Myrtle Rust and the Chatham Islands

On May 4 2017 Myrtle Rust (Austropuccinia psidii) was first detected in the North Island of New Zealand (Beresford et al. 2018). Its arrival had been anticipated; New Zealander’s had nervously watched the relentless spread of this rust as it skipped across the world from the Amazon to Florida (USA), then to Hawai’i, South Africa and eventually Australia. Once it reached Australia it was really only ever a matter of time before it reached New Zealand, rusts have always freely scooted across the Tasman Sea (Close et al. 1978).

So, what is a rust anyway? Most people who have gardens or grow crops commercially will know them as annoying diseases that can seriously disfigure or even kill plants under their care. Rusts are fungi, they take their common name from the usually orange, orange-brown, or reddish spores that they produce, usually on the undersides of the leaves or on the stems of the hosts they infect. The first sign of a rust infection is usually a small circular blemish, often expressed as a series of concentric rings of red, orange and green – known in the trade as ‘traffic lights’. When the rust is mature, spores are then produced from the infected host tissue.

The Chatham Islands has its own rusts, some are endemic to the islands, and even though they can be disfiguring they are an important part of the island’s natural history and ecosystems. One such rust is endemic to the Chatham Island sow thistle (Sonchus (Embergeria) grandifolia), known as Puccinia embergeriae, that rust was discovered by Dr(s) Eric McKenzie and Peter Johnston at Kaingaroa Point, in the early 1990s and described by them in 2004 (McKenzie & Johnston 2004). To date this innocuous rust has still only ever been found in the wild at Kaingāroa and, in 2018 at Ocean Mail, on account of an ongoing decline in its only known host Chatham Island sow thistle at Kaingāroa, this rust is now very close to extinction.

Puccinia coprosmae on Coprosma macrocarpa

Myrtle rust however, is something else altogether. Originating from the Amazon Basin, where it infects a range of hosts in the myrtle family (Myrtaceae) it has proved a nasty surprise wherever it has arrived. Why? Myrtle rust is rather unusual as rusts go. The typical rust is usually very particular about which host it infects, some like Puccinia embergeriae are extremes as that rust has only ever been found on one particular strain (genotype) of sow thistle, others, like the one that infects karamu (Coprosma spp.), Puccinia coprosmae mostly attacks Coprosma macrocarpa, C. robusta and on the Chatham Islands C. chathamica. Infact rusts are so particular about the hosts they infect that you can often identify the species simply by the host it is found on. Myrtle rust however, bucks the trend, it doesn’t just do one kind of myrtle, it seems to ‘jump’ across myrtle hosts. Since it’s arrival in New Zealand it has been found on 30 different hosts so far, 17 of these indigenous and 13 of them exotic (Toome et al. 2020). Infected hosts include such well known plants as pōhutukawa (Metrosideros excelsa), bottle brush (Callistemon, Melaleuca spp.), feijoa (Acca sellowiana) and mānuka / kāhikatoa (Leptospermum scoparium) – on the Chatham Islands it can be expected to seriously impact not only pōhutukawa and manuka but also Chilean guava (Ugni molinae) which would be somewhat of a mixed blessing depending on your point of view. Either way, as the rust takes hold we can expect that it will increase its host range – New Zealand has 28 native myrtles and an even larger more diverse array of introduced myrtles for the rust to sample.

It needs to be stressed here, that the ability of myrtle rust to attack multiple hosts from different genera is very unusual for a rust, rusts don’t normally skip so freely across distantly related hosts. So, this is why myrtle rust is such a concern. Not only can we not predict what myrtles it will infect, overseas experience has shown that it can, and does unexpectedly ‘jump’ from one host or host group of myrtles it is then hammering to another no one had thought it would touch.

The other issue with myrtle rust is that it can make its host very ill, and it can and often does kill its victim. Death is ‘by a 1000 paper cuts’, the rust invades new growth; developing flowers and young fruits are also favoured, soon covering the new growth with dark maroon / wine-red spots, from which erupt bright yellow powdery pustules of sporulating tissue (urediniospores). Each infection cycle kills the associated host tissue, such that over a short time the infected plant can no longer produce new growth. Once that has happened, all that is required is for the older mature leaves to age and die, and the plant can no longer make its own food, at which point it collapses and dies. Death of host plants is happening but even worse extinctions of host species is also now occurring (Fensham et al. 2020).

Already in New Zealand we are seeing this happening, there a favoured host is Lophomyrtus which is an endemic genus of two species, ramarama (Lophomyrtus bullata) and rohutu (L. obcordata). Neither species is native to the Chatham Islands but in New Zealand they are a locally important component of indigenous vegetation. Both species and the hybrids that form between them are also popular horticultural subjects on account of their unusual, bullate (‘bubbly’) leaves which in some selections can be coloured very dark maroon to almost black. Based on the last three years of myrtle rust observations, we are now beginning to suspect that, unless a cure is found, Lophomyrtus may eventually go extinct.

Even if the host is not killed, the repeated infection cycles will reduce flowering and fruiting, and that in turn will have a flow on effect into the ecosystems the trees occupy (Galbraith & Large 2017), as well as spoil one’s enjoyment of them in urban parks and gardens. Myrtle rust is serious, and as it has still actively spreading throughout New Zealand we really have no idea what the extent of its impact will be – though early indications are that it is looking pretty grim.

