Dr Brett SummerellMyrtle rust – as bad as we feared?

It is now a little over four years since Myrtle Rust was first reported in Australia, when it was found at some properties in the central coast of New South Wales. Since then the fungus that causes the disease has spread up and down the east coast of Australia and is of major concern in Victoria, New South Wales and Queensland. Fortunately it has not spread to any of the other states of Australia as yet.

Myrtle rust on Waterhousea floribunda (syn. Syzygium floribundum)

Myrtle rust on Waterhousea floribunda (syn. Syzygium floribundum)

So what have we learnt in that time and is it the huge threat that it was originally thought to be? As you might have guessed there is no simple and easy answer to that and it depends on where you are and the situation you are interested in.

Myrtle rust on Syzygium jambos (Rose apple) (Photo by R. Makinson)

Myrtle rust on Syzygium jambos (Rose apple) (Photo by R. Makinson)

From a purely horticultural perspective the impact is not as severe as first anticipated. Most of the plants in the Myrtaceae that are popular in the horticultural industry don’t appear to be severely impacted by the disease. There are some species of lillypillies that are quite susceptible and will show symptoms that range from mild to severe depending on the weather. You can expect that some of these species and cultivars will drop out of use because they just won’t perform well enough. Control with fungicides is not really practical because of the need to continually respray to give effective control. However it is reasonably clear at this stage that it is not the end of the world but that some plants will become less common in gardens and nurseries and that the situation will vary between northern and southern Australia.

[Click here to see a list of susceptible species, as compiled in the Nursery and Garden Industry of Australia Myrtle Rust Management Plan, 2012 and you can also find out an affected species list at the Invasive Species Compendium]

Stand of Rhodomyrtus psidioides defoliated by myrtle rust, Ewings Dale, Byron Bay, New South Wales (Photo by K. Kupsch)

Stand of Rhodomyrtus psidioides defoliated by myrtle rust, Ewings Dale, Byron Bay, New South Wales (Photo by K. Kupsch)

Out in the bush it is a much more complex story. Fortunately the impact on Eucalypts has not been significant, at least at this point. However there are a number of species that are being knocked around quite severely by the disease. In these photos you can see the impact the disease has on Rhodomyrtus psidioides (native guava), and the situation is just as bad on a number of species of Rhodamnia, such as Rhodamnia rubescens (Scrub turpentine). The impact of the disease is so bad in some of these species that we are concerned that the disease could lead to the extinction of these species of plants.

Rhodomyrtus psidioides defoliated by Myrtle rust, north-eastern New South Wales (Photo by K. Kupsch)

Rhodomyrtus psidioides defoliated by Myrtle rust, north-eastern New South Wales (Photo by K. Kupsch)

Melaleuca quinquenervia affected by myrtle rust (Photo by R. Makinson)

Melaleuca quinquenervia affected by myrtle rust (Photo by R. Makinson)

Some species of paperbark, especially Melaleuca quinquenervia (broad leaved paperbark), are also very susceptible and are quite likely to be very adversely affected. As these species are extremely important in wetlands, where obviously conditions will be ideal for the spread of the pathogen, there are significant concerns about the functioning of these ecosystems and the fauna that depends on them. It highlights that a plant disease will not only affect the plant but also all of the other species that depend on that plant for food and habitat.

Dieback in Melaleuca quinquenervia caused by myrtle rust (Photo by R. Makinson)

Dieback in Melaleuca quinquenervia caused by myrtle rust (Photo by R. Makinson)

Future prospects
There is still an enormous amount that we do not know about this disease and its potential impact on native ecosystems in the long term. We need more information about the host range, how this varies with the age of the plant and with changes in environmental conditions. Some early work at the University of Sydney indicates that there are a range of susceptibilities in individuals in the same species – i.e. some individuals are more susceptible to the disease and some more resistant. This offers some hope that some plant species will be able to adapt to the presence of the disease.

Here at the Gardens one of the things we are focussing on ensuring that those species worst affected by the disease are preserved in the seed bank at the Australian PlantBank. However more research is needed on all aspects of the disease – what is the full host range, what impact will the disease have on susceptible species recovering from fire, and what impact will environmental and climate change have on the frequency and severity of the disease.

[Editor’s Note – please let GardenDrum know, by leaving a comment below, if you’ve grown or seen particular plants affected by myrtle rust. Queensland readers can also report MR sightings to DAFF]

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Dr Brett Summerell

About Dr Brett Summerell

Dr Brett Summerell is the Deputy Executive Director, Science and Conservation at the Botanic Gardens and Centennial Parklands in Sydney. He is also an Adjunct Professor at the University of Sydney, the University of New South Wales and at Kansas State University. He has a Bachelor of Science in Agriculture (Hons 1) and a PhD from the University of Sydney. Brett has been employed at the Sydney gardens for nearly 26 years, initially as a plant pathologist and mycologist. For the last ten years he has led the science and conservation programs there, including the National Herbarium of New South Wales and the recently completed Australian PlantBank. He maintains a research interest in plant diseases and the systematics of fungi and has published nearly 150 refereed papers in this area.

4 thoughts on “Myrtle rust – as bad as we feared?

  1. Bernard Chapman on said:

    I have a gardening and landscaping business on the North Shore of Sydney, and have only had one incident with myrtle rust on two topiary cones made from Syzgium ‘Bush Christmas’ at Northbridge. This annoying plant (not chosen by me), also gets very bad psyllid damage if it is not sprayed. Obviously the plants were small enough, so it was easy to spray. The infestation has been cleared up with Triforine. Only one application was necessary. This is around fifteen months ago now. This garden is full of lillipillis and I always use different hedge clippers for them to the other plants, so as not to spread the problem. My only other experience of Myrtle Rust is in a neighbouring garden to one I do in Pibrac Ave, Warrawee (Sydney). I believe the plant was Syzgium jambos. It has been treated after I pointed it out to the owner.

  2. Julie Parker on said:

    I have been growing Austromyrtus Blushing Beauty for a few years now. I love the plant and keep it despite its propensity to get Myrtle Rust a couple of times a year. I prune it back and spray with Copper oxychloride to protect the new growth.

  3. Eugene on said:

    I am sick of caring. When I realised my frown would have no impact on the scheme of things, my load lightened.

    Interesting fact I read the other day – since colonisation we have introduced approx. 27,000 new species of plants to this country. I think, perhaps wrongly and of course there are some colossal blunders, but, overall, in terms of bio diversity, this could be seen as a good thing?

    • Brett Summerell on said:

      Well the 27000 species increase diversity, and from a horticultural perspective had a wonderful impact in our gardens, but I think the impact on our biodiversity would have to be considered a negative taking into account the impact of weeds and diseases.

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