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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 show ed
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.
Pakiri Scenic
reserve PTA infestation
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.
Oospore (soil-borne spores) of kauri
dieback – Phytophthora “taxon” Agathis (PTA).
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.
Kauri dieback symptoms - 'stagheads'
and dead trees.
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
Phytosanitary (barrel and grate) station.
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 |
