Writer Germaine Greer argues that unpopular measures will need to be taken to save kauri from the deadly pathogen PTA.
Germaine Greer, photo Rex Features
The initials PTA should strike dread into the hearts of all New Zealanders. They still stand for Parent Teacher Association, but everyone should be aware that since 2008 they also stand for Phytophthora taxon Agathis, a pathogen dedicated to annihilating the iconic North Island tree: the New Zealand kauri.
So far no common name for the disease has stuck; some call it kauri dieback, others kauri collar rot. Even PTA is a provisional name for an organism that is still to be ratified as a separate species of Phytophthora, the plant destroyer.
Every kauri infected with PTA will die, some very swiftly, others very slowly. So far, PTA has affected no other plant species; when it comes to kauri it is 100% lethal. That in itself is an odd situation, for when the last kauri dies, PTA will die with it – unless it manages to mutate and diversify, which phytophthoras seem to have a propensity to doing.
Before the discovery of a phytophthora that exists to obliterate kauri, the story of the deforestation of New Zealand was already a terrible one. Before the arrival of Europeans, forest cover had been halved, and the Europeans destroyed half the remaining 50% within 50 years. In 2000, all logging of native forest in public ownership was banned, but not before a new kind of devastation had been set in train.
In 2006, people living near Piha on the Maungaroa Ridge Track reported that regenerating kauri were showing symptoms of disease: their foliage was yellow, their crowns thin, and some branches and some entire trees seemed to be dead. In 2008, when scientists were convinced the cause was a phytophthora that had not previously been described, it was given the provisional name of Phytophthora taxon Agathis, PTA.
It has since been found in the Waitakere Ranges, where it is being studied at Cascade Kauri, Karekare, Anawhata and Huia. It has also been found on Great Barrier Island and in soil in Northland’s Trounson Kauri Park and Waipoua Forest, and in and around Auckland, in Franklin, Pakiri, Albany and Okura. Auckland Council is funding preliminary research and is doing its best to restrict access to infected sites. Disinfecting footwear is an option, and the evidence is that New Zealanders are being scrupulous about it, but it has not been shown to have any impact. And keeping feral pigs away is impossible.
The New Zealand kauri (Agathis australis) is an ancient Gondwanan survivor that has been on Earth since before the evolution of flowering plants; its ancestors have been found in Jurassic fossils of 190 to 135 million years BCE. A full-grown kauri is an unforgettable sight, a natural Tower of Pisa. I can still remember my first glimpse of Tane Mahuta, which seemed too big to be a tree, its huge trunk closing off the approach path like a wall. Though I peered upwards till my neck ached, I could only barely see the aerial gardens among the canopy branches 50m above me.
Everything about the kauri is beautiful – its dark green phyllodes, the logarithmic spiral of its cones – but most of all I love its trunk. The steel-grey bark is glossy and hard, and patterned like hammered metal. The flakes of bark it sheds end up piled at the base where the highly acid leaf litter contributes to the generation of nutrients that feed the fine root system that in turn nourishes the tree, which is buttressed by peg roots that pin the mat of fine roots to the soil.
Waitakere Ranges sign, photo Glenn Jeffrey/NZH
Most of the work going on all over the world is seeking to identify Phytophthora species and ways of attacking them. However, the impression that phytophthoras are exotic aliens that invade virgin ecosystems is probably wrong and certainly misleading. Oomycetes – which include the Phytophthora group – are found in all kinds of soils and in water.
Because the symptoms of dieback were found to be spreading from areas disrupted by logging and road building, disturbance itself was thought to be the cause. But instead, what was happening was that people and machines were spreading the infection. In the case of Western Australia, it is possible that in 1927 when soil containing the necessary beneficial fungi was imported to stimulate growth in plantations of Pinus radiata (monterey pine), Phytophthora cinnamomi came with it.
The role of Pinus radiata may turn out to be central to the emergence of New Zealand’s kauri dieback problem as well. In 1970, a student thesis submitted for a master of science degree described an unidentified phytophthora found under Pinus radiata growing on pumice near Tokoroa. This is now thought to be Phytophthora kernoviae, first described and named in Cornwall in 2003.
In March 2006, Biosecurity New Zealand announced the finding of a new phytophthora in an abandoned cherimoya orchard in Northland. The infecting agent turned out to be P kernoviae. The same organism was also found in soil in the Trounson Kauri Park. It has subsequently been found in Auckland, the Bay of Plenty and Taupo.
