A treatment programme now being rolled out on Waikato’s Coromandel Peninsula is bringing new hope to trees infected with kauri dieback.
This month Waikato Regional Council has begun a phosphite treatment trial programme in the Whangapoua area, injecting phosphite into 2000 trees, with contractor BioSense.
It is one of six sites in the Waikato region – all of them on the eastern Coromandel Peninsula – where kauri dieback has been confirmed. Auckland Council has been delivering phosphite to sick kauri in the Waitakere Ranges for the past two years, as have private landowners through the Kauri Rescue programme.
“It’s exciting to see this work being undertaken in our region to help kauri trees in Whangapoua, as well as provide valuable information on the treatment’s effectiveness for the national effort to save this iconic tree species,” said Waikato regional councillor Dal Minogue.
Council biosecurity officer Kim Parker added: “The injections aren’t a cure, but it does help support a kauri’s defence system to fight back against the disease. Once treated the trees remain alive, buying time for them,” said Miss Parker.
In the meantime, limiting the movement of soil remains the best way of protecting trees, she said. This can be achieved by ensuring all gear is dirt free before entering the bush, as well as fencing off kauri stands from stock.
Kauri dieback is caused by a microscopic soil-borne organism called Phytophthora agathidicida (PA) that kills infected kauri. The organism can survive in the soil, away from kauri, for many years and can be spread in small amounts of soil.
Trees with kauri dieback can have bleeding at the base of the trunk and thinning canopy that eventually becomes bare as the tree dies in the later stages of the disease.
Dr Ian Horner from Plant and Food Research has been leading the phosphite research trials since 2011, which have shown it has “great potential” as a tool against kauri dieback.
“Through our initial greenhouse trials and then forest treatment trials in Northland and Auckland we’ve seen a halt in lesion spread – in most cases, lesions have healed.
“While phosphite injections don’t permanently cure kauri of the disease, or remove PA from the surrounding soil, the treatment does temporarily stop or reduce its harmful effects, and give the tree a chance to recover.
“This is great news for our kauri,” said Dr Horner.
Phosphite trials on PA-infected kauri of various sizes and ages have explored applications of different concentrations, injection spacing around the trunks and treatment intervals.
Retreatment is required, with the ideal intervals the subject of ongoing research.
The research results to date have informed a best practice phosphite treatment protocol that Waikato Regional Council is using in Whangapoua.
Phosphite is a low toxicity, biodegradable chemical that has been used since the 1970s to protect crops like avocado, pineapple and cocoa against diseases caused by other species of Phytophthora.
It has also been trialled and utilised overseas to control the spread and impact of other similar organism infections by injection of infected trees and aerial application of entire plant communities.