Kauri Dieback:  An emerging biosecurity threat to New Zealand's Kauri forests?

by Dr Nick Waipara -  biosecurity scientist and plant pathologist at Auckland Council.

Environmental News Issue 25 Autumn 2011

Growers and gardeners have long known about the perils Phytophthora diseases on their crops, nurseries and ornamentals. Phytophthora infestans was unknowingly introduced to Europe and wrecked havoc on potato crops, causing the Irish potato famine and mass human migration of the 1800’s and thus also pioneering the study of plant diseases (plant pathology). Phytophthora’s are commonly known as “water moulds” and comprise some of the most destructive plant diseases known. Without careful attention and a variety of control methods used by both conventional and organic growers, these microscopic pathogens readily destroy most crop plants.

Unfortunately these destructive Phytophthora diseases have also been unwittingly introduced to many native forests throughout the world where they are not only killing millions of canopy trees but also impacting upon whole ecosystems that rely on the trees. From ‘Sudden Oak Death’ in North America, to declining cork forests in Portugal, new disease sites in the Scottish heathlands and catastrophic phytophthora diebacks in Australia, forest pathologists are in no doubt that many of the worlds forests are under attack in the same way Europe’s potato fields were. You can read more of such diseases at the websites listed in the reference section at the article’s end.

So what of the Phytophthora situation in New Zealand’s kauri forests?

A total of five Phytophthora species have been associated with kauri, but only two species have been demonstrated to kill trees, Phytophthora cinnamomi (PC) and Phytophthora taxon Agathis (PTA). Some of the world’s first research into Phytophthora diseases in native forests was conducted between the 1960’s and 80’s in the Waitakere Ranges by eminent Phytophthora experts Professor Frank Newhook, who alongside Dr Frank Podger and Dr Ian Horner, undertook studies(1,2) that showed P. cinnamomi was a problem for kauri in environmentally stressed sites. On sites with soil compaction, root damage, nutrient or water stress it can damage and sometimes kill young (50–150 year old) ricker kauri trees. Without these environmental triggers PC remains as a minor root nibbler that all trees with good root health can deal with. Subsequent studies have shown P. cinnamomi has been introduced to most native ecosystems across New Zealand but its overall impact is still poorly understood. It has been recorded infecting several hundred plant species in New Zealand and over 2000 worldwide.    

So what do we know about PTA?

In 1974 forest pathologist Dr Peter Gadgil first described what we now know as PTA in a small NZ forest service block on Great Barrier Island. His work showed this pathogen, which he called Phytophthora heveae, could kill kauri, but the mortality was thought to be an isolated incident caused by an unusual set of environmental triggers on the island(3). Roll on 30 years when an unusual and alarming number of kauri in the Waitakere Ranges, Awhitu and Rodney districts were found dead and dying. In 2006 the work of the late Dr Ross Beever at Manaaki Whenua Landcare Research, isolated PTA from these sites. Using advanced molecular DNA diagnostics his team was able to show that PTA was a valid species and new to science.(4) Further tests showed PTA was an aggressive primary pathogen which can infect and kill all life stages of kauri. Consequently not only were kauri seedlings and rickers at risk but also our large iconic and taonga trees(5,6). This new research led to PTA being declared an unwanted pest by MAF Biosecurity New Zealand in 2008 and a precautionary preventative long term management programme was initiated nationwide. Subsequent kauri dieback surveys between 2008-20011 identified PTA is not just along tracks or highly disturbed stressed sites but also on ridgelines and other areas away from such disturbances. Like all diseases, the worse the condition of the infected host, the quicker PTA will infect and kill, but PTA is a virulent killer of kauri and doesn’t necessarily need disturbance by people, pigs or tracks to do so.
 

PTA produces microscopic motile waterborne spores that can potentially swim like a tadpole through the soil water, eventually getting into streams and whole water catchments. The best and only protection for our kauri at the moment is to stop PTA spreading to healthy stands, particularly until the research programme can learn more about it and how to control it. Research to develop a treatment for this disease is presently underway by Dr Ian Horner at Plant & Food Research to trial conventional methods already used against related Phytophthora diseases in the agricultural sector.

Does PTA infect other species?

Luckily for us, PTA is not similar to the generalist P. cinnamomi, as the other native and crop trees in NZ appear to be resilient to PTA infection(6), including some of the related Araucariaceae tree species. So, while this research is still incomplete, testing to date shows PTA to be a highly specific pathogen of the Agathis tree genus which includes NZ’s kauri. A current theory is that PTA’s original host may be one of the other 21 Agathis tree species that grow throughout the Pacific and South East Asia and it has been introduced to NZ via trade or on a dirty boot. But a lot more research is required to determine the origin of PTA.

Where is Kauri Dieback on Great Barrier Island?

