DNA barcoding to prepare for the invasion

Puccinia psidii or better known as myrtle or guava rust is a fungus with a wide and expanding host range within the tree family Myrtaceae, with over 440 host species currently known, 250 of those were reported from Australia only. It is native to Brazil but since the late 1970s it has spread into continental US, most of the Hawaiian Islands, Japan, Australia, China, South Africa, and Indonesia. 

Unlike other plant pathogens, rust usually affects healthy plants. An infection may cause deformities such as growth retardation, witches brooms, stem canker, or formation of galls. Plants with severe rust infection may appear stunted and discoloured. Rusts are considered one of the most harmful pathogens to agriculture, horticulture and forestry, and they are very hard to treat.

New Zealand has been afforded the opportunity to prepare for a myrtle rust invasion. The Ministry for Primary Industries' (MPI) High Risk Site Surveillance programme is tasked with providing early detections of unwanted plant pests of tree species present in New Zealand, and the programme has been put on a myrtle rust alert. The identification of species of Myrtaceae by morphological means can be time consuming and often requires a combination of characters, which may not all be present at a particular time. The more rapidly species of Myrtaceae can be identified, the more rapidly surveying for myrtle rust and a response to an incursion can take place. Accurate surveillance of infected hosts will enable MPI to consider appropriate biosecurity measures in order to protect threatened plant species and vegetation communities and to reduce the risk of spread to new areas by workers and visitors.

In an attempt to prepare for a potential but also likely incursion to New Zealand a team of researchers has started to assemble a DNA barcode reference library for more than 100 species of Myrtaceae occurring in New Zealand. The team used the internal transcribed spacer (ITS), the external transcribed spacer (ETS) and the barcode standard marker matK. New Zealand has only 24 native species of Myrtaceae but a large number (386) of non-native cultivated Eucalyptus species that are very susceptible to this pathogen.

The Myrtaceae DNA barcode database will enable rapid identification of large numbers of myrtaceous specimens in the event of a myrtle rust incursion. This will help MPI to accurately survey the spread of the disease and thereby enable timely decision making and consideration of biosecurity measures in order to protect threatened plant species and reduce the risk of spread to new areas.