Myrtle rust growing Canterbury threat
Myrtle rust has spread into Canterbury and the West Coast, and hundreds of shrubs on the Chatham Islands are now infected too.
The plant disease has been detected at six sites in Christchurch, as well as near Rolleston, Westport, Greymouth and Hokitika. On the Chatham Islands, there were six official reports, but a senior scientist earlier this year reported ‘‘many hundreds’’ of shrubs had been infected.
Dr Peter De Lange was on the
Chathams last week and said there were infections ‘‘as far as the eye could see’’. He is an associate professor in the School of Environmental & Animal Sciences at Unitec in Auckland and worked as a scientist and senior adviser for the Department of Conservation for 27 years.
Myrtle rust was a fungal disease that spread its microscopic spores via the wind, although the spores could travel in soil and clothes. It only affected myrtles, but that plant family was large and included pōhutukawa, rātā, mānuka and ramarama along with commercially valuable exotics such as eucalyptus and feijoa.
The rust was a huge threat on the North Island and top of the South Island. Canterbury and southerly provinces were susceptible, but at less risk due to cold.
Myrtle rust discoveries were supposed to be reported on iNaturalist NZ, a ‘‘social network of people sharing biodiversity information’’. The exact location of species can be obscured, usually for privacy reasons or to protect threatened things.
iNaturalist generally relied on experts and amateurs judging photos of species to make identifications.
A Timaru case was reported in November last year. A woman said she bought pō hutukawa from a nursery in Napier and the plants arrived at her door obviously infected. The iNaturalist community identified the disease as myrtle rust and the plants were disposed of.
Six cases were reported in Greymouth, but the iNaturalist users thought only four were ‘‘research grade’’, meaning members generally agreed on the identification.
The Chatham Islands infections were entirely on a pest plant called chilean guava, which had spread invasively in the peat bogs of the northern end of main island.
Last week, de Lange did not have to stop driving to see the damage. Myrtle rust attacks the young shoots of the guava, killing the top 5cm to 10cm of the shrub.
‘‘It’s very, very active,’’ de Lange said.
‘‘Myrtle rust in this case is doing a good thing . . . because we didn’t have any other way of controlling [chilean guava].’’
Chilean guava was still sold on the main New Zealand islands, sometimes with the name NZ cranberry.
iNaturalist became the public repository for the spread of myrtle rust after the Ministry for Primary Industries was overwhelmed by observations. The airborne pathogen was first discovered in New Zealand in May 2017.
From April 2018, the Government’s focus moved to research to find ways to manage the disease and mitigate impacts in the longer term, Biosecurity New Zealand pest management manager John Sanson said.
It also educated and supported regional council, agencies such as Manaaki Whenua Landcare Research, nurseries, and landowners.