Papua New Guinea (PNG) was one of the few countries in the world where potato crops remained free of potato late blight. That changed in February 2003 when reports of a disease devastating potato crops were received from the Sirunki area in the Enga province in the Western Highlands of PNG. Examination of leaf samples by scientists from NARI (National Agricultural Research Institute) and NAQIA (National Agricultural Quarantine and Inspection Authority) confirmed the identity of the pathogen as Phytophthora infestans (reported 5 March 2003 in ProMED-mail [http://www.isid.org], International Society for Infectious Diseases). The disease had spread to the Western Highlands, Mt Hagen, by late February 2003, then to in the Chimbu Province by March 2003, the Eastern Highlands Province by the end of April 2003 and by late May 2003 the disease was present in all five highland provinces (FPDC, 2003). The pathogen was probably of the A2 mating type because of the proximity to Indonesia where A2 mating type appears to be dominant (Pitt and Wicks, 2003).

The quick spread of potato late blight within the time span of a few months from west to east across the highlands of PNG had disastrous consequences for the potato producers in these areas. The combination of an aggressive pathogen, very susceptible host (cv. Sequoia), suitable environmental conditions and the absence of chemical control strategies brought serious hardship to almost all potato producers. The devastation of potato crops by late blight was such that there was no marketable yield from the crop at all (Pitt and Wicks, 2003).

In June 2004, the Australian Centre for International Agricultural Research (ACIAR) funded a trip by a team of four scientists to investigate the late blight situation and provide recommendations. The team consisted of Dolf de Boer (Primary Industries Research Victoria, Knoxfield, Victoria, Australia), Andre Drenth (Cooperative Research Center for Tropical Plant Protection, Indooroopilly, Queensland, Australia), Greg Forbes and Fernando Ezeta (International Potato Center (CIP), Lima, Peru). John Konam from the Cocoa and Coconut Institute (Rabaul, East New Britain, PNG) provided assistance. The following is a summary from their report.

The impact of the late blight epidemic on the PNG economy, and the smallholders in the high altitude regions is difficult to gage and cannot be understated. One of their few and scarce sources of cash income and, to a smaller degree, of a food crop in time of food shortages, has all but disappeared as a result of this epidemic. Farmers on very small holdings have not been able to grow potatoes because of both a lack of confidence and the lack of cash to buy fungicides and spray equipment. NGO’s and government officials are seeking ways to help small farmers get access to fungicides and spraying equipment.

To lessen the impact of late blight on potato production, changes are needed in the production system in PNG. New varieties with a greater level of host resistance to late blight, which require significantly fewer fungicide sprays than current varieties and greatly reduce the risk of crop loss, should be introduced. Late blight resistant clones are available from CIP. These have been selected for high levels of field resistance to potato late blight. Some of these clones are successfully grown in other tropical highlands of the world, such as Costa Rica, Bolivia, Ecuador, Colombia, Panama and Peru. A small number of these materials will need to be evaluated in the highlands of PNG.

Some commercial growers have been able to successfully grow potato crops to maturity by applying copper fungicide (cuprous oxide or copper hydroxide) sprays at intervals of 3-4 days. Some chlorothalonil was available but not widely used. Potato workers consistently mentioned that the primary fungicides in use were copper based. The two agricultural supply stores in Goroka had only these two copper fungicides and two formulations of chlorothalonil, which was much more expensive. The costs in June 2004 were: Nordox (58% cuprous oxide) US$ 5.52 per kg, Kocide (50 % cupric hydroxide) US$10.52 per kg, Barrack (72% chlorothalonil) US$27.68 per liter, Bannis (40% chlorothalonil) US$ 14.13 per liter.

The rational use of fungicides may have an additional benefit that has been observed by some farmers in PNG as well as in many other places of the world. Fungicides applied to protect the crop from late blight also have benefits for the control of other leaf fungal diseases such as early blight caused by Alternaria solani. This disease damages the foliage toward the end of the season shortening the growth cycle by about one month. PNG farmers using fungicides now are getting higher yields than ever before thanks to an extended growth season. A wider spectrum of fungicides needs to be tested and some input from suppliers may be needed to determine which products can be made available locally at acceptable prices. Since late blight in potatoes is a new disease in PNG little hands-on experience and capacity is available to manage this disease.

The team of four scientists prepared a project proposal aimed at introducing new resistant materials and assisting in the training of PNG researchers and extension workers. The proposal was submitted to ACIAR and is now being considered for funding.

Literature cited

FPDC. 2003. Potato late blight disease: A survey report, 10-13 March 2003. Goroka: Fresh Produce Development Company, Papua New Guinea.

Minemba, D. 2003. Potato late blight survey in Oksapmin area, Sandaun Province, unpublished survey report (NARI).

Pitt, A.J., and T. Wicks. 2003. Late blight of potato, Phytophthora infestans: Ag Challenge pty ltd, South Australian Research and Development Institute. Technical Assistance : 28 May to 6 June 2003, unpublished consultants report.