The original 1970’s ‘Gadgil site’ at Whangaparapara (above) has long been known on the island. Subsequent testing and survey work has confirmed kauri dieback at two other sites at Kaiaraara (Port FitzRoy), and Okiwi. However, islanders have also reported symptomatic trees at other locations on the island and further testing and surveys will be required to delimit all locations. One of the best methods used for kauri dieback across the Hunua and Waitakere Ranges has been aerial surveillance. This method has now been recommended to complete the survey work on the island as it allows systematic and widespread coverage of remote less accessible areas. Aerial methods look for the classic dieback symptoms in the kauri canopy which are thinning sparse canopies, dead and dying branches (stagheads) and dead trees. Ground-based parties are then sent in to assess symptoms on the trunk (freshly bleeding lesions at the base of the trunk) and collect samples for laboratory testing to confirm PTA presence.
Management of kauri dieback – how can you help stop the spread?

Biosecurity for our native environment is an important and growing activity. Basic hygiene and management practices to control, contain and stop introduced pests and diseases ruining economic land and crops are crucial to New Zealand’s export economy and trade. But we must also ensure protection for our native forests and wetlands from an ever increasing number of current and future pests that can do similar damage to our native plants and animals.

Biosecurity measures underway in the South Island with the “check, clean, dry” campaign to contain spread of didymo are one example where education with simple actions can protect our precious environment. Cleaning stations and kits installed at park entrances are there to assist visitors undertake the same basic biosecurity for our kauri forests. One of the ways PTA spreads is via its soil-borne spores. These microscopic spores have a resistant cell wall, similar to a plant’s seed-coat, which allow it to survive periods without its kauri host or water. The spores can therefore lay dormant but alive for month’s or even years. Someone walking in kauri dieback zones can therefore unintentionally move these spores with the soil adhering to their footwear. Any movement of infested soil into kauri root zones will enable PTA to germinate and find new healthy trees to infect and kill. By promoting the “clean shoes in and out” message to all visitors at kauri forests we are aiming to reduce kauri dieback along the tracks already affected but also to protect other areas of healthy kauri forest. We know many visitors could be in the Auckland forests one day and in the Coromandel or Northland in the following day, weeks or months, this is how this terrible disease is continuing to spread between our kauri forests locally, regionally and nationally.

The trigene disinfectant used in the kits is 100% biodegradable, safe for human use as directed but also helps inactivate the soilborne spores of PTA. However, you don’t need a kit to help stop PTA, the use of soapy water and ‘elbow grease’ to scrub your shoes, mountain bike tyres and other dirty equipment free of soil will not only eliminate PTA spores, but other potential nasty pests, such as weed seeds, Didymo, Chytrid disease of frogs and other common fungal diseases of plants. The cleanliness message is long overdue for our native forests and has wider relevance than just the Phytophthora kauri dieback issue.

Kauri is a magnificent iconic tree that many New Zealanders love to see, but as visitors we now also need to be vigilant and start practising the simple biosecurity measures to protect it against this proven and deadly disease. For more information on PTA and kauri dieback and what you can do to help refer to the website: www.kauridieback.co.nz

References
1 Podger FD, Newhook FJ 1971. Phytophthora cinnamomi in indigenous plant communities in New Zealand. New Zealand Journal of Botany 9: 625-638.

2 Horner IJ 1984. The role of Phytophthora cinnamomi and other fungal pathogens in the establishment of kauri and kahikatea. MSc thesis, University of Auckland.
3 Gadgil, P.D. (1974). Phytophthora heveae, a pathogen of kauri. New Zealand Journal of Forestry Science 4: 59-63.
4 Ramsfield TD, Dick MA, Beever RE, Horner IJ, McAlonan MJ, Hill CF 2009. Phytophthora kernoviae in New Zealand. Pp 47-53 in: Goheen, E.M.; Frankel, S.J., tech. coords. Proceedings of the fourth meeting of the International Union of Forest Research Organizations (IUFRO) Working Party S07.02.09: Phytophthoras in forests and natural ecosystems. Gen. Tech. Rep. PSW-GTR-221. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station.
5 Beever RE, Waipara NW, Ramsfield TD, Dick MA, Horner IJ 2009. Kauri (Agathis australis) under threat from Phytophthora? Pp 74-85 in: Goheen, E.M.; Frankel, S.J., tech. coords. Proceedings of the fourth meeting of the International Union of Forest Research Organizations (IUFRO) Working Party S07.02.09: Phytophthoras in forests and natural ecosystems. Gen. Tech. Rep. PSW-GTR-221. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station.
6 Beever RE, Tsai S, Waipara NW, Dick MA, Ramsfield TD 2010. Pathogenicity of Phytophthora taxon Agathis (PTA). 5th IUFRO Phytophthoras in Forests and Natural Ecosystems Auckland and Rotorua, New Zealand, 7-12 March 2010.

International Websites;
http://www.suddenoakdeath.org/
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7498.html
http://www.dwg.org.au/index.cfm
http://www.dieback.net.au/
http://www.phyto2010.com/index.html