Five years on and diseased kauri injected with phosphite are growing strong

Tuesday, 7 February 2017

Dr Ian Horner says the use of phosphite in kauri is promising, but not a cure.

Injecting kauri with a single treatment of phosphite halts dieback disease, at least for five years.

The country's foremost scientist on the issue, Dr Ian Horner, visited an infected west Auckland stand recently to see what effect the chemical is having half a decade on.

The results at Lower Huia Dam are exciting, as they are at three other trial sites around the North Island, the tree pathologist says.

Dr Ian Horner of Plant and Food Research.

'We've found that where we've injected the trees with phosphite the lesions have pretty much stopped spreading, and healed. In comparison, the trees that we didn't treat, the lesions have kept growing in most cases.'

Healing could be seen after six months with fresh bark now showing underneath old lesions that have crumbled away.

This kauri has responded well to phosphite, with new bark showing under a crumbled lesion.

Canopies though are not showing much sign of regrowth, yet – but it takes a long time to turn a kauri around, Horner says.

The common agricultural chemical appears to spur the tree's immune system, and Horner is optimistic about new trials that are testing much lower rates of the chemical.

A drill and syringe used to inject kauri with phosphite.

Testing has also finally started on large kauri of up to about three metres in diameter, at the Cascade Kauri park in Auckland, the Trounson Kauri Park near Dargaville and a site near Kerikeri.

'We are trying to work out what rates we need to treat trees of that size, because a lot of it is guess work. Educated guesses, but you are still taking a bit of a risk.'

An old injection hole in a kauri, five years on.

Although the results in smaller trees is exciting so far, with the phosphite actually stopping the spread of the disease inside the tree, it's still only 'a bandaid'.

At some point the kauri will be need to be retreated, Horner says.

Dr Ian Horner grew up in west Auckland – where kauri dieback disease is rampant in the Waitakere Ranges.

'It doesn't eradicate the pathogen, it's still there in the forest.

'We don't know how long it will take to reactivate, but we haven't seen any re-activation so far over the five years.'

The public should hold off treating their own trees with the common agricultural chemical until the correct rates for different tree sizes and stages of infection is worked out. The wrong dose can kill a kauri, Horner says.

To get the science to the stage where the public can inject their own trees, a citizen science initiative called Kauri Rescue is being launched at a meeting in Titirangi, west Auckland on February 9.

All trees infected with Phytophthora agathidicida, also known as kauri dieback disease, will naturally die.

No cure has been found.

Scientists say the disease is being spread around the country in dirt attached to human shoes.

In west Auckland's Waitakere Ranges, about one in five kauri are now infected – nearly three times more than five years ago.