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.

NZ Gardener - - Contents -

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 (kau­rires­ 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.

Waitakere Ranges.

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