Current thinking is that it must have been around for at least 50 to 60 years, judging by a slight divergence from the European genotype. It is beginning to look very much as if we have always lived with oomycetes in soil and water. What we have to find out is when and how they turn into agents of destruction.
In 1972, Peter Gadgil, a pathologist working for the Forest Research Institute (now renamed Scion), found a phytophthora in dying kauri saplings on Great Barrier Island that was identified by the Commonwealth Mycological Institute as P heveae. It was also found in areas where there were no signs of disease, so the assumption was made that it presented no significant threat to the regeneration of kauri. Gadgil recently argued P heveae and PTA are the same thing, and it is only pathogenic under certain conditions.
However, the clue here lies in the word regeneration. Like most other rainforest trees, kauri depend on fungal associations for their nutriment. If the trees are felled and/or burnt, the associated beneficial fungi are destroyed. Kauri seeds in the soil store might germinate, but in the absence of these fungi they struggle to synthesise the nutrients that are essential if they are to thrive.
Oomycetes in healthy soils have to contend with other micro-organisms that predate them, but in poor soils, affected by leaching, for example, they can become dominant. Great Barrier Island was deforested in 1930-31; the plagues affecting regenerating kauri, which on Great Barrier Island include several different phytophthoras, are themselves the consequence of the wanton destruction of the forest. Tangata Whenua Roopu, a Maori group that provides advice to the Kauri Dieback Management Team, has already suggested that the way forward is to study whole forest ecosystems to arrive at an explanation of the imbalance that is menacing the survival of the kauri.
Waipoua Forest, where PTA has been detected, and the adjoining forests of Mataraua and Waima contain the largest remaining stand of kauri in the world. If kauri are to survive, the area of rainforest in conservation must be extended. That extension should proceed outwards from nuclei of healthy forest, from where the fungal agents present in intact soil/plant associations can spread to inoculate the seedlings and saplings.
Though there are chemicals that can kill oomycetes, they also disrupt the essential fungal associations needed for a healthy forest and are better not used. The only solution to kauri dieback is restoration of the forest with all its species. For that to happen, unpopular decisions will have to be made; the Hunua Ranges, which are apparently untouched by PTA, should probably be closed to the public. Relying on people to clean their boots and gear before and after visiting will not be enough.
Germaine Greer will be a guest at Writers & Readers Week, March 9-14, during the NZ International Arts Festival, Wellington.
The plant destroyer
Phytophthora infestations around the world cause billions of dollars of damage each year.
Kauri dieback in the Waitakere Ranges, photo ARC
The name Phytophthora was coined by German surgeon Heinrich Anton de Bary in 1875 for the organism that had caused the collapse of potato crops in mainland Europe, Ireland and Scotland. He was the first to realise that the lethal pathogen wasn’t one of the opportunistic fungal infections that showed up on the diseased tubers but a different kind of organism altogether, an oomycete or water mould.
An oomycete is not itself a fungus because its cells have walls of cellulose, whereas fungi have cell walls of chitin. But for convenience’s sake, most people tend to refer to the various phytophthoras as fungi. De Bary’s plant destroyer is more closely related to other water-borne organisms like brown algae and diatoms. Other oomycetes, known to us mainly through the horticulture industry, are Pythium, downy and powdery mildews, and white blister rusts.
De Bary called the species he identified Phytophthora infestans; it is actually much less dangerous than many other phytophthoras because it has only a couple of host species. Even so, P infestans is still responsible for about $6 billion of damage to potato and tomato crops every year. Potato farmers have had to increase their chemical inputs by as much as 30%, and still the mould survives in the soil.
Other phytophthoras are turning out to be capable of infesting thousands of hosts. By 2000, 55 species had been discovered; in the 12 years since then, the number of known species has doubled. Never before had botanists witnessed the destruction of whole ecosystems by a single pathogen; now similar pathogens are appearing in 70 countries so far.
It has been suggested that phytophthoras probably arrived in New Zealand with human settlers about 600 years ago, but what should not be forgotten is that there are oomycetes in sea water, and prehistoric floods in New Zealand almost certainly deposited phytophthora-like organisms on young soils. In 1971, Phytophthora cinnamomi was discovered in stands of regenerating kauri on Great Barrier Island; since then the infected area has increased five- or tenfold. One clue to why it’s there is the systematic deforestation of the island. In the wild, pathogenic Phytophthora infestation is always associated with disturbance, particularly logging. Proteaceous species are particularly susceptible to Phytophthora cinnamomi; in New Zealand rewarewa (Knightia excelsa) is the worst affected.
