A new highly threatened enigmatic rust recognised from Chatham Islands forget-me-not (Myosotidium hortensia) on the Chatham Islands

Chatham Island forget-me-not (Myosotidium hortensia), Kaiangaroa Point. Image: Peter J. de LangeChatham Island forget-me-not (Myosotidium hortensia), Kaiangaroa Point. Image: Peter J. de LangeDuring January 2007 the Auckland Botanical Society visited the Chatham Islands. During their visit the late Dr Ross Beever, then a mycologist working for Landcare Research discovered a strange, orange rust growing on cultivated plants of Chatham Islands forget-me-not (Myosotidium hortensia) within the visitor car park gardens, Department of Conservation offices, Te One, Chatham Islands (Beever 2007).

The rust Beever discovered presents somewhat of an enigma. For a start, despite much survey effort it has to date not been found anywhere else on the Chatham Islands in wild or cultivated occurrences of Chatham Islands forget-me-not. The rust has also not been found on cultivated plants in New Zealand or reported from elsewhere in the world where this plant is grown. Another oddity is that while urediniospores have been seen no teliospores have been found making accurate determination of the rust, at least on morphological grounds difficult. Finally, the new rust represents the first record of any rust infecting Chatham Islands forget-me-not, and that despite 63 species of rust fungi having now been recorded for the Chatham Islands group (McKenzie 1991; McKenzie & Johnston 1999).

Last year, the mystery rust was finally described following DNA sequencing of it, which showed that it is a species of Puccinastrum. The rust is now known as Puccinastrum myosotidii (Padamsee & McKenzie 2014), and so far the only place it has been recorded from in the whole wide world remains the visitor car park gardens, Department of Conservation offices, Te One, Chatham Islands.

 

The sole Chatham Island forget-me-not (Myosotidium hortensia) at Te One, Department of Conservation visitor car park gardens known to be hosting Puccinastrum myosotidii. Image: Peter J. de LangeThe sole Chatham Island forget-me-not (Myosotidium hortensia) at Te One, Department of Conservation visitor car park gardens known to be hosting Puccinastrum myosotidii. Image: Peter J. de LangeAt this site, on the undersides of leaves of Chatham Islands forget-me-not, the rust is evident as small somewhat elongated orange pustules erupting from discoloured lesions on the leaf surface. These orange pustules produce the fine orange powder which is the urediniospores of the new species. As of July 2015 only one infected host plant was found supporting the rust, making this – at least for now – one of the most threatened rust fungi in the world. Not surprisingly, Padamsee & McKenzie (2014) wondered whether Puccinastrum myosotidii might be an introduced species. However, for now at least their analyses suggest it is an endemic of the Chatham Islands.

This means that rust, although undoubtedly a problem for Chatham Islands forget-me-not needs conservation attention, a view that might surprise some people. The idea that a plant disease is worthy of conservation is however not unusual, and indeed the Chatham Islands has similar, though less serious conservation issues with embergeria rust (Puccinia embergeriae) which parasitizes another threatened Chathams endemic Sonchus grandifolius. It is suggested that the Puccinastrum, with such a clear host specificity has been present on Chatham Islands forget-me-not for a long time, and such is contributing part of the unique Chatham Islands biota.

While people may not want this rust infecting their plants, until it can be found associated with wild populations of Chatham Islands forget-me-not the occurrence at Te One needs to be managed carefully to ensure the survival of the rust and its host.

 Puccinastrum myosotidii urediniospores (orange pustules) on the undersides of the leaves of Chatham Island forget-me-not (Myosotidium hortensia). Image: Peter J. de LangePuccinastrum myosotidii urediniospores (orange pustules) on the undersides of the leaves of Chatham Island forget-me-not (Myosotidium hortensia). Image: Peter J. de LangePuccinastrum myosotidii urediniospores (orange pustules) on the undersides of the leaves of Chatham Island forget-me-not (Myosotidium hortensia). Image: Peter J. de LangePuccinastrum myosotidii urediniospores (orange pustules) on the undersides of the leaves of Chatham Island forget-me-not (Myosotidium hortensia). Image: Peter J. de Lange

References

Beever, R.E. (2007) Notes on Chatham Island fungi and some plant pests. Journal of the Auckland Botanical Society 62: 28–31.

Padamsee, M.; McKenzie, E.H.C. 2014: A new species of rust fungus on the New Zealand endemic plant, Myosotidium, from the isolated Chatham Islands. Phytotaxa 174: 223–230.

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