How does it spread?
Microscopic spores in the soil infect kauri roots and damage the
tissues that carry nutrients within the tree.
Kauri dieback life cycle
Kauri dieback spores are produced in the following
1. Oospores (resting spores) are introduced into an area
2. Oospores germinate to form sporangia (a structure
which produces zoospores)
3. These zoospores are released during and immediately
after heavy rain
4. Zoospores (mobile spores) swim through soil-water to
kauri roots, attach to the root surface, germinate to produce
mycelia which infects the kauri root
5. The organism grows through the root system to
affect tissues at the base of the trunk (damaging tissues that
transport nutrients and water to canopy)
6. More sporangia are formed from areas of infected
root…..which release more zoospores during/after heavy rain
7. Oospores form within infected tree tissue and are
release into the soil as tissue decays
The oospores are like the 'seeds' of this disease, with a
hard outer shell they can sit dormant in soil for up to three years
or more. These spores live in soil and are spread with soil
movement. Dirty footwear, animals, equipment and vehicles are
responsible for the large scale spread of this disease - between
different areas of kauri.
The introduction of spores to an area of kauri can lead to a new
area of infection. We don't yet know what "inoculum load"/number of
spores are required for an infection to occur - however, as the
spores can reproduce/multiply once introduced to an area, a minute
amount of soil with a tiny amount of spores can result in a new
area of disease.
In 2011, Simon Randall, Auckland University completed his
Master's thesis on
stream based water detection of Phytophthora species.
Simon assessed if current stream based detection methods
could be used to detect kauri dieback within a catchment.
Interestingly, PTA was not detected in any water catchments in the
Waitakere Ranges (where areas of high infection were known). This
does not mean that kauri dieback doesn't spread via water
catchments (we know this species produces water borne spores in the lab) just that we don't
have the right tools to detect it yet. Simon did detect a number of
other phytophthora species in the streams - which proves the
'spread pathway" (ie. high likelihood that kauri dieback also
spreads via streams) it seems that kauri dieback is just a little
trickier to pin down.
Dr Cheryl Krull (University of Auckland) successfully completed
her PhD "Feral Pigs In A Temperate Rainforest Ecosystem;
Ecological Impacts and Management" in 2012. Part of Cheryl's
work looked to determine if feral pigs were
spreading PTA spores in kauri forests. This study detected 19
species of plant pathogens in the soil on pig trotters and snouts,
including a different Phytophthora species
(Phytophthora cinnamomi). However, no PTA was isolated
from the samples. This is thought to be due to the detection
methods used at the time (methods have since been improved). Cheryl
concluded that it is highly likely that kauri dieback is also
spread by feral pigs.
Work is now underway by the programme to assess what risks
livestock/cattle present to spreading the disease across rural
kauri stands. We expect results from this study to be
finished later in 2013.
A study is underway by the Kauri Dieback Management Team
Planning & Intellingence team (Dr Tony Beauchamp, Will Ngakuru
and John Beachman) investigating the historic pathways and spread
of kauri dieback in New Zealand. This study aims to determine if
kauri nursery, planting and silviculture that was undertaken form
1930s until the 1980s contributed to the spread of the
disease. To date several kauri plantations including those on
Great Barrier and Northland have been confirmed as being PTA
positive. This work involves interviewing scientists,
foresters, nurserymen and kaumatua who were involved in this work
to gain their knowledge of activities and projects which may have
unwittingly also introduced kauri dieback to these planted stands.
So far, this work indicates kauri dieback has potentially been in
NZ since the 1950s.
Research Associate Monique Wheat is currently researching
where PTA exists in an infected tree (ie. how far up the trunk
and into the wood). This work will help answer if we can safely
harvest and recycle the wood of dead trees and how we can safely
dispose of dead and dying tree. Monique is taking
wood/bark/tree samples from infected trees at various affected
kauri stands and is being helped from a number of dedicated
volunteer arborists and tree climbers. Results from this work
should be completed by the end of the year.