Samoan farmer Lomitusi Brown is happy to hear research has shown exports of fresh taro to Australia pose a very low risk of transferring taro leaf blight—and adopting the right harvest practices could reduce this further.
While fresh and frozen taro is exported from Samoa to New Zealand and the USA (including Hawaii), only frozen taro is currently permitted into Australia.
‘If the Australian market opened up for fresh taro it would give us as farmers a reason to plant taro at a commercial level,’ says Mr Brown from his farm at Falealili.
The findings come from an ACIAR-funded Small Research Activity (SRA) being delivered by the New Zealand Institute for Plant and Food Research Limited (PFR), the Scientific Research Organisation of Samoa (SROS) and the Samoa Ministry of Agriculture and Fisheries (MAF).
Taro is the most commonly grown vegetable crop in Samoa, eaten as both flesh and leaves and processed into flour. Taro leaf blight (TLB) is primarily a leaf disease which is capable of totally destroying the canopies of susceptible plants.
Exports of Samoan taro from varieties that are largely resistant to TLB are valued at A$3.1 million.
Exporting fresh taro to Australia is a priority for the Samoan Government as a more valuable and cost-effective option than frozen taro with its high refrigeration costs.
In 2011 the then Australian Department of Agriculture decided that fresh taro posed too high a risk of introducing Phytophthora colocasiae, the causal agent of TLB, and ruled that imports should be permitted only from areas declared free of the disease (in this case, requiring official ‘country freedom’ status).
But the SRA study completed in May 2019 has provided new information on how long P. colocasiae can survive on taro corms. Joy Tyson, PFR Senior Scientist (Plant Pathology), says the project studied the availability of viable inoculum from P. colocasiae in the field, its ability to infect and colonise corm tissue under different scenarios and, in the event corms were infected, its ability to survive the conditions of simulated transport to Australia.
P. colocasiae is temperature sensitive and does not grow in conditions below 10⁰C or above 35⁰C.
‘The only infective propagule of P. colocasiae is the zoosporangium, which is produced on leaf lesions at night under wet conditions such as rain or heavy dew, but our studies showed these sporangia are very fragile and die immediately they are dehydrated,’ Ms Tyson says.
‘Artificial inoculation of harvested corms with P. colocasiae showed that it could establish and progress through wounded corm tissue, but in field trials no such infections occurred.
‘The likelihood of infections via this pathway at harvesting is dependent on the availability of inoculum at harvest time, which is very low if harvest is carried out under dry conditions to eliminate viable sporangia in the canopy.’
Project leader Dr Bob Fullerton says that by adopting a systems approach to producing, harvesting and handling taro, Samoan farmers could reduce the risk of corm infection to negligible levels.
‘This includes only using varieties that are tolerant to TLB; reducing the sources of infection by regularly removing affected taro leaves; harvesting for export only during dry weather; and preliminary washing and cleaning of taro in an area that’s remote from plantations and any infected crops,’ Dr Fullerton said.
Other actions that would further minimise the risk of P. colocasiae surviving include washing with high pressure water as soon as possible after harvest; using a hot water dip—something that is already being evaluated as a treatment to eliminate mites and nematodes; and drying taro after treatment.
‘Existing import conditions for large-corm taro into Australia require topping to remove all leaf bases and growing points to prevent the corms being propagated, and this is also a vital component of the systems approach,’ Dr Fullerton says.
Dr Seeseei Molimau-Samasoni, SROS Head of Plant and Postharvest Technologies Division, says the Samoa MAF has submitted a copy of the report to Biosecurity Australia and is seeking its opinion on the level of risk and the proposed systems approach for reducing this further.
Dr Molimau-Samasoni says the need for more research on some of the critical steps in the pathway will depend on Biosecurity Australia and what it sees as being necessary to eliminate the risk of P. colocasiae entering Australia either on or inside fresh taro corms from Samoa.
These steps may include investigating the temperature/time tolerance of P. colocasiae to define the conditions required for successful heat treatment; conducting further studies on the survival of sporangia under different conditions in the growing environment; and verification that the corms cannot become infected during the growing season.
‘Ultimately, having the ability to market our taro into Australia means more opportunities for our farmers, which translates to better monetary returns,’ she says.
Dr Tyson says the Samoan community is already benefitting from the research as it has increased the capacity of Samoan scientists to study plant pathology of all crops, not just taro.
‘The ACIAR funding enabled us to upgrade the capacity of the SROS laboratory and install goodquality microscopes, cameras and other equipment, and reference books,’ she says.
‘The team of Samoan scientists are now very capable in general plant pathology and their ability to research post-harvest diseases, not just those that apply to taro, has been massively increased.'
ACIAR Small Research Activity: Defining the biotic constraints to fresh taro from Samoa gaining market access to Australia, HORT/2017/014.