diseased trees, checked to see that they have picked up MLOs, then put on to healthy trees to see whether those trees become infected.

Results are not expected for many months, possibly years.

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