MYRTLE RUST                                                                                               by Christine Corbett
              Agonis flexuosa "Afterdark" infected with Myrtle Rust (Uredo rangelii)
Myrtle rust (Uredo rangelii)
           Photo: Dr. Louise Morin ©CSIRO

Myrtle rust (Uredo rangelii) is a newly described fungus that belongs to a fungal group known as the ‘eucalyptus/guava rust complex’.  It is native to South America.  The fungus is only known to infect plants in the family Myrtaceae.  Infection on highly susceptible plants may result in plant death.  Myrtle rust could have devastating effects on Australian ecosystems. 

Myrtle rust was first detected in New South Wales, on the Central Coast, in April 2010.  However, observations in state forests and nature reserves indicate that the disease may have been present in Australia for at least two years.  How the rust fungus entered Australia is not known. 

Myrtle rust attacks young, soft, actively growing leaves and shoot tips and young stems.  It can also affect flower buds and fruits.  Myrtle rust is distinctive in that it produces masses of powdery bright yellow or orange-yellow spores on infected plant parts.  On leaves, it produces spore-filled lesions.  Leaves may become buckled or twisted and may die as a result of infection.  Sometimes these infected spots are surrounded by a purple ring. Older lesions may contain dark brown spores.

Myrtle rust produces a large number of spores in the pustules. These may be carried to new host plants by wind, water splash or by insects such as bees.  Rust spores travel very long distances on the wind and may infect stands of susceptible plants many kilometres from the original infestation.  These natural means of spread are extremely difficult to control.  However, humans can also easily spread myrtle rust on infected plant material including cut flowers and nursery stock, on clothing and dirty equipment including containers and pruning shears, and on contaminated timber products.  Spores can survive for up to 3 months in the environment if conditions are favourable.

It appears that the main routes of spread in Australia have been infected nursery plants and spores on infested clothing and equipment.

Where is it?
A quarantine area for 26 coastal local government areas in New South Wales (all but two of the coastal municipalities) was established, but the rust quickly spread beyond this area despite containment actions.  Myrtle rust is now confirmed in a high number of areas in New South Wales as well as in 46 sites in Queensland.  The sites include production and retail nurseries, residential, business, government and public land locations. The infected sites in Queensland are from south of Brisbane up to Cairns.

Myrtle rust is currently not known from Tasmania.  There is an import restriction enforced by Quarantine Tasmania banning “any live plants, fruit, seed, tissue culture, pollen, cut flowers, foliage and stems of any plant of the Family Myrtaceae that has been grown or packed in any part of Australia outside Tasmania”. That declaration took effect on 21st July 2010 and “will remain in force until further notice” (Biosecurity factsheet on the DPIPWE website). 

On 22 December 2010 it was agreed that it is not technically feasible to eradicate the rust from New South Wales. “This decision was based on our understanding of the behaviour of the disease, its increasing host range and its spread beyond the New South Wales Central Coast to a large number of domestic, commercial, public and recreational sites” (Dept. Agriculture, Fisheries and Forestry, March 2011).

Due to the potential for Myrtle rust to have an ongoing impact on the natural environment, the community and potentially affected industries, the Myrtle Rust Coordination Group is coordinating projects and directing actions including education and awareness, activities for slowing the spread of the disease and collation and analysis of information about the behaviour of the disease and its impacts on natural ecosystems.  Members of the Myrtle Rust Coordination Group include Wildflowers Australia (but apparently not APS) and representatives of other non-government organisations have also been asked to contribute their skills and expertise. 

Recognising the importance of ongoing chemical control, the Myrtle Rust National Management Group has agreed that securing product registration for chemical use for all jurisdictions beyond the emergency response is an immediate priority.
 
What genera are susceptible?
Myrtle rust has now been diagnosed on 30 species of myrtaceous plants, including many plants that are commonly found in gardens.  Plant genera cultivated in Australia that are known to be susceptible to this disease include Agonis, Austromyrtus, Backhousia, Callistemon (two species and a variety), Chamelaucium, Choricarpia, Decaspermum, Eucalyptus (four species, none native to Tasmania), Eugenia, Gossia, Lenwebbia, Leptospermum (two species), Lophomyrtus, Melaleuca (seven species), Metrosideros, Myrtus, Pilidiostigma, Rhodamnia, Syncarpia, Syzygium, Tristania, Uromyrtus, Waterhousea and Xanthostemon.  Myrtle rust was first detected in NSW on the foliage plant Agonis flexuosa, cultivar ‘Afterdark’ (commonly known as willow myrtle).  This plant cultivar is used as an ornamental plant in gardens and landscaping and as a source of cut foliage for floristry in Australia.

It is currently unknown which other Myrtaceae genera may be susceptible to myrtle rust under Australian conditions.  However, further research is being conducted.  Because Australian Myrtaceae have not been exposed to this disease before, it is unclear how severe its impact will be. 

What conditions are likely to favour the disease?
Rust spores will germinate on a susceptible host plant only under certain conditions. Overseas, rusts in the guava rust group need high humidity or leaf wetness (for more than 6 hours), and temperatures of 15-25°C.  Darkness for some hours after the spores have landed increases the likelihood of successful infection.  It is quite likely that myrtle rust will need similar environmental conditions. 

Exactly which climatic regions are most likely to host myrtle rust in Australia is not known at this stage.  However, Australian rust experts have predicted that areas most likely to favour this rust include the east coast of Australia from Sydney to the tip of Cape York. 

The rust pustules can mature and release infective spores in as little as 10-12 days.  The fungus requires a living host in order to survive and reproduce.  The rust can be asymptomatic (i.e. there are no visible signs of it) for some weeks before it is expressed on a plant.

Restrictions on moving plant material
Currently (March 2011) Victoria, Tasmania, Western Australia and South Australia have restrictions on the importation of products from the family Myrtaceae from all of NSW.  In Tasmania we have a ban on imports of any Myrtaceae material from anywhere else in Australia.  The requirements for all States and Territories should be checked prior to exporting anything.

What can we do?
Myrtle rust is an emergency plant pest and should be reported.  If you think you have found it, ring the Emergency Plant Pest hotline on 1800 084 881.  There are pictures for identification on the NSW Department of Industry and Investment web page; http://www.dpi.nsw.gov.au/biosecurity/plant/myrtle-rust.

This article is a compilation of information available on the NSW Department of Industry and Investment web page (http://www.dpi.nsw.gov.au/biosecurity/plant/myrtle-rust), in particular the following notesheet;

Gollnow B, Carnegie A, Horwood M and Driessen S 2010 Myrtle rust –Uredo rangelii, Primefact 1017, 2nd edition, Department of Industry and Investment, NSW. 

There is also information from the Tasmanian Department of Primary Industries, Parks, Water and Environment Biosecurity webpage; http://www.dpipwe.tas.gov.au
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