Image: Rhodamnia dumicola with Myrtle Rust August 2019
By Spencer Shaw
Next year will be the 10th anniversary of Myrtle Rust being brought into Australia. Note that I say “brought into” Australia, as the terminology usually used is “arrived” or “was first detected”. The latter could be taken as inferring that Myrtle Rust arrived in Australia all by itself. This is clearly not the case; Myrtle rust was brought into the country as result of the global trade and travel networks of human activity. We brought it here and we also need to be the ones responsible for preserving the amazing species diversity of Myrtaceae that has evolved in this land and is now threatened by Myrtle Rust.
So just how are we going to preserve the diversity of Myrtaceae in Australia? I can only speak of our personal experiences in observing the life cycles and propagation of our local Myrtaceae in SE QLD, but I think we are onto something. And that is, that preserving species that are threatened by Myrtle Rust, can be tackled at a grass roots level (or at least at a tree and shrub level – pardon the pun), through the work of nurseries propagating Myrtle Rust resistant plants.
Back in 2011, when Myrtle Rust arrived in SE QLD, you could have been forgiven for thinking that nurseries were the only source of Myrtle Rust and that going near a nursery was potentially dangerous to you own health! Sure, nurseries could potentially speed the spread of Myrtle Rust due to interstate plant transport, but once established in NSW it was only a matter of time before trillions of microscopic spores blew across the landscape and spread rapidly to colonise whole new areas. To try and stem the tide, Biosecurity then quarantined a few nurseries, but the horse was well and truly bolted. Overnight the Myrtle name was tarnished, as the fear of Myrtle Rust spread. Some production nurseries disposed of all their Myrtaceae stock rather than trying to manage the disease, as the potential damage to their business from growing Myrtaceae outweighed the cost of throwing them all away. For those who persisted with growing Myrtaceae fungicide regimes became compulsory – although I’d argue that they mask the problem rather that treating it. What I’m trying to get to, in a an albeit circuitous way, is that nurseries were perceived as the problem, when in fact I believe that nurseries and horticulturalists (native ones in particular) have the tools to help assist in preserving the Myrtaceae species that are under threat.
Luckily in Australia, Myrtle Rust has yet to have a significant impact on the dominant tree and shrubs of our woodlands e.g. Eucalypts, Corymbia, Angophora, Lophostemon, Melaleuca, Leptospermum (although locally on the Sunshine Coast we have observed some impacts on Melaleuca quinquenervia). However, many of our rainforest Myrtaceae have been significantly affected. These include Gossia spp, Lenwebbia spp, Rhodamnia spp, Backhousia spp. Rhodomyrtus psidioides, Uromyrtus lamingtonensis, Decaspermum humile, Archirhodomyrtus beckleri and Acmena smithii. These species have been affected to varying degrees, with populations and individuals within species showing variable degrees of susceptibility. Species such as Rhodomyrtus psidioides are bordering on functionally extinct, with the majority of individuals struggling to maintain foliage, let alone produce flowers or fruit. At the other end of the spectrum, species such as Austromyrtus dulcis and Acmena smithii (which have only shown susceptibility over the last few years) only appear to be affected under irrigation in nurseries and not affected (at least as much) when planted.
Personally, given nearly 10 years of observations of Myrtle Rust and its impacts on our flora, I believe the only real hope for assisting in the preservation of Myrtaceae species that have demonstrated that they are vulnerable, is to assist those individuals and populations within a species that are demonstrating resistance and resilience. We (nurseries, horticulturalists, nature lovers in general) can assist these species by actively searching for, identifying and then propagating and cultivating those individuals. This cultivation whether it be in gardens, farms or revegetation projects will then assist their resistant and resilient genes to spread through their populations. Resistance to Myrtle Rust does appear to varying degrees within the species listed above. For example, in Acmena smithii vulnerability appears to be the exception, but Archirhodomyrtus beckleri is about 50/50 in our area. Our cultivated Rhodamnia dumicola and Gossia acmenoides can keep good leaf cover but are yet to produce viable fruit. Rhodomyrtus psidioides, which as mentioned before, appears to have very limited resistance across the majority of its population is really struggling.
Species that we have personally had success with so far include Decaspermum humile and Lenwebbia sp. blackall range, both of which we have selected and propagated from resistant individuals. We’re also working on a few Rhodamnia spp. – so fingers crossed there too! Another observation we’ve made is that if plants that have been affected by Myrtle Rust and can be nurtured to produce flower, fruit and then seed, then the resulting seedlings appear to be more resistant than their known parent. We don’t use systemic fungicides as part of our selection work, as discussed earlier it would only mask the problem. Selection of resistant individuals is the key to producing Myrtle Rust resistant / disease free stock. We do however have to use systemic fungicides to comply with our Biosecurity requirements with regards movement of Myrtle Rust infected stock… These are the only systemic pesticides we use in our nursery and on our property, to be honest we would much rather not (but have to, to meet compliance requirements). They have no benefit other than to kill potentially present Myrtle Rust on stock that will then be sent out into an environment where Myrtle Rust is present. Any beneficial fungi present (which could be a key player in controlling Myrtle Rust) will also be killed.
Nearly 10 years on, there is still talk about addressing Myrtle Rust on a national level and mainly just that – talk. Cynical as I am with regards the slow wheels and lack of focus nationally, I don’t rule out how much we could be doing including: stopping the next “myrtle rust” type disease from being brought into the country; co-ordinating the work done by nurseries and horticulturalists to select and breed myrtle rust resistant stock (e.g. funding research); and perhaps given the potential resources available federally, we could do the research that can help understand resistant genetics and maybe genetically engineer resistance into species that we would otherwise lose.
For the time being, however, the real action is taking place out in the bush as evolutionary processes select those individuals that are resistant Myrtle Rust to survive and reproduce. The best that we can do for now is get behind nurseries and horticulturalists who are passionate about our native flora. They are the front line in preserving our vulnerable Myrtaceae species. For 10 years we’ve seen major impacts to our rainforest ecosystems through the damage to Myrtaceae species and the loss of flowering and fruiting abundance they provided for our fauna, let’s hope we can improve that situation somewhat over the next 10 years…
