Myrtle Rust

May 30, 2012

Pest control

Greg Pollard and James Martens-Mullaly

Myrtle Rust (Uredo rangelii) is a recently described anamorphic (asexually reproducing) rust fungus (Simpson et al. 2006), that is closely related to the Eucalyptus/Guava rusts (Puccina psidii), a pathogen native to parts of South America and the cause of Guava rust.  Although morphologically distinct from P. psidii, DNA sequencing has placed it in the P. psidii complex (Carnegie et al 2010), a group of pathogens that uses plants of the family Myrtaceae as hosts.

Australia, including its offshore island territories, has 88 genera and about 2253 native species of plants belonging to the family Myrtaceae, representing around 10% of Australia’s native flora (CHAH Australian Plant Census, unpublished data, B. Lepschi pers. comm. 24 Aug. 2010, in Major, R, 2011).  The discovery of Myrtle Rust at a cut-flower growing property on the central coast of NSW in April 2010 sparked concern from a range government bodies, industries and individuals for Australia’s dominant plant family – Myrtaceae.  Within weeks a National Management Group met to consider the implications of the disease’s presence in Australia.

In May 2010 the Federal Department of Agriculture Fisheries and Forestry (DAFF) released a statement describing Myrtle Rust.  The statement accepted there was little knowledge of exactly how the fungus might behave under Australian conditions.  The distribution of the similar Eucalyptus/guava rust, which has caused serious disease among Australian myrtaceous plants in nurseries and plantations in South America, southern USA and Hawaii, and which has been well documented, provided useful data for predictive modeling of the potential spread of Myrtle rust in Australia.

Despite early advice that it was probably not technically feasible to eradicate the disease, an interim response plan aimed at continuing to suppress it, with a view to eradication, was agreed to by authorities on July 02, 2010.  By August the disease had been detected at six different properties affecting eight different species but at that stage had not been found on the genus Eucalyptus and efforts to suppress the outbreak and gather additional information continued.  In mid-September evidence suggested the disease was still largely confined to cut-flower producers and both wholesale and retail nurseries although it had spread to at least thirty-two sites from Nowra to the mid north coast in NSW.  Identification of the disease at an increasing number of sites indicated that it had probably been present for at least two years and by December 2010 it was conceded that eradication was not a realistic goal.  Instead, efforts were to focus on mitigating impacts of the disease on both the natural environment and industries which rely on Myrtaceae.

Myrtle rust continued to be detected over a wider area throughout 2011 including into Queensland and more recently in Victoria and has now been found on over 150 plant species in a variety of nurseries, parks, gardens, streetscapes and bushland.  Booth, C. (2011) in an article published in the CSIRO’s ECOS magazine, quoted scientists who had looked at the disease from the points of view of both potential host range as well as climate suitability.  It was suggested that other rusts are known to cause more severe damage to plants, but that Myrtle rust appeared to be capable of infecting a greater number of species.  However, it would appear that in many species there is some level of natural resistance to the disease, as from tests conducted on over one-hundred taxa, it was found that in around two-thirds of cases not all individuals became infected.  It is becoming apparent that variation in susceptibility exits between species, provenances and individuals within a species.

Myrtle rust needs at least three hours of leaf moisture to germinate and grows best between 14 and 25 degrees Celsius.  In terms of potential distribution within Australia, recent predictive modeling undertaken by the CSIRO (Kriticos (2011 unpubl) in Cannon, A., M., (ALIEMI Pty Ltd) 2011), indicates natural areas east of the dividing range are most at risk because optimum conditions for germination and growth exist.  Less favorable climatic areas exist west of the dividing range though the spread west is expected to be contained by arid inland environments.  Coastal Victoria, the east and central coast of South Australia, south-west Western Australia and the north and east coast of Tasmania have also been identified as providing suitable climatic conditions for disease development.  In addition to its likely distribution in natural areas, plant nursery conditions which provide similar conditions will favour the disease.

The Myrtle rust pathogen attacks young soft actively growing plant parts, namely leaves, shoot tips, and young stems as well as fruits and flower parts of susceptible plants.  Disease signs vary between host species; though in general the first signs of infection are the appearance of small raised lesions, brown to grey in colour, often with red–purple haloes.  Up to fourteen days after infection, the lesions produce masses of yellow or orange-yellow spores (DPI, 2012).  Infected leaves may become pocked, buckled or twisted and severe infestation in young trees may kill shoot tips. Anecdotal evidence suggests that the spores can last up to three months until conditions favour germination (ATTIA, 2009).

Rusts are readily transportable because they produce large numbers of small and light spores. Myrtle rust spreads easily via spore, though can also be dispersed through the transport of infected plant material, and consequently Australia is presently witnessing the rapid spread of Myrtle rust through the climatically suitable regions.  Spores can be spread by humans, on vehicles, clothes, tools, equipment, etc.  Birds, bats, native marsupials and other mammals, and insects are also vectors.  Under suitable conditions, spores may become wind-borne traveling many kilometers, and are also spread by water (wind driven rain and irrigation).

