My ancient Greek is a tad rusty now, but I recall from when I studied it at university (really) that the word phyton means plant, and the word phthora means destruction.
Jo McCarroll reports on the latest from the fight against kauri dieback
So the word phytophthora literally translates as “plant destroyer”, which sounds like a character in a vegetarian version of The Avengers. It’s an appropriate name though, as this family of fungus-like pathogens is both aggressive and destructive (it was a phytophthora, Phytophthora infestans, that caused the Irish potato famine).
A phytophthora that’s been in the news more recently is Phytophthora
agathidicida aka kauri dieback.
This incurable disease infects our native kauri ( Agathis australis), causing root rot, weeping lesions on the bark, yellowing leaves, defoliation and eventually, death. It was first spotted on a kauri tree on Great Barrier Island in the 1970s, but it wasn’t until the early 2000s, when we started to see kauri trees dying, that scientists realised it was a phytophthora that was causing the problem.
And actually we still don’t know how long it has been here, where it came from or how it got here. ”But the suspicion is it arrived some time in the last few hundred years,” says Dr Ian Horner, a plant pathologist heading the Plant & Food Research team trying to protect this taonga species. ”The argument is, if it had been here for millennia it would have already eradicated kauri trees.”
We also don’t know if this phytophthora affects any other hosts, Ian says, or if there are other species that harbour the pathogen without symptoms. ”But very few phytophthora have just one host, so it would be unusual if that were the case.”
As gardeners know there are all sorts of phytophthora out there.
Commercial apple and avocado growers often manage the impact of the phytophthora that affect those crops through the application of phosphorous acid or phosphite.
So around 10 years ago, Ian and his team started to investigate whether that would also be an effective way to treat kauri dieback. ”We started off with in vitro trials in the laboratory to see whether phosphite would have any effect against the organism, then we moved to testing it with infected kauri seedlings in a glasshouse,” Ian says. ”We got stunning results where all the seedlings that had been inoculated with the phosphite lived, and all the seedlings that were untreated died.”
So field trials began with infected trees at four sites around the North Island being inoculated with phosphite – it’s almost like an injection at the doctor, with a small hole being drilled in the trunk, and the solution injected, via a spring-loaded syringe, and taken up through the tree’s transpiration process. ”A tree can absorb a syringeworth – that might be about 20ml – in just five to 10 minutes,” Ian says.
Five years after that first application, the progress of the disease in the treated trees had been halted. ”Lesions stop advancing,” Ian says. ”You see the cracking of the bark around the lesions and the tree eventually pushes the disease out. In many cases there would be healthy bark underneath.”
Scientists know that the phosphite has a direct effect on the phytophthora. But Ian says his research has found that with kauri, this treatment has an impact beyond what you might expect from the direct result of the dose applied, suggesting the phosphite is triggering the tree’s own natural defence systems. More recently this research has focused on getting the dose right – too high and there is a risk of phytotoxicity, Ian says. But results are immensely promising and this treatment is being rolled out by Auckland Council at some infected sites.
But phosphite suppresses, rather than eradicates the phytophthora.
So a treated tree could eventually be re-infected, although Ian’s results suggest one treatment would be effective for up to five years. And obviously this method requires labour, with each tree needing to be treated individually. ”I see this as a Band Aid,” Ian says. ”It’s not the solution. But it allows us to save certain trees. And it’s a stop-gap that might buy these trees some more time.”
There’s a citizen science project that’s part of it too: if you have infected trees at home in Auckland, Coromandel or Northland, register as part of Kauri Rescue (kaurirescue.org.nz). You get instructions on how to treat your trees and are required to feed your observations back into the programme. But we can all help stop the disease’s spread, Ian says. People are a major vector for kauri dieback (in the Waitakere Ranges, 70 per cent of the infection is on the track network). ”So use wash stations,and respect track closures and ra¯hui. It’s quite upsetting to be out doing trials and see how many people ignore signs and walk around barriers.”
Kauri dieback on trees on Lone Kauri Rd, Waitakere Ranges.