Catching a killer

Stan Bellgard says innovative science will help catch a pathogen killing kauri forests.

In Western Australia, where Stan Bellgard is from, they call Phytophthora a ‘biological bulldozer’. Catchy name, isn't it? And Phytophthora agathidicida (or PTA), more widely referred to by New Zealanders as ‘kauri dieback’, is proving equally destructive.

The pathogen has infected New Zealand’s kauri forests in the upper North Island, threatening the viability of this iconic taonga. Bellgard and his colleagues at Landcare Research have a unique connection to the dieback story: they made the critical discovery that PTA is a species new to science.

In 1972 Phytophthora (putatively identified by IMI/CAB International as Phytophthora heveae) was linked to dead and dying kauri on Great Barrier Island. Symptoms included yellowing of foliage, canopy thinning and occasional tree death. Alarm bells started to ring in 2006 when the late Dr Ross Beever FRS, an esteemed Landcare Research scientist, identified Phytophthora ‘taxon Agathis’ in diseased kauri in Trounson Kauri Park in Northland, and in kauri forest west of Auckland.

Beever’s discovery challenged the identification of P. heveae as the causative organism. The DNA barcode sequence of the isolate obtained from Great Barrier Island and those from the mainland were identical to another species, P. castaneae, but there were many morphological traits of the kauri pathogen that made it appear to be a different organism.

Confirmation the kauri Phytophthora was a new species within Clade 5 of the genus came when Bellgard and his colleagues carried on Beever’s work and formally identified the Phytophthora organism as Phytophthora agathidicida – the ‘kauri-killing’ Phytophthora.

The killer had a name. The Landcare Research team had made an important taxonomic discovery. But the push to stop the killer had only just begun. Bellgard said, ‘We established a base-camp: there was still the mountain to consider and it was clear that we needed to find an innovative solution, so we, here at Landcare Research, decided to adopt the holistic Māori world-view and started asking whether there are any beneficial microbes and plants associated with healthy kauri that can be added to the soil to combat the pathogen and aid kauri’s inner health.’

Landcare Research mycologist Dr Maj Padamsee recently described the fungal partners that live in the short root nodules of kauri. These helper fungi (mycorrhizae) are known to be associated with plant protection against root pathogens.

Bellgard explains: ‘Together with our research partners, Scion, Plant & Food Research and Auckland University, we are also looking at whether any resistance in the remnant kauri population could provide clues on how to stop it. Would, for example mycorrhizal fungi, colonising roots and nodules of New Zealand kauri provide an effective barrier to the pathogen, or are there other beneficial plants which contribute to help buffer kauri from this disease? Or could the application of rongoā herbal remedies to heal lesions provide some answers? Both western science and mātauranga Māori’s holistic view direct us to the same conclusion, that returning biological diversity to the soil and plant ecosystem will assist with kauri’s well-being.’

Phosphite treatments cause diseased trees to produce callous tissue, walling off the infection. That has given hope that other plant growth promoters, ‘super-charged’ with probiotics, may help too. Equally important is the need to offer fast, accurate and portable methods of detecting the pathogen in soil and other substrates. Traditional molecular methods of testing, such as the soil bioassay, take 10–18 days to give a diagnosis. The development of new polymerase chain reaction (PCR) tests to use on soil samples has reduced the time taken to deliver a diagnosis, which helps land managers make decisions efficiently.

Improvements to increase the efficiency and accuracy of diagnosis are being investigated, and Bellgard and his colleagues from Massey University and Palmerston North have recorded impressive results from a duplex assay, which incorporates elements of the soil bioassay and delivers the result via a LAMP test.

Landcare Research has developed isothermal DNA amplification technology with their research partners at Massey University. A portable hand-held device uses a premixed conjugate solution that allows one-step extraction from the soil baits recovered from the soil bioassay. This approach cuts down the assay time by 10 days.

Bellgard knows it may take a decade to discover a cure for kauri dieback, so community and cultural partnerships are needed to increase awareness of the risks and uptake of management interventions. Bellgard said, ‘That also means finding the next generation of forest guardians/scientists. We have a partnership with Ark in the Park, an eco-restoration project in the Cascades Kauri Park, and two projects with Rongomai Primary, South Auckland, and Mercury Bay School, Whitianga.’

Student volunteers helped re-design a device called a bait cassette, which they can set in streams at two sampling stations. The cassettes contain kauri leaves, and after two weeks they are collected and sent to Landcare Research, where Bellgard can isolate any collected organisms. The work is funded by an MBIE project, Unlocking Curious Minds, and is proving invaluable. Bellgard said, ‘They have discovered a couple of new records of Phytophthora species not previously found here in New Zealand, and one hybrid. The study is creating a following and awareness at the community level. While the solution to curing kauri dieback will come from science, the community have an important role to play in the translation of the scientific findings into actions.’

He continued, ‘Understanding how the disease arrived in New Zealand, and where it came from, is another important strand to this story. Is it an exotic incursion? Is the kauri-killer continuing to evolve as it spreads into new forests in New Zealand? Is the disease benign in its country of origin and has it co-evolved with kauri here?’

Bellgard and Auckland University master’s graduate Simon Randall have also found a progenitor that may be related to Phytophthora,which they hope will provide insights for possible scientific solutions. ‘Therefore we have been thinking about the other places that Agathis kauri grows. For example, New Caledonia and Fiji have problems with their kauri. Science will provide the answer, but the solutions will require both local and international collaboration to expedite the successful control measures. We do not have the luxury of gradualism. The threat is real and apparent, and there is the need for innovative thinking, drawing knowledge from both western and Māori scientists, embracing the whakataukī “Mahi ngātahi” – resilience through collaboration.’

NOTES ON FUNDING: Stanley Bellgard’s Phytophthora research is funded by MBIE’s Prosperity From Trees: Protection from Current and Future Disease programme ($250,900 in 2016, $173,160 in 2015 and $80,740 in 2014). He also received $14,000 from MBIE to support the Kauri Litter Fungi Water Group, and $45,000 from an MBIE Unlocking Curious Minds bid for the 2017 financial year to engage communities in dieback prevention.