Phytophthora cinnamomi, commonly known as cinnamon fungus because it was first identified in cinnamon trees in Western Sumatra, kills plants by damaging two important hormones in their root system – cytokinins and abscisic acid – leaving the roots no longer able to transport water to the rest of the plant. This damage is done within two days of infection, long before any signs of disease become apparent.
Swimming spores carried by water encyst on the root, sending out a germ tube towards the centre of the root, usually in the spring. As the weather grows colder, the spores become resistant and overwinter in the root, ready to send out swimming spores again the next spring from spherical sporangia on the outside of the root. The same capacity for resistance enables the oomycete to survive drought. The highly mineralised and poor soils of most of Australia are ideal hosts for an organism that in damper, more humus-rich environments is susceptible to attack by other soil micro-organisms.
In 1965, when the cause of dieback in the jarrah forests of southwestern Western Australia was identified as Phytophthora cinnamomi, it must have been clear that the infection was killing the under-storey plants as well as the trees, and that its progress through the entire ecosystem was marked by a clear boundary between infested and clean ground. But the focus was on the cash crop, the timber trees.
The possible destruction of an entire ecosystem by a single pathogen was not contemplated. Eventually it became obvious that dieback was affecting the understorey plants as well, and a further study of affected areas has revealed an associated collapse in fauna populations. Leading Australian plant pathologist Gretna Weste spent most of the 1990s mapping the depredations of cinnamon fungus in Australia and produced a 250-page report listing the thousands of species that were infected, but it was never made public. She had made the fundamental error of producing a diagnosis of a disease for which there was neither treatment nor a cure. Its inexorable progress since then has forced a recognition of the seriousness of the threat to global biodiversity. – Germaine Greer
WE HAVE RECEIVED THE FOLLOWING LETTER IN RESPONSE TO THIS STORY
Germaine Greer’s plea for the rescue of kauri from the pathogen currently known as Phytophthora “taxon Agathis” (PTA) would have more credibility if its emotive appeal was backed by a balanced account of current knowledge about this disease. The article says, “In 2008, when scientists were convinced the cause was a phytophthora that had not previously been described, it was given the provisional name of Phytophthorataxon Agathis, PTA.” This gives the impression that a new pathogen of kauri was found recently. In reality, Landcare Research scientists have shown that PTA found in the Waitakere Ranges in 2006 is in all morphological and molecular characteristics the same organism as the kauri pathogen found on Great Barrier Island in 1972 by Forest Research Institute (now Scion) scientists and identified as Phytophthora heveae (Beever and others, 2009: pp.74-85 in General Technical Report PSW-GTR-221, USDA Forest Service). Both names refer to a single organism. A full description of the Great Barrier isolate with illustrations and proof of pathogenicity was published in 1974 (Gadgil, 1974: New Zealand Journal of Forestry Science 4: 59-63).
The fact the kauri pathogen has been present in New Zealand for over 40 years requires a more sober assessment of its potential for causing damage. There is no indication from Great Barrier Island that this organism is “annihilating the iconic North Island tree”. The recent spate of discoveries in kauri forests does not necessarily mean the pathogen is spreading rapidly across Northland. These new records may simply reflect a lack of data from the time when no methodical surveys were conducted in that area. Nevertheless, given the national significance of kauri, it is important to continue to monitor the situation to see if it is worsening. If so, practical field solutions need to be developed.
Why has the disease become more obvious in recent years? Disease expression is always the outcome of interaction between the host, the pathogen and the environment (the disease triangle). Disturbances such as logging activity, road-building and pig-rooting may have made trees affected by the disturbance more vulnerable to an organism that was already present but not acting as a pathogen.
At present, we do not have good information about the distribution of PTA. This can only be gained by an intensive survey of all kauri forests, whether diseased or not. If the organism is already widespread, disinfection of footwear and locking up the Hunua Ranges will serve no useful purpose. There is also an urgent need for identification of the conditions that favour the disease-causing organism.
Greer implies that Pinus radiata might somehow be involved in kauri dieback. This is a mischievous suggestion, unsupported by any known facts.Phytophthora kernoviae is not known to cause disease in kauri. There are other inaccuracies in the article. For example, PTA is not an obligate parasite – like all other Phytophthora species it is a saprophyte and can survive without its host. PTA will not “die” when the last kauri dies. Also, botanists will know that kauri has true leaves, not “phyllodes”.
Unfortunately, Greer has not “researched the research” properly. Her article is an example of persuasive science writing that has potential to mislead the public. Her interest in the threat to kauri is welcome, but it is important any advice is based on science evidence.
Brian Richardson PhD
General manager, forest science, Scion