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Ever-present danger - late blight

The fungal pathogen that triggered the Irish Potato Famine in the 1840s is still a formidable crop killer, striking fear into the hearts of potato growers worldwide. Late blight (Phytophthora infestans) destroys an estimated 15 per cent of the annual potato crop worldwide; in developing countries alone the disease costs about US$3.25 billion per year in lost production.

Late blight quickly attacks the foliage of potato plants, and tubers (David Shaw)
Late blight quickly attacks the foliage of potato plants, and tubers
David Shaw

The ever-mutating late blight fungus has the power to appear out-of-the-blue and wipe out countries' entire potato industries in a few weeks. In 2003 it destroyed a trade in Papua Guinea then worth around US$11 million. Previously free of the disease, the country was one of the world's few remaining safe havens for growing potato. But spores from the neighbouring Irian Jaya region of Indonesia moved west-to-east across the highlands leaving a trail of destruction. Yield losses caused many smallholders, who relied on potato as a valuable cash crop, to withdraw from production and potato prices in the country shot up by 300-400 per cent. Even developed countries are not immune: 2007 was one of the worst years for late blight in the UK, when a particularly wet summer saw an unprecedented 300 outbreaks.

Be very afraid

"There is no doubt about it," explains David Shaw of the Sárvári Research Trust, who has spent a lifetime studying the disease, "farmers are scared of late blight, partly because it develops extremely quickly. It's able to spread from 0-100 per cent blighted foliage in just two-to-three weeks." Late blight thrives in warm, wet conditions and affects potato seed, foliage and tubers. It is a year-round threat in frost-free areas of tropical highlands and, in more temperate regions, invades crops following summer rains. The disease attacks on two fronts: leaf blight destroys foliage before tubers develop, while tuber blight causes potatoes to rot in the ground or in storage. The disease overwinters in infected seed tubers causing a new outbreak the following season.

During the growing season, late blight spreads when infected leaves produce sporangia, small lemon-shaped spores that become detached from infected plants by rain or wind. Windborne sporangia are able to travel distances of up to a few kilometres and, within a few hours of landing on wet potato leaves, penetrate cell walls. A second generation of spores can develop within four days followed by a third, fourth and maybe more; spreading from field-to-field, the disease is very quickly out-of-control. Little wonder that farmers are scared.

In developed countries potato crops are generally protected by regular applications of chemical fungicide, which kills spores landing on foliage before they penetrate the leaf. But, while the European Union's US$9 billion potato industry is underpinned by the US$1.5 billion farmers spend on fungicide, many farmers in developing countries, including Papua New Guinea, are unable to spray every three-to-five days as required. "The main problem for subsistence farmers is the poor availability and high cost of spray," continues Shaw. "So the tendency in the developing world is for farmers not to spray at all, or only a few times in a season instead of every week. In these cases it's far more important to adopt avoidance strategies, such as using early-maturing varieties, harvested before the blight arrives." This is no easy task.

Preparing for the worst

Tuber damage caused by potato late blight (CIP)
Tuber damage caused by potato late blight

Potato breeders have spent years working on varieties with late blight resistance only to find themselves outsmarted as new, more aggressive strains of the disease emerge. Shaw himself discovered a new form of late blight in 2005, a particularly destructive strain called Blue 13, which has since become a common form of the disease in the UK and Western Europe. Blue 13 accounted for 70 per cent of blight infection in the UK in 2007.

As well as the need for continual innovation in developing blight resistance, clean seed from disease-free stock is essential. In Vietnam, where good quality tuber seed is scarce, True Potato Seed (TPS) progenies - botanical seed from the berries of healthy parent plants - have proven a successful, cost-effective weapon in the late blight fight. The seeds are free from all soil- and tuber-borne diseases and transportation and storage is considerably cheaper than that of seed potatoes. However, the lack of uniformity in TPS tubers means it is difficult for Vietnamese farmers to reach the standards required for export and processing markets, but for food security and local trade, they have become an important part of potato production.

The best strategies to prevent, control and contain late blight are likely to combine several different approaches: resistant varieties and earlier maturation (in some regions) together with affordable, healthy seed, farmer training and on-farm monitoring to help raise the alarm quickly when a blight attack is imminent. In the meantime, the race to stay ahead of the ever-mutating disease is sure to keep the scientists on their toes and farmers on their guard.

Date published: September 2008


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