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Old enemies: potato blight

December 2009

When ‘potato blight’ struck northern Europe in the 1840s the world changed forever. Ireland was worst hit, where more than a million died of starvation and a million more left for America, which changed the population demographics of Ireland and the fledgling US, and caused a political storm that is still felt today.

Potato blight has caused misery to millions

More than 150 years later, we still don’t have the measure of Phytophthora infestans, the potato blight pathogen. Current annual worldwide potato crop losses due to late blight are conservatively estimated at $6.7Bn.

So far, it has not been possible to breed resistance into crops because natural variation in P. infestans has thwarted the potatoes’ defences. "What we have seen is an evolutionary arms race between a pathogen and its host and, so far, the pathogen has been winning," explains Professor Paul Birch of at the Scottish Crop Research Institute (SCRI), based at the University of Dundee.

Symptoms of Phytophthora infestans attack.
Image: SCRI

P. infestans is a curious enemy. It is commonly referred to as a water mould or water fungus, but is an oomycete, meaning ‘egg-fungus’, which is also inaccurate as the organism is more closely related to brown algae than fungus – their cells walls include cellulose, rather than chitin, for example.

The pathogen secretes ‘effector’ proteins which manipulate the target cells to establish disease and prevent a fightback. Decoding the entire P. infestans genome revealed that these fast-evolving effector genes are localized to highly dynamic and expanded regions of the genome, which is thought to underpin the rapid adaptability of the pathogen to host plants.

Research at SCRI and the University of Dundee has focused on finding effectors that P. infestans uses to enter cells. The RXLR protein is one of them, and genome analysis has revealed more than 500 effector genes, which scientists hope to use to find new ways to control potato blight.

The infestation (bright green filaments) reaches deep into tissues.
Image: Petra Boevink/SCRI

However, scientists have not given up on natural resistance. The SCRI team will also search through The Commonwealth Potato Collection for plants that contain surveillance proteins that recognise key P. infestans effectors.

And it’s not just potato blight on the radar. The Phytophthora genus also includes organisms that cause sudden oak death and soybean root rot. Related oomycetes cause the widespread downy mildew diseases in a number of food and commercially valuable crops, such as crucifers (mustard, cabbage), grapes and vine-type vegetables, such as hops.

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