The potato late blight disease is successfully controlled in this healthy sequoia potato patch in Tambul with the use of Echo, a Chlorothalonil-based fungicide.
The potato late blight (PLB) disease, caused by a fungus, Phytophthora infestans, is considered one of the most economically-important diseases of potato in the world.
It is a century old disease and was responsible for the great potato famine reported in Ireland in the 1840s.
This airborne fungal disease is reportedly present in almost all the countries where potato is cultivated.
Papua New Guinea was free from the PLB disease until January 2003.
It was first reported in the Surinki plateau, Enga province, and within few months it moved very quickly into the Western Highlands Province and eventually to other parts of the highlands.
By May 2003, it was found in all parts of the PNG mainland wehre potato crops are grown.
It must be accepted now that the PLB disease is here to stay, and effective, affordable and sustainable management methods are needed to control it.
Nature and effect of PLB in PNG
PLB is a poly-cyclic disease (it can complete a single life cycle within three to five days and can have as many completed life cycles in one potato cropping season, producing a large number of disease spores).
The disease spores are easily transported by wind and can be carried over long distances in a short period of time.
This was evident in early 2003 where many potato gardens were completely destroyed within few months, causing the fall of the PNG potato industry which was estimated around K20 million - based on annual seed production figures.
One reason for this massive negative impact was that the industry entirely dependent on a single potato variety, sequoia, which is highly-susceptible to the PLB disease.
Research efforts in controlling PLB
The presence of the PLB disease in the highland has affected the livelihoods of many farming communities.
Many rural growers could not continue to cultivate potatoes.
Those that live in the high altitude regions (areas about 1800 – 2400 metres above sea level) suffered most as potato is considered their major source of food and cash income.
They were disadvantaged by lack of knowledge and unavailability of adequate resources to manage the disease.
Potato has then become a rich man’s crop; only those who can afford the costs associated with the use of chemical fungicides are able to grow potatoes.
Since the disease outbreak, NARI mobilised its resources with funding support from AusAID and the Australian Centre for International Agricultural Research to develop a cost-effective and integrated disease management strategy for PLB for potato farmers in PNG.
Cost effective fungicide use
Use of chemical fungicides was the option immediately available to control the PLB disease in PNG after its outbreak in 2003.
Many farmers still had stocks of sequoia seeds and needed a control method against the blight.
At that time, available chemical fungicides listed for use on PLB were tested on sequoia with the aim to develop a highly cost-effective fungicide application system.
Cost-effective in the sense that it is profitable, user friendly and can be sustained over a long period of time.
The evaluations were specifically conducted to assess the optimal application rates of different fungicides and wetting agents, the effective spray frequencies, effectiveness of fungicides with curatives properties against those with systemic mode of actions and the effectiveness of integrating systemic and protective fungicides.
Results from these trials showed that the Chlorothalonil based fungicides such as Barrack®, Echo® and Banis® were highly effective in controlling the disease. Although this group of fungicides are more expensive compared to copper-based fungicides such as Copper Nordox® and Kocide® , higher gross margins are achieved, hence higher profits for potato growers.
The Chlorothalonils effectively controlled the disease when applied at three to seven days interval.
The copper-based fungicides also worked well when applied at three to seven days but needed thorough coverage on plant canopy to ensure that the potato plants are protected well from invasion of PLB.
This requires a lot of time, skills and efforts which proved difficult for many farmers when potato plants grow into larger canopies.
The project also assessed the fungicide Agri-Phos® which has a systemic mode of action.
This group of fungicides seemed to have worked well on potato varieties with increased levels of resistance against PLB with much longer spray intervals at 14 to21 days.
The findings from the fungicide trials presented the opportunity to integrate different fungicides in a manner that can substantially reduce the cost incurred by fungicides in potato production.
Integrated disease management
The ultimate aim of the project on PLB management in PNG by NARI is to develop an integrated disease management (IDM) strategy.
This IDM would involve the combination of different disease control methods such as cultural control, sanitation (e.g. removal and proper disposal of plant debris) and fungicide application but central to the IDM concept is a suitable potato variety with good level of resistance to the disease.
NARI therefore imported 56 potato clones, specifically bred for PLB resistance from the International Potato Centre in Peru, South America.
These clones were evaluated under PNG’s growing condition and selections were made based on characters comparable to Sequoia which was used as a standard variety.
|A NARI research trial in Tambul showing rows of sequoia potato badly infected by PLB vs imported potato clones resistant to PLB.|
The performance of the CIP clones were assessed based on set characteristics acceptable for potato markets available in PNG under field condition through to post-harvest.
To date, eight different potato clones have been selected for farmer evaluation.
On-farm trials have been established in 12 different locations in Western Highlands, Enga, Eastern Highlands and Morobe provinces.
Once suitable potato clones are selected, an appropriate IDM strategy will be developed to control the PLB disease.
This should reduce the cost of potato production significantly and enable subsistence farmers to grow more potatoes again.