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  • Leaf 'stamp' could detect crop diseases

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Crop diseases could be detected earlier and more easily if a new method that stamps a leaf with a colour-changing biosensor is successful.

The Bill and Melinda Gates Foundation awarded US$100,000 to Hideaki Tsutsui of the University of California, Riverside, earlier this month (9 May) to develop the early warning system for crop diseases.

Tsutsui's idea is to print biosensors directly onto maize leaves to detect pathogens such as aflatoxin. If contamination shows up, farmers will be able to isolate a plant to prevent its spread.

"The long-term goal … is to detect multiple plant pathogens or their markers by chemical reactions that turn areas of leaves different colors," Tsutsui told SciDev.Net.

He likened the basic mechanism to home pregnancy tests, which have areas that turn blue to indicate a positive result.

Fungi such as Aspergillus and Fusarium produce mycotoxins such as aflatoxin. Mycotoxins stunt children's growth, cause liver cancer and suppress the immune system. The UN Food and Agriculture Organization estimates that mycotoxins infect around a quarter of the world's crops every year, wasting one billion metric tons of food.

Tsutsui said that paper-based colorimetric tests already exist, but they can be used only once. He plans to develop a self-inking stamp that delivers the chemicals that detect the toxins directly into plant leaf veins, possibly via fine needles incorporated into the stamp to ensure the reagents get inside.

He estimates that a stamp that can be used 200 times might cost US$10.

Martin Kimanya, a researcher at the Tanzania Food and Drugs Authority who works on mycotoxin contamination, said: "When I read [about] this idea, I said 'Wow, this is good!'" He estimated that around 30 per cent of maize in Tanzania is contaminated with aflatoxin.

"They are approaching these issues very, very scientifically. If you can detect this early, then you can prevent this earlier … Post-harvest, the harm is already done."

Initially, Tsutsui plans to focus on maize, which is one of the most widely grown staple crops in Sub-Saharan Africa. The veins in its leaves run in parallel, and it is therefore especially suitable for simultaneous monitoring of different pathogens in adjacent veins.

The Gates Foundation's Grand Challenges Explorations fund provides grants of US$100,000 for innovative 18-month projects. Around ten per cent of phase one grant recipients are sufficiently successful to receive further funding of US$1million over two years.

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