Science – Dying shame

Within the rainforest on our Kaipara lifestyle property, the largest and most impressive tree is a majestic kauri that may have been seeded half a millennium ago – only a few hundred years after Maori arrived and settled in Aotearoa, and about the time Abel Tasman became the first European to set foot on this land. The scale of this kauri, with its girth approaching three metres, never fails to awe friends who see it for the first time. This magnificent kauri survived the age of axes and saws but now faces its greatest threat – a subterranean organism, the fungus-like pathogen Phytophthora agathidicida, commonly known as kauri dieback (KDB).

Kauri trees can be infected well before they show any signs of the disease. Most, if not all, infected trees die. The pathogen spreads by moving through the soil and attacking the root systems of the trees. The agents of its transmission (vectors) include contaminated footwear and other clothing and gear and vehicles or digging equipment that penetrate the soil surface.

Animals that disturb soft or muddy soil with their hooves, including wild pigs and stock, can also transmit KDB.

The KDB organism and its history is not yet completely understood by scientists, despite many investigations. The high tree mortality rate and its rate of spread suggests it is a relatively recent invasive species, but its actual pathway into New Zealand is unknown. It was first detected on Great Barrier Island in 1972 and was later detected in the Waitakere Ranges in 2006. In 2015, the organism was finally and formally identified with the name Phytophthora agathidicida. In August 2018, all Forest and Bird reserves including North Shore reserves were closed as a precaution. Vaccination of individual trees by phosphite has been trialled with some success, but this does not provide a blanket protection for clusters of kauri or forests.

No single precaution will defeat the KDB organism, but adoption of a full range of precautions informed by a basic understanding of its transmission by the public is more likely to be effective. Some public parks in the Kaipara have been closed to protect significant kauri. Access to private forests should be supervised and maintain a no-go zone around kauri. Protection of kauri clusters by fencing to prevent the movement of stock and pigs transferring the organism is important.

Ensuring that any footwear being worn in forests is cleaned thoroughly using Trigene biocide, will help minimise human transmission. Managers of civil engineering or roading projects should ensure that their digging equipment is cleaned and hygienic before and after use.

Systems biology approaches have replaced species-specific responses, and a national interdisciplinary research programme (led by Dr Nari Williams, Scion Research Ltd) brings Crown Research Institutes, universities and industry together to combat the KDB challenge. A new deeper scientific strategy is our best hope of finding a comprehensive solution to kauri dieback.

Professor Ralph Cooney
r.cooney@auckland.ac.nz