So, what does this mean for the Chatham Islands? The natural flora of the Chatham’s probably doesn’t have indigenous myrtles, and myrtle rust will only infect plants in the myrtle family. Of the few New Zealand myrtles found on the islands, only mānuka is possibly native. Nevertheless, the sporadic occurrences of it that I have seen around Waitangi, Henga, and Te Whanga are of plants that we have mapped back to past deliberate plantings of North Island stock. As any islander who has tried to grow manuka knows, it doesn’t actually do that well on the islands. We are not sure why, but one mycologist (fungal expert) the late Dr Ross Beever had suggested in 2006 that this may in part happen because the soil fungi that mānuka need to thrive are scarce on the Chatham Islands. This does suggest that there are no natural Chatham Island occurrences of mānuka but in fact in several small gullies draining off Monument Hill, above Point Somes, I have seen small stands of manuka, and I have heard of a few other such occurrences. The plants at Monument Hill are not the same match as the plants that I have seen elsewhere at Waitangi, Henga, and Te Whanga. So, I suspect that those little shrubs on Monument Hill may be natural. All the other New Zealand myrtles I have seen on the Chatham Islands are either planted or naturalized from plantings, e.g., pōhutukawa and northern rātā (Metrosideros robusta).

As of 2020 Myrtle rust has yet to be found on the Chatham Islands. It is however, really only a matter of time. It will arrive, or rather it will eventually be detected by someone, and based on the New Zealand experience, that detection will most likely happen in someone’s garden. Whilst it’s arrival won’t mean much for the island’s natural flora, islanders can expect to see declines in pōhutukawa. Also, I suspect that the long-standing plague of Chilean guava (Ugni molinae) in the northern Chatham Island is going to be affected, as myrtle rust likes that species in New Zealand (Toome et al. 2020). So that will be the plant for people to watch for myrtle rust, as the bright yellow sporulating tissue will be easy to see on its dark green red-tinged leaves.

The real issue for Chatham Islanders to consider though is what will happen to those myrtles that have been planted as timber or firewood, e.g., gum tree’s (Eucalyptus spp.) on the islands. Also, the suggestion that planting mānuka for mānuka honey on the islands may need careful consideration. Mānuka is attacked, and its close relatives in eastern Australia are now being seriously damaged. Whilst establishing a commercial crop of mānuka on the islands may sound like a good idea, there are already natural constraints affecting mānuka growth, even if these are overcome, what will happen if myrtle rust then takes hold? We don’t know for sure yet, but we can expect a reduction in flowering, and possibly, as a worst-case scenario, the wholesale death of plantings.

Current control options are also limited as the effectiveness of sprays is unclear. We know that frequent spraying needs to be done, and studies on spray efficacy in Australia were not that convincing (Carnegie & Pegg 2018).

So, what should you be looking for? Myrtle rust starts out as fairly innocuous maroon / wine-red ‘spotting’ on young leaves, sprouting buds flower buds, flowers and immature fruits. At this stage it is not easily detected, however, once it starts to produce spores it is easily seen as the sporulating tissue is bright yellow. The best way to look for myrtle rust is to select a range of accessible myrtle species and monitor them daily for signs of the rust but be sensible in your selection as it is simply too hard to check every host, especially if it’s a big tree. The tricky bit is not to touch the host tissue, and this is especially important if you think your plant has myrtle rust. If you think you have found myrtle rust then try and take some clear pictures and either post these on the iNaturalist (https://inaturalist.nz/home), phone the MPI Biosecurity Hotline 0800 80 99 66, or report the find to the Department of Conservation, Te One Office 03 3050098.

Remember, myrtle rust will only attack plants in the myrtle family.

References

  • Beresford, R. M., Turner, R., Tait, A., Paul, V., Macara, G., Yu, Z. D., Martin, R. 2018: Predicting the climatic risk of myrtle rust during its first year in New Zealand. Rust Pathogens 71: 332–347.
  • Carnegie, A.J.; Pegg, G.S. 2018: Lessons from the incursion of myrtle rust in Australia. Annual Review of Phytopathology 56: 457–478.
  • Close, R.C.; Moar, N.T.; Tomlinson, A.I.; Lowe, A.D. 1978: Aerial dispersal of biological material from Australia to New Zealand. International Journal of Biometeorology 22: 1–19.
  • Fensham, R.J.; Carnegie, A.J.; Laffineur, B.; Makinson, R.O.; Pegg, G.S.; Wills, J. 2020: Imminent extinction of Australian Myrtaceae by fungal disease. Trends in Ecology & Evolution. https://doi.org/10.1016/j.tree.2020.03.012
  • Galbraith. M.: Large. M. 2017: Implications for selected indigenous fauna of Tiritiri Matangi of the establishment of Austropuccinia psidii (G. Winter) Beenken (myrtle rust) in northern New Zealand. Perspectives in Biosecurity 2: 6–26.
  • McKenzie, E.H.C.; Johnston, P.R. 2004: Puccinia embergeriae sp. nov. on Chatham Islands sow thistle (Embergeria grandifolia) and a note on Miyagia pseudosphaeria on sow thistles (Sonchus spp.) in New Zealand. New Zealand Journal of Botany 42(4): 657–661.
  • Toome-Heller, M.; Ho, W.W.H.; Ganley, R.J.; Elliott, C.E.A.; Quinn, B.; Pearson, H.G.; Alexander, B.J.R. 2020: Chasing myrtle rust in New Zealand: host range and distribution over the first year after invasion. Australasian Plant Pathology 49: 1–10.

Associate Professor (Botany, Ecology, Plant Conservation, Biosystematics) at Unitec in Auckland and a former Department of Conservation scientist. Peter has been visiting the Chatham Islands since 1996 and is a current member of the Chatham Islands Conservation Board.