An exotic fungus that attacks native plant species including eucalypts, threatens to become endemic after thwarting containment on the NSW central coast and reaching North Queensland this summer.
Biosecurity Queensland announced last week that plants infected with Myrtle Rust, or Uredo rangelii had been found at a Cairns nursery. It was the first time the disease has been identified so far north.
Due to its rapid spread, the Australian Quarantine and Inspection Service now says the fungus is unlikely to be eradicated.
Myrtle rust is derived from the group of fungi that includes guava rust and eucalyptus rust. It which originated in South America but was first identified in Australia in a nursery on the Central Coast of NSW in April 2010.
Authorities have been unable to determine exactly how the fungus first arrived in Australia, but it is thought spores may have been carried on imported equipment, or even long distances on wind currents.
Recently published research in the CSIRO's Australasian Plant Pathology describes how the fungus group is relatively harmless on native South American plant species but has been very harmful to eucalypts cultivated there.
Powdery yellow spores
Myrtle Rust in particular afflicts plants in the Myrtaceae family, which includes many native species such as Bottle brush, lemon myrtle, lilly pilly and blackbutt.
It first appears as purple spots on young leaves, and these develop into powdery yellow or orange spores. Once disturbed, the spores are easily carried long distances by wind, on the soles of shoes, or on gardening equipment.
Affected leaves often die and it compromises a plant's ability to thrive and reproduce. Those plants which are especially vulnerable - particularly young plants - may eventually die.
Plant protection authorities are studying the disease to determine which plants it affects and which conditions help or hinder the fungus.
Dr Satendra Kumar, director of plant biosecurity with the NSW Department of Primary Industries says that the fungus does particularly well in humid conditions.
"That's why we have found rust along the coast in New South Wales and now Queensland - mostly in all the coastal areas where the environment is much more humid.
He says it's unlikely that plants grown in extreme arid conditions will be affected.
Since it has been accepted that the rust is likely to become endemic, research has focussed on determining which plants are going to be worst affected; and the National Senior Biosecurity Officers Group is looking how to manage the rust in Australia.
Fungus puts industries at risk
In the short-term, the greatest danger is for those industries that rely on Myrtaceae species, such as nurseries, the cut-flower industry and some crops, including tea tree.
Associate professor Andres Drenth, a plant pathologist at the University of Queensland's School of Biological Sciences, says no one really knows what the long-term impact is likely to be. But he says anything that affects eucalypts in Australia is likely to cause significant change.
"It's likely to have a substantial long-term impact. It will affect reproductive rates for infected eucalypts. In the next generation, those resistant species will become more dominant and slowly over time you will get a change of species. This will also affect the animals that are dependent on these species. "
But Kumar is more cautious, saying it's still too hard to make any accurate predictions of what the impact will be on the Australian bush.
"We can only extrapolate what we've seen similar rusts doing overseas and, while there are species that will be quite susceptible - there are also ones that will be resistant, and others that will be in the middle. The damage is still a bit hard to predict at this stage."