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

life cycle

Kauri dieback spores are produced in the following steps: 

1.  Oospores (resting spores) are introduced into an area of kauri
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 zoosporespores 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.