Scientists working on the kauri die-back programme have found a way to arrest the disease that is killing trees from Auckland to the Far North.
Dr Ian Horner, from Plant and Food Research, said the horticultural chemical, phosphite, was proving very effective against phytophthora: the water mould that infects kauri through their roots.
He said avocado growers used phosphite to ward off a similar disease in their orchards, and over the past three years, researchers had been using the chemical in field trials in four kauri forests.
Dr Horner said the results were promising.
"In the trees that had not been treated, many of the lesions, these big bleeding cankers at the base of the tree, were spreading and advancing up the trunk and will eventually cause death. In the trees that we had treated, in almost all cases, the lesions had stopped spreading and had started to heal, which is a very encouraging sign."
Dr Horner said in glasshouse trials, phosphite also prevented infection in kauri seedlings in glasshouse trials, while the untreated seedlings all died.
Dr Horner stressed that phosphite was not a cure for phytophthora, and it was not known how often a tree would need to be injected to keep it healthy. That is a question to be tackled in the next stage of his research, for which the Government has made funds available.
He said phosphite was cheap and easily available, and treating a tree would cost between $1 and $10, depending on its size.
But he warned kauri lovers not to rush out and start jabbing their favourite kauri.
"There is a down-side. Phosphite can cause the leaves to yellow and fall off if the dose is not right. We are still experimenting with the correct dosage and we need to refine the technique before we start rolling it out en masse."
Dr Horner is one of a number of speakers at a symposium on kauri die-back this weekend in Hokianga - where tangata whenua and tourist operators are worried about the fate of Waipoua kauri forest and famous ancient kauri like Tane Mahuta.
The giant trees are showing no sign of infection so far, but the disease has killed a number of other kauri in Waipoua and in nearby Trounson Park.
Phytophthora talon agathis is now widespread in kauri stands throughout Auckland and the Coromandel peninsula.