Pathogen Profile #7

Hello Robigalia Reader and welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inbox!

This week’s newsletter was written during a recent trip to Cairns where I attended the Plant Biosecurity Research Initiative (PBRI) symposium. Situated in Tropical North Queensland, Cairns is home to the world-heritage listed Great Barrier Reef and the world’s oldest rainforest, the Daintree. Scroll down to learn about this week’s pathogen. If you’re a Robigalia subscriber, you also have access to my online database and today’s title song!

This ancient rainforest is home to nearly 80 rare or threatened plant species which are under threat from various pests and diseases. One major concern highlighted in several talks at the PBRI conference is myrtle rust.

Myrtle rust is caused by the rust fungus Austropuccinia psidii and infects plants within the family Myrtaceae. Rust fungi have been around for centuries, with myrtle rust first being described in Brazil in the late 1800s. It is believed to have coevolved on Psidium guajava (guava) and began infecting other native and non-native Myrtaceae species. The fungus uses brute force to infiltrate the new leaves of hundreds of hosts and extract the nutrients from the leaves. After multiple cycles of repeated infection, this can deplete the nutrients required for the leaves to survive. This causes the die-back of whole branches and in severe cases eventually leads to whole tree death.

The thousands of microscopic spores produced on a single leaf spread easily via wind, pollinators and anthropogenic means (on clothing, transport of infected material, etc). These factors, along with the broad host range, has facilitated the global spread of myrtle rust which is now present in the US, Japan, China, South Africa, Indonesia, New Caledonia, The Caribbean, Singapore, and New Zealand and Australia.

In Australia, where Myrtaceae are dominant, many plant species are under threat of extinction as a direct result of myrtle rust. The death of keystone plant species not only reduced food and shelter availability for native animals but alter native ecosystems allowing non-native plant species to thrive. Myrtle rust also impact forestry sectors, including honey production. In New Zealand, it is estimated the economic impact of mānuka/kānuka forests will be over $155 million over 20 years. The impacts extend beyond the environment and forestry. Many Myrtaceae in Australia and New Zealand have important cultural significance to indigenous populations.

Thankfully, there is hope in the face of myrtle rust, as many species are able to survive myrtle rust infection. By studying the resistance response to myrtle rust infection, researchers are working to identify which genes may be responsible for plants fending off infection. This knowledge may provide future opportunities for breeding efforts and restoration. On the other side of the coin, research is being conducted to understand why the pathogen, unlike other rust fungi, is able to infect such a broad host range. This may shed light on potential control measures in the future.

If you’re interested in deep-diving into the science of myrtle rust, I’ve summarised a few articles below. If you’re a subscriber, scroll down to access my online database which includes full summaries of these, and all other Robigalia featured articles.

Austropuccinia psidii papers

Collateral damage: epiphytic orchids at risk from myrtle rust

• This short communication piece discusses the impact of myrtle rust on epiphytic orchids in Australia. The researchers identified 73 epiphytic orchid species that rely on myrtaceous hosts, with seven species highly dependent on these hosts and at significant risk due to myrtle rust. This highlights the decline of Myrtaceae due to myrtle rust has broader ecological consequences, including habitat loss for epiphytic orchids.

A Diagnostic Guide for Myrtle Rust 

• This diagnostic guide from researchers in Brazil provides methods for myrtle rust diagnosis. It includes fantastic images of myrtle rust infection on different hosts, along with microscopic images of the spores.

Early Detection of Myrtle Rust on Pōhutukawa Using Indices Derived from Hyperspectral and Thermal Imagery

• This research out of New Zealand focussed on the use of thermal and hyperspectral indices for monitoring myrtle rust. The research concluded hyperspectral indices were most effective for pre-visual and early disease detection while thermal indices were most effective for disease detection after the onset of disease symptoms.

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Until next week,

Alyssa