Sigatoka Leaf Disease A diagnostic timeline

It’s been nine years since New Scientist published an article declaring the possible demise of the world’s favourite fruit from pests and diseases within a decade (‘Going Bananas’, New Scientist (2003), Issue 2378, pages 27-29). By Dr Juliane Henderson, Queensland Alliance for Agriculture and Food Innovation, University of Queensland.

THIS ARTICLE STIRRED the emotions of banana scientists dedicated to the ongoing protection of banana. The published opinion was a hot topic of conversation in horticultural circles across the globe and public responses emerged from some very prominent banana researchers. Meanwhile, in Australia we got on with the tasks of crop protection and safeguarding banana health and production, buoyed by the now added challenge of proving the sensationalist New Scientist article wrong.

Australian bananas enjoy a ‘clean green’ reputation which is largely due to the diligence of growers and the industry as a whole of keeping a number of important pests and diseases out of the country.

Banana producing countries from around the world envy Australia’s track record and regularly seek assistance and advice from our plant pathologists with prevention, control and management of disease problems. Legislation underpinned by surveillance, diagnostics for the fast detection and identification of pathogens and preparedness in case of incursions, also play an integral role in keeping Australia disease-free .

Australia had its last brush with Black Sigatoka in 2001. A well-coordinated effort by growers, surveillance crews and diagnostic staff resulted in the swift eradication of the disease and area freedom from Black Sigatoka has been enjoyed again since. It is this effort that has been recognised worldwide and Australian experts are frequently asked to provide advice to international banana growing countries hoping to replicate our success.

Diagnosis of Black Sigatoka during the 2001 eradication campaign relied on a combination of traditional skills of plant pathologists experienced in recognising symptom and fungal structures under the microscope, and molecular methods which are designed to recognise the pathogen’s genetic material, or DNA.

High rainfall in Tully washed the fungal spores from the lesions on the leaves causing up to half of the 15000+ samples collected to need confirmation by DNA testing. This highlighted the importance of having both traditional and DNA methods available.

Ms Kathy Grice (DEEDI, Mareeba) using microscopy to diagnose black Sigatoka during the 2001 Tully eradication campaign. Microscopy remains a vital part of the diagnostic process.

ABOVE & CENTRE: Advanced diagnostic methods are required to differentiate between closely related pathogens of the Sigatoka leaf disease complex. Yellow Sigatoka (above) is difficult to distinguish from Eumusae leaf spot disease (centre), especially when fungal structures are not present on the leaf.

ABOVE RIGHT: It is important to collect diagnostic testing and surveillance data to prove area freedom from diseases such as Black Sigatoka. This field shot, taken in Ecuador in 2010, shows the impact of the disease in countries where it is endemic. DNA-based technology was used to diagnose more than half of the 15000 samples collected during the 2001 Tully black Sigatoka eradication. These methods have now been succeeded by less labourintensive and more reliable DNA technologies.

In addition to Black Sigatoka (caused by Mycosphaerella fijiensis), a very closely related disease, Eumusae leaf spot (caused by Mycosphaerella eumusae) is spreading throughout southeast Asia and in some cases causes more damage to bananas than black Sigatoka. Since we have Yellow Sigatoka (caused by Mycosphaerella musicola) already in Australia, it is very important that we can reliably and rapidly distinguish between these three pathogens.

Since the Tully outbreak in 2001, we have continued to upgrade our DNAbased diagnostic formats to improve the quality and speed of diagnosis, and to include detection of the exotic threat, Eumusae leaf spot disease.

Fortunately, these tests have not yet been needed in an emergency response but they are being used for routine surveillance of samples collected from the Torres Strait Islands, Papua New Guinea and other offshore locations as well as for onshore disease surveillance. Not only is it important to be able to detect if diseases are present in Australia, it is equally important to show which pathogens we do not have in Australia. These days documented proof of absence is required to make the statement that a particular pathogen is known not to occur in Australia. The availability of tests which can detect these exotic pathogens, coupled with surveillance in banana growing regions and in Northern Australia in general, linked with proven diagnostic performance of these tests on samples collected in countries where these pathogens occur, enables us to form a solid foundation in this area.

Diagnostic technology has developed rapidly over the last decade and we need to stay abreast of new technologies in this area. We also continuously check the quality of our assays and keep a watch on closely-related leaf spot pathogens which may cause similar symptoms or be found co-infecting in leaf lesions.

These related pathogens can compound diagnosis by presenting as cross-specificity and rare false positives, and can compromise the ability to quickly and reliably recognise our target pathogens. To address this, starting in 2012, investigation of a new DNA-based diagnostic method will begin as part of the Banana Plant Protection Program. Using a technology called pyrosequencing, we hope to be able to quickly identify specific pathogen DNA signature sequences in a sample of leaf.

Development of this technology for Sigatoka pathogens is possible due to the availability of a large database of DNA sequences collected over time by Australian researchers and their overseas collaborators.

Sequencing an organism’s DNA is the ultimate diagnostic tool, but has not been cost-effective in the past. Pyrosequencing has now emerged an attractive option due to its high throughput, relative low cost, high reproducibility, superior accuracy and low labour outputs. Currently used in clinical applications including oncology, neurology, gastroenterology, cardiology and neurology, pyrosequencing will allow a level of accuracy not previously seen in banana diagnostics.

You’ll be hearing more about pyrosequencing, as well as progress in other fungal, bacterial and viral disease research as diagnostic activities progress further in the new Banana Plant Protection Program.