Talk explores rust disease threatening rare native plants

Ecologist Graeme Platt stands by the rare Bartlett's rata at Virginia Lake. It's at risk from myrtle rust. Photo/supplied

Neither Colin Ogle nor Sara Treadgold have ever seen the fungus disease myrtle rust - but they want to talk about it.

Their talk is at 7.30pm on July 4, in Whanganui's Davis Lecture Theatre, as part of a Whanganui Museum Botanical Group meeting.

Myrtle rust arrived on mainland New Zealand in May, probably blown across the Tasman Sea in tiny spores. It's been in Australia since 2010.

It affects plants in the myrtle family - including pohutukawa, rata, manuka, kanuka, bottlebrush, eucalyptus and feijoa - and can kill them.

On Wednesday Radio NZ reported there were 54 confirmed cases in northern Taranaki, and a plant movement ban has been imposed there to contain the disease. Elsewhere in New Zealand smaller outbreaks may have been controlled already.

Plant ecologist Mr Ogle tried to get someone from the Ministry for Primary Industries or a Crown science institute to come and talk about the disease. He says there's been lots of speculation and misinformation, and not many facts.

Everyone he tried was too busy, so Mr Ogle and Whanganui biodiversity ranger Sara Treadgold will speak instead.

He can explain what's now known about the disease, what it has done in other countries and what plants could be affected here.

Miss Treadgold led Whanganui's myrtle rust programme. She helped search for it in Taranaki and knows what to do if it is found. She'll talk about that experience.

Mr Ogle said the rust disease has mainly been found in nurseries, where experienced people keep an eye on plants, and there's likely to be more of it in New Zealand than we know.

"All the predictions are that it's going to get throughout the milder climate parts of New Zealand and there's nothing we can do about it."

It's very unlikely to kill all the plants in the myrtle family. After all some plants survive rust diseases, as New Zealand poplars have.

But it may kill rare and endangered native plants - including ones that have not yet been named or described.

In the Kermadec Islands pohutukawa are badly affected, and RNZ reports that an affected hedge of mature pohutukawa at Waitara has been destroyed. There are no cases on rata so far.

In Australia the rust hasn't killed off whole eucalyptus forests, but it affects young plants and natural revegetation.

A species related to kanuka has been affected in Australia, so some species of kanuka and manuka may be at risk. In Florida paperbarks are badly affected.

Feijoa plants have not been touched in New Zealand so far, but Mr Ogle has seen pictures of affected feijoas elsewhere.

Precious and individual plants can be protected from myrtle rust by spraying with copper-based and other fungicides. Otherwise the only protection is to collect seed and store it for the future.

That process has already begun, with a national seed bank at Massey University. Myrtle seeds were among the first collected, Mr Ogle said, because people knew the family was vulnerable to myrtle rust.