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.
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.
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]
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.
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.
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]