Understanding the enemy

Photo: Assembling the myrtle rust genome. By Peri Tobias, University of Sydney*

Host plant susceptibility to myrtle rust

Seedlings of six New Zealand myrtle plants (mānuka, kānuka, rawiri mānuka, pōhutukawa, ramarama, rohutu) were grown from seeds collected from different parts of New Zealand, and then infected with spores of the myrtle rust pathogen (Austropuccinia psidii).

Researchers say that all six species should be considered susceptible and there is a need to conserve as much plant genetic diversity as possible while we still can.

What they found

Disease development on leaves, stems and shoot tips was monitored and assessed. Initial results revealed that there is resistance to this pathogen in mānuka, kānuka and rawiri mānuka, and some seedlings did not develop any myrtle rust symptoms during the trial.

However, researchers found there are also substantial levels of susceptibility. Only one resistant pōhutukawa seedling and no resistant ramarama or rohutu seedlings have been found to date.

A regional effect was also found - mother plants growing in some regions produced more resistant seedlings than those from other regions.

Download the report: Identification of native and important exotic host species susceptibility to myrtle rust, including variability within species

How New Zealand's climate impacts myrtle rust

The objective of this research was to better understand the threat from myrtle rust to key myrtle species in New Zealand’s climate.

Experiments on key factors affecting infection and development of myrtle rust were studied to understand regional risk of disease development and identify management opportunities. The experiments were undertaken on pōhutukawa and a Lophomyrtus hybrid under controlled temperatures in Brisbane and in the field in Auckland.

Modelling results showed that low winter temperatures in the lower North Island and upper South Island can halt myrtle rust development, but that leaf emergence on Lophomyrtus can continue, slowly, providing a seasonal opportunity to out-grow rust development.

For pōhutukawa, leaf emergence stops at a higher temperature, so there is less opportunity to outgrow the rust. Fortunately, pōhutukawa shows some field resistance to rust attack.

Implications for management

Differential host and pathogen development rates indicate a rust control opportunity by managing irrigation, fertiliser and pruning in managed or restorative plantings. Specifically:

Avoid promoting growth flushes in summer when conditions favour infection
Establish plants and encourage rapid growth in late autumn as temperatures become too cold for infection
Avoid irrigating, fertilising and pruning in spring to minimise growth flushes as temperatures rise and become suitable for infection

Download the report: Identification of asymptomatic periods

The South African strain of myrtle rust - an even greater threat

The pandemic strain of myrtle rust that is present in New Zealand is the most widely distributed strain globally. It’s also different to the myrtle rust strains present in South America and South Africa - those strains have different host ranges and severity.

If these other myrtle rust strains were introduced into New Zealand this could lead to additional myrtle species becoming infected, making myrtle rust an even more serious threat to the natural diversity of New Zealand.

In order to better understand the risk that these other myrtle rust strains pose to New Zealand, seeds from mānuka, kānuka, mānuka rawiri and pōhutukawa from different regions were collected and sent to South Africa where they were exposed to the myrtle rust strain present in that country.

The seedlings were assessed for disease severity and the results showed that all the species are susceptible. All indications are that the South African strain of myrtle rust poses a threat to New Zealand forests. This work highlights the need for continued vigilance at the borders to prevent further introductions of new myrtle rust strains.

Download the report: Assessment of other myrtle rust biotypes

It's all in the genes - tolerance to myrtle rust

Plant tolerance to disease is encoded in their genes.

A full genome of mānuka was recently developed by Plant & Food Research. Researchers looked at 76 mānuka populations and identified genetic variation that showed differences between local provenances within New Zealand, and between New Zealand and Australian related species.

Next, seeds were collected from the same trees that were genetically sequenced and grown into seedlings. The seedlings were inoculated with myrtle rust.

Researchers found that mānuka shows susceptibility and tolerance to myrtle rust. They will now look for correlations between seedling tolerance/susceptibility to myrtle rust and the genetic variations that they previously identified. This will enable them to discover if any of the genetic variations are associated with tolerance to myrtle rust. This knowledge could be used to select more tolerant types of mānuka for planting.

Download the report: Initial identification of genetic markets linked to resistance

Can microbes help myrtles resist myrtle rust?

The community of microbes living in New Zealand myrtles is diverse and can inhibit germination of rust spores

All plants have microbes living on them and in them. These microbes can help plants resist diseases. This study wanted to find out if the microbes living in New Zealand myrtles could help the plants resist myrtle rust.

The new season’s growth (young tissue) is always more heavily infected by myrtle rust than old season’s growth. Researchers therefore investigated whether microbes differed in young and old plant tissue.

Three plant species were studied – pōhutukawa (iconic species), mānuka (emerging economic strength) and ramarama (highly susceptible to myrtle rust). Working with Māori, researchers sampled four sites, 10 plants of each species at each site (where possible).

What they found

The research confirmed the hypothesis that all three plant hosts – pōhutukawa, mānuka and ramarama – contained communities of fungi and bacterial within their foliage. Thus, there is a resource of microbial communities from which those that improve tolerance to myrtle rust can be selected.

The results showed that the community of microbes in new foliage was different from that in older tissue. An experiment showed that some microbes from the old tissues could inhibit the growth of rusts. This work supported a role for the microbial community in tolerance to infection by rusts.

Download the report: Relationship with endophyte populations

Decoding the myrtle rust genome

Unlike other rust fungi, myrtle rust occurs on a very broad host range (over 500 plant species), making it a particularly dangerous plant pathogen.

In only nine years since it was first found in Australia, myrtle rust has caused near extinction of at least three species, caused the decline of at least one keystone species, and impacted commercial production of species such as tea-tree and lemon myrtle.

To combat myrtle rust we need to understand the mechanisms it uses to infect its hosts. To study and understand these mechanisms researchers assembled a high-quality genome based on the latest sequencing technology. They determined that it is the largest assembled fungal genome to date. *See the picture above. If you printed the letters in 12 point font onto A4 sheets and stacked them on top of each other they would be 2km in height (lined up they would go all the way from Sydney to past Auckland and back and then back again). It’s far larger than other fungal genomes.

Now researchers are unravelling its complexity to address the impact on iconic plants such as eucalypts and pōhutakawa.

How can these research outcomes be used?

Improved resistance breeding for myrtle rust in economically important trees using developed markers
Enhanced natural land management strategies
Improved biosecurity management by monitoring pathogen populations to detect sexual recombination, new incursions and populations shifts
Support for an international drive to understand the infection process from this globally significant fungus

Download the report: Austropuccinia psidii De Novo genome sequencing

Download the full report, including all the above reports, and Seedbanking and germplasm research strategy, and Appendix 1: Example of landowner consent form.