Denham Bay canopy.

Image: DOC

Introduction

What we have learnt from our myrtle rust findings on Raoul Island.

What we have learnt from Raoul Island

Myrtle rust was first discovered in New Zealand in March 2017 on Raoul Island in the Kermadecs, where most vegetation is pōhutukawa forest.

After looking at the extent and intensity of the infestation, we decided that it was too widespread and too technically difficult to attempt eradication throughout the island.

Between October and December 2017, we set up four short term and 40 medium term monitoring sites on Raoul Island which continue to be monitored.

Unrelated to the decisions being made on Raoul Island, in May 2017 myrtle rust was discovered on mainland New Zealand in Northland.

The short-term monitoring (monthly assessments) provides data on the speed of transfer and how the leaves, buds and fruit are contaminated on Raoul Island pōhutukawa. The medium-term monitoring (annual or bi-annual assessments) helps to understand the impact of myrtle rust on the foliage of mature plants, and also the rate of regeneration.

Kermadec pōhutukawa (Meterosideros kermedecensis) is the only host species for myrtle rust on Raoul Island. This has some advantages for a monitoring programme as there are no complicating factors caused by multiple species – which is typical with mainland monitoring sites.

The canopy collapse at Denham Bay

Our scientists made several visits to the island between March 2017 and March 2018 to set up and monitor myrtle rust on the island.

Myrtle rust is a potential threat to the pōhutukawa canopy under the right environmental conditions. Although canopy dieback was seen in 2017, it was not clear at the time that myrtle rust was the cause of this canopy loss in Denham Bay. A site visit in March 2018 concluded that although myrtle rust was present in the epicormic growth (less than 2 m from the ground), the canopy loss seen was not caused by the disease. (See image at the top of this page). 

Unlike mainland pōhutukawa, Kermadec pōhutukawa has stem and leaf growth before flowering and while seed sets. We understand that if myrtle rust were to cause canopy collapse on Raoul Island, it would need to infect both periods of leaf 'flush'. Between these flushes, the leaves age and harden off, becoming less susceptible to an infestation.

In contrast, mainland plants (including pōhutukawa) generally have a single flush. Therefore, if myrtle rust is active during that flush period, the impact of the disease could be more significant. 

We are unable to know for sure the exact cause of the canopy collapse at Denham Bay. It may be from localised volcanic activity such as ground warming, or a toxic gas vent.

The impact of insects

It appears that insects may be playing several roles relating to myrtle rust on Raoul Island. Leaf curl (caused by caterpillars) seems to create micro-habitats enclosing and protecting the rust within the foliage. There is also evidence that crawling insects move infected material within epicormic material within one plant or between plants.

Summary of our key findings

We understand that the severity of a myrtle rust infestation is dependent on the temperature, the amount of rust spores in the area (inoculum levels), humidity, the stage of plant development (phenology) and the host species. The most infested plants appear to be seedlings, and the most infested part of older trees appears to be the epicormic growth.

It is too early to tell how the climate during the summer of 2017-18 may have affected myrtle rust infestation on Raoul Island or how it compares to the mainland. Myrtle rust needs to be monitored across several seasons to determine any seasonal impacts and annual climate effects. It may take several years to fully understand the impact.

With the discovery of myrtle rust in New Zealand, we have recently classified all myrtle species, see Conservation status of New Zealand indigenous vascular plants, June 2018 (PDF, 8,580K), as being threatened. Thirteen of the 37 species of myrtle now have the unenviable classification of Threatened - Nationally Critical, including Raoul Island pōhutukawa.

We continue to monitor the short and long-term impacts of myrtle rust on Raoul Island and at selected sites (mainly) on public conservation land on the mainland.

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