The Science of Horticulture
Learning to live with Myrtle Rust
In 2010, a new fungal disease was detected in Australia - Myrtle Rust. Affecting plants of the family Myrtaceae, the potential for harm in Australia was enormous. Not only do myrtaceous plants constitute a very significant part of our native flora (including Eucalyptus), many industries such as forestry and apiculture are reliant on them. Many native and some exotic Myrtaceae are also widely used in ornamental horticulture in Australia.
So, how is ornamental horticulture coping with the myrtle rust threat so far? Should we even continue to plant myrtaceous ornamentals in Australia?
How bad is it?
Myrtle rust has spread extensively in Australia since it's first sighting, and at the time of writing, has been recorded on 143 species .
The good news is that, so far, the impact of the disease has not been as bad as initially feared, and the nursery and garden industry may be able to cope .
There are chemicals now registered in Australia for control of Myrtle Rust. While they may be useful in maintaining clean nursery stock, existing landscapes or valuable specimens, they are not a long-term solution to the problem. The general trend worldwide is to reduce use of chemicals and in the case of Australian natives, low inputs and environmental values are among the reasons to plant them.
Selection of resistant lines
Looking for resistance, whether in currently available cultivars or in the breeding of new ones, will be an important strategy for living with Myrtle Rust going forward.
Australian plant breeding and marketing company Ozbreed have been working with The University of Sydney to evaluate some of their myrtaceous lines for susceptibility to the rust. There was considerable variation across the range of species tested, and within species. Callistemon viminalis, for example, appears to be generally more resistant than some other Myrtaceae, but the seven cultivars tested tested exhibited a range of reactions from moderately to highly resistant .
Ozbreed stresses that susceptibility will also vary with environmental conditions. They recommend that in tropical and subtropical climates only resistant or highly resistant cultivars be planted, given the severity of rust infections in those regions.
Nevertheless, the results indicate that there is potential for future selection and breeding of new rust-resistant myrtaceous ornamentals. Many resistant cultivars may already be in the marketplace, but testing and promotion are required to give consumers confidence in using this material. Rust resistance could be used as a marketable feature, just as psyllid resistance has been for many lilypilly cultivars.
An alternative is to avoid myrtaceous plants altogether, but this would leave a big whole in the garden designer's palette, especially when environmental responsibility (e.g. providing food for native birds and insects) and utility (e.g. low water and fertiliser demands of many species) are important.
No doubt there are many under-exploited natives from other families which could be useful additions to cultivated landscapes, but selection, evaluation and commercialisation could take many years. This could be present an opportunity for some nurseries, however. It could also lead to greater diversity in the landscape, which is helpful in reducing the impact of any disease. Who knows what's around the corner?
As myrtle rust is such a new disease, there must be much still to be learnt about it that could affect its management, besides further screening and breeding efforts. As far as the garden industry is concerned, the process of adapting to myrtle rust has only just begun.
Acknowledgment: Thank-you to Ozbreed Pty Ltd for providing information and feedback on this article.
References and Further Reading
 Myrtle rust
Queensland Department of Agriculture, Fisheries and Forestry
For the latest information on movement restrictions and control options, contact the appropriate government authority in your state. In Queensland, the Myrtle rust
section of the Department of Agriculture, Fisheries and Forestry website is good place to start.