Kauri Dieback Basics

What causes kauri dieback?

  • A microscopic water mould Phytophthora agathidicida which has two spores. 
    1. Oospores: can survive for years in dried soil before germinating on contact with water. Upon contact with water they become zoospores.
    2. Zoospores: can move within water film of soil to find kauri roots. They then germinate and infect kauri roots.
  • It kills kauri, regardless of the health status and age of the trees

Is climate change the cause of or contributing to kauri dieback?

  • If climate change was the cause of kauri dieback we would see dieback everywhere, but we don’t. The disease is present in some forests and absent in others. The disease would also be evenly spread through forests, which it is not.  
  • Climate change may be stressing our forests due to changes in rain patterns but this is not the “cause” of the disease. 
  • Unfavourable climate conditions can add to the effect of tree diseases, killing trees that might have otherwise survived.

Do kauri die for other reasons too?

  • They don’t like their roots being wounded or compacted with driveways and buildings placed on them. 
  • They are affected by other diseases. 
  • Like climate change, anything that compromises the health of a tree, or damages the environment in which it is growing, will make it more susceptible to disease.

How is it spread?

  • Mainly by spores in soil and water being moved by people (and equipment) and large animals like pigs and dogs. 
  • Water ways.
  • Without help the pathogen will only move within the soil up to 3 metres a year.

Why are the rahui, hygiene and track closures and upgrades so important?

  • We human beings are the only way kauri dieback can be moved from an infected forest or place to another forest or place that has not been infected yet.
  • Within a forest or park people are the main way the disease can be spread from an infected area to a healthy area.
  • If a track has infected kauri on it, then there is a risk the whole track is contaminated with Phytophthora, spread by walkers, so it either must be closed or upgraded.

What is the evidence that people are the main spreaders of dieback?

The Waitakere Ranges Monitoring Report showed 71% of the kauri dieback infected trees and groups of trees are within 50 metres of a track. Please refer to Studies and Maps at the end if you

  • want to know how the study was done 
  • have seen material showing Council Geomaps that dispute people are not the main spreaders

The Waitakere Ranges is also the worst affected forest no doubt owing to its many human visitors.

Why is there kauri dieback away from tracks?

  • Pigs, dogs, hunters 
  • People walking off track or on informal tracks and bait lines. 
  • Streams and smaller waterways 

Why do many street kauri not suffer from dieback when there are people walking past them?

  • The road or footpath is usually a solid surface and people are walking in clean street shoes, not muddy boots, and they are not in contact with the tree roots.  
  • Many urban and rural landowners with kauri are working hard to ensure any visitors, including contractors, are not arriving on their properties with dirty footwear, equipment and vehicles.   

What about birds, rats and possums?

Small animals move only tiny amounts of soil and are considered to be a very low risk.

Won’t kauri recover like the cabbage tree did?

There is no similarity between cabbage tree decline and kauri dieback other than both are caused by unrelated plant pathogens.  

  • The disease that killed thousands of cabbage trees during the outbreaks of the 1980s-1990s was caused by a bacterium spread by sap-sucking insects such as the passionvine hopper. 
  • Kauri dieback is caused by a water mould called Phytophthora which literally means “plant destroyer”.  

Are there similar diseases in other countries?

Phytophthoras cause many plant diseases across the globe including the devastating potato blight in Ireland (Phytophthora infestans), sudden oak death in California (Phytophthora ramorum) and Jarrah dieback in Western Australia (Phytophthora cinnamomi).

Is there treatment for infected kauri?

Injecting ailing kauri with phosphite slows the disease in the treated trees. Similarly phosphite is used to successfully treat avocado trees suffering from another kind of Phytophthora infection. Phosphite is widely used as a fungicide and is not to be confused with phosphate which is a fertiliser.

Phosphite is not a cure for kauri dieback, but it does enable us to keep sick trees alive while we hope the research will provide other solutions in the future. 

How can I get help to protect trees on my property?

The Kauri Rescue project is working with landowners on private land to treat their own trees and monitor how the trees respond to the treatment. You can contact the Kauri Rescue team at www.kaurirescue.org

Is there hope for kauri?

Kauri dieback is not everywhere yet. There are many uninfected healthy kauri trees and forests, and these are the hope for future generations but it is critical everyone helps to protect these kauri from kauri dieback.  

There is also some promising research.

Science and Research

Are some kauri resistant to dieback?

