Family lines of kauri seedlings germinated at Scion’s research nursery in preparation for screening with Phytophthora agathidicida.

Kauri (Agathis australis) are the slow growing giants of our northern forests revered by all New Zealanders, particularly Maori.

In recent years, these iconic trees have been struck down by kauri dieback, a prolific wasting disease caused by the soil-borne fungus-like organism Phytophthora  agathidicida, which grows within its host’s roots and slowly chokes it to death. To date, there are no effective methods of managing kauri dieback and given the slow regeneration time of kauri, this insidious pathogen is causing major concern.

P. agathidicida is a newly described species of Phytophthora, and at this stage it is unclear whether our trees have any inherent resistance to the pathogen. Finding this out is a key focus of Scion’s ‘Healthy trees, healthy future’ (HTHF) research programme, which aims to identify markers and establish screening methods to assess the breadth of disease resistance. In doing so, we hope to understand more about the mechanisms of disease resistance.

The science behind the scenes

Key to our research, says Research Leader Dr Nari Williams, is the question of what resistance will look like, and whether it will last for the lifespan of the tree. Is it genetic or biochemical, or a physiological response in the tree that slows the rate of growth of the pathogen? Does the host produce inhibitors that delay or interrupt the pathogen’s reproduction and limit its spread?

To date, laboratory trials show P. agathidicida to be an extremely efficient pathogen that can kill non-resistant two year old kauri seedlings a mere 20 days after inoculation. Scion’s HTHF pathologists have developed a technology that enables them to view the pathogen growing within the host’s roots; it reproduces rapidly and forms specialised structures that help it to survive even when the host is dying.

Our chemists are focusing on understanding the chemical mechanisms of resistance by analysing the biochemical reaction that occurs at, and following, the time of infection. These data will provide us with a characteristic ‘chemical fingerprint’ of a healthy plant, and of one that is being challenged by a pathogen. Further analyses will then be able to identify the actual chemical changes that occur at the time of infection. This could prove to be a unique diagnostic test for the disease prior to a tree showing signs of disease and give indications of the mechanisms involved in pathogen defence. Otherwise it can be years before the tree shows any outward signs of the disease.

Screening for success

Preventing the spread of P. agathidicida and finding resistant strains of kauri from which to breed are a major focus for Scion. The first phase of the HTHF programme involved the research team using robust science to establish screening protocols for a range of Phytophthora species. This is now complete and the team is working with regional mana whenua groups to start screening kauri sourced from within their rohe. This year, the team has successfully raised 3000 kauri seedlings in a specially contained area of Scion’s research nursery. The seed was collected from five mana whenua in Northland where there is a high rate of kauri dieback.

“We’ll do another seed collection this year to extend the collection and geographic range of seed, or provenance. This will also include further mana whenua groups in the screening programme,” says Nari.

“The key to this whakapapa line work is how resistance varies between siblings. For example, the seed we collect from a single mother tree has been pollinated by a pollen donor somewhere upwind. If one seedling proves to be resistant, there is either a resistant pollen donor in the area, or the mother is resistant. This is where we need to understand the structure of resistance – is it one gene, or a group of genes, and what does the pathogen do when it infects a seedling?

“Early stages of screening have already shown different responses to Phytophthora infection across different kauri lines. While promising, there is still a great deal we don’t know about how our various screening assays relate to disease susceptibility as trees age and across environments, or provenances.”

Nari and the team have begun a screening assay in collaboration with Landcare Research that involves inoculating the current lines of seedlings with P. agathidicida in controlled conditions, then monitoring them for signs of infection using established genetic and biochemical screening protocols. The surviving seedlings will be inoculated in a further 6, 12 or 18 months’ time.

Pending the results, surviving seedlings will be planted in areas where the disease is present in order to challenge them under natural conditions. Each seedling will be carefully monitored for disease development over time.

Where to from there?

Once Nari and the team have identified a line of kauri that is resistant to P. agathidicida the future will look a lot healthier for our majestic giants of the north. The next step will be to launch a dedicated genetic breeding programme. That, says Nari, is a few years away yet.

Help stop the spread of Phytophthora in our forests

Plan and prepare:

• Obey all signage in natural areas and stay out of areas with kauri dieback
• Check for suitable walking or cycle tracks, and keep to the tracks
• Avoid walking or cycling in forests if soil is wet or muddy
• Ensure footwear, bike and all other equipment are free of soil or mud before entering the forest
• Make sure your vehicle is clean before entering the forest area
• Park your vehicle in designated car park
• Dispose of waste properly
• When you leave the forest, avoid transporting soil and mud on your shoes, bike or camping equipment by using the designated wash stations, or your own hygiene kit. Scrape off any soil, wash boots or bike tyres with fresh water, and spray with diluted methylated spirits to sterilise the soles of shoes.

For further information visit
www.healthytrees.co.nz