Myrtaceous plants are both ecologically and economically important in Australia.  Myrtaceous plants dominate many Australian terrestrial ecological communities.  Although at present, it seems the main threat of Myrtle rust is more likely to reduce the vigour of established plants rather than result in their outright death, Myrtle rust has the potential to threaten susceptible communities through tree mortality and retarding generational succession by impeding plant development and reproduction, in particular regeneration after adverse environmental events such as fire or flood as young plants and new foliage are seriously affected.

As with many plant health ailments, the effects of the rust may, however, lead to a greater susceptibility to secondary pathogens and repeated infections may have more serious consequences.  Indirect effects to the ecology include the reduction or loss of native floral and faunal habitat and food sources, increased impact of fire, and abiotic effects as a result of canopy decline including erosion, and reduced soil water retention.
Economically, Myrtle rust may potentially have a serious adverse impact on those commercial industries using myrtaceous plants, including the cut flower, nursery, garden, native forestry, tea tree oil and bee keeping.  In an industry paper recently published by the Forest & Wood Products Australia, a rough estimate of annual plantation timber volume loss due to Myrtle rust infection was placed at 1,497,908 m3, or 10.5% of total annual production by volume (Major, R, 2011).  Indirectly, the tourism industry may be adversely affected should significant degradation of myrtaceous vegetation communities particularly in national parks and state forests, occur.

It is not possible to prevent the spread of the disease through naturally occurring plant populations; geographic and climatic barriers should however impede its spread to isolated though otherwise suitable climatic regions.  In an attempt to slow the spread of the disease, Government agencies and industry bodies have introduced Myrtle rust management plans.  Businesses trading in Myrtaceae plants should adopt the national nursery industry management plan for myrtle rust (www.ngia.com.au).  In commercial plant growing operations, particularly those operating in the more confined or controlled situations, such as nurseries, outbreaks are likely to be controllable with fungicides and appropriate cultural practices.  Some threatened industry sectors are already taking proactive steps to mitigate the potential economic impact through research to ascertain the degree of susceptibility of economically important species and to test for and develop resistant stock.

A number of permits have been made available, authorised by the Australian Pesticides & Veterinary Medicines Authority (APVMA) to allow chemical treatments to be applied, over and above the emergency response, to combat the spread of myrtle rust.  These permits specify which chemicals can be applied and under what circumstances and include options for commercial enterprises to decontaminate host material and for homeowners to treat their myrtaceous garden plants for the disease.

While more definitive information is gathered about the effects of myrtle rust in Australia, it would be prudent for those who work or visit regions with a climate which suits the disease to be familiar with it’s signs and symptoms, be mindful of the ease with which it can be spread and to observe the State based restrictions that have been imposed on the transportation of host material.  Suspected cases of myrtle rust should be reported by phoning 1800 084 881 and while plant material should not be moved, close up photographs can be useful in confirming the disease’s presence.

References and further information

Australian Pesticides and Veterinary Medicines Authority, accessed at: www.apvma.gov.au/

Australian Government Department of Agriculture, Fisheries and Forestry (DAFF) (2009) http://www.daff.gov.au/aqis/quarantine/pests-diseases/plants/eucalyptus-guava-rust

Australian Tea Tree Industry Association (ATTIA) (2009) http://www.attia.org.au/myrtle_rust.php2000 – 2009

Booth, C. (2011) Myrtle rust: how big a threat to native plants?, http://www.ecosmagazine.com, CSIRO Publishing.

Cannon, M. A., (ALIEMI Pty Ltd)  (2011) Myrtle rust – forest industry issues paper, project no. PRC218-1011, Forest & Wood products Australia June 2011

Carnegie, A. J., Lidbetter, J. R., Walker B. J., Horwood, M. A., Tesoriero, L., Glen, M., and Priest,  M., J. (2010) Uredo rangelii, a taxon in the guava rust complex, newly recorded on Myrtaceae in Australia, Australasian Plant Pathology, 2010, 39, 463-466, CSIRO Publishing

Department Agriculture, Fisheries and Forestry,

canadian pharmacy

http://www.daff.gov.au/aqis/quarantine/pests-diseases/myrtle-rust/myrtle-rust-270711

Major, R.,(2011) Introduction and establishment of exotic rust fungi of the order uredinales pathogenic on plants of the family Myrtaceae – Proposed Key Threatening Process Listing, New South Wales Department of Environment and Heritage  2011, http://www.environment.nsw.gov.au/determinations/exoticrustPD.htm

NSW Department of Primary Industry; http://www.dpi.nsw.gov.au/biosecurity/plant/myrtle-rust/resources

Simpson, J. A., Thomas, K., Grgurinovic C. A. (2006) Uredinales species pathogenic on species of Myrtaceae. Australasian Plant Pathology 35: 549-562

Victorian Department of Primary Industries; http://www.dpi.vic.gov.au/forestry/pests-diseases-weeds/diseases/myrtle-rust

Victorian Department of Primary Industries (2012), Myrtle Rust Fact Sheet – Myrtle Rust in the Home garden  March 2012, Victorian Government.

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James

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