  • There is positive research being done growing kauri in greenhouses that indicates that some kauri seedlings may be more tolerant than others, but this research is still ongoing with no resistant kauri being confirmed to date. 
  • More good news is that in infected kauri zones there are apparently healthy kauri still growing next to sick kauri - maybe these kauri are more resistant or maybe there is a different mix of beneficial plants or microorganisms in the soil around them which are protecting them against kauri dieback – more research is required.

Why are scientists still worried about apparently healthy kauri?  

Scientists have no idea of the lag time between the infection of the roots and appearance of disease in an infected tree. So while trees may look healthy for years they may already be infected. 

Kauri have very large root zones, more than 3x the dripline of the tree, and so within a grove of kauri all trees are likely to be in root contact with each other.  So once some trees are infected, then the other trees are very likely to eventually become infected also.

So only time will tell if the apparently healthy kauri stay healthy or eventually succumb.

Why are kauri especially vulnerable to diseases?

  • Kauri have enormous life spans compared to pathogens like Phytophthora which may mutate or evolve over the years and then kill the currently more resistant trees. 
  • The vast majority of kauri were logged (over 96% of original kauri stands were clear felled) and there may be little natural genetic variability in the remaining fragments of kauri making them even more vulnerable to attack in the long term.

Can we re-plant our kauri forests in future?

  • Even if scientists develop very resistant kauri seeds to replant our forests, if most kauri die it will be a long time before our forests look like they do now.
  • Kauri are most vulnerable at the tiny seedling stage so even resistant seeds may not establish in contaminated soil.  Therefore it is very important to stop the disease spreading to new areas.
  • Young kauri are more vulnerable to drought so climate change may hinder regeneration.

What other research is being done?

Recently released research has shown that leaf extracts from kānuka disable the zoospores of Phytophthora agathidicida and prevent them from germinating and infecting kauri roots in the lab. This is very encouraging but is at an early stage.

What about Climate Change?

Extreme drought is thought to be the biggest threat to kauri but research at the University of Auckland has found that kauri are efficient water users. 

  • Kauri save water by closing their leaf pores (stomata) early in the day. 
  • Kauri have deep roots so they can access deep soil water during dry conditions. 
  • Dry soil can stimulate root growth in kauri so they can access as much soil moisture as possible. 
  • Kauri store large amounts of water in their sap wood and this acts as a buffer against drought.
  • Kauri will lose leaves during dry periods to save water. 

No evidence has been found of kauri dying of drought alone.

Studies and Maps

How was the Waitakere Ranges Monitoring Report done?

  • An aerial survey in 2011 systematically flew over the entire Waitakere Ranges to identify suspected diseased trees from the air. This was repeated in 2016. 
  • A ground team then went in to establish whether the trees were showing kauri dieback symptoms and to take soil samples.  
  • These samples were analysed in Plant and Food Research’s diagnostic Phytophthora laboratory to confirm the presence of kauri dieback.
  • The field teams also investigated and sampled symptomatic trees off and away from tracks.
  • Note that the survey was not conducted by only walking along the tracks and looking for sick trees as some have suggested. 

Why was the Waitakere Ranges Monitoring Report done?

The purpose was to establish the extent of the disease and how quickly it is spreading, hence the systematic method to identify exactly where the sick trees were and not just do random plots.

What about the Council GeoMaps?

There has been material circulating showing Auckland Council Geomaps that seem to show that kauri dieback is more prevalent away from tracks. There are two things very wrong with this material:

  1. It uses the Geomaps “kauri dieback” dataset which is limited to kauri that have been assessed either by surveying 10 metres either side of the track or via aerial survey focusing on sick looking kauri. Hence it appears that most kauri in the Waitakere Ranges are adjacent to tracks and that a high percentage of the few kauri shown away from the tracks are suffering from dieback.  If you want to see how very widespread kauri are in the Waitakeres you need to look at the Geomaps “ecosystem” dataset.
  2. It analyses groves of kauri in which there are both sick looking trees and healthy trees. In this case the Phytophthora is all over the grove but some trees are holding off the disease for reasons speculated on earlier. Therefore it is invalid to try to point out that there are healthy trees by the track and sick trees away from the track.

 

November 2019

Prepared by:

Christine Major MB ChB - tramper of 27 years (Auckland TC, West Auckland TC) and amateur botanist/science enthusiast who has been following kauri dieback for 10 years

With assistance and contributions from:

Joanne Aley MSc (Social Science Advisor, DOC)

Dr Cate Macinnis-Ng (Assoc. Professor, University of Auckland)

Dr Mels Barton (Secretary, The Tree Council)

Dr Nick Waipara (Senior Scientist, Plant and Food Research)

Hugo Geddes (Senior Bio Information Analyst [Geomaps], Auckland Council)