Low-Tech Solution to Nematodes Shows Promise

Published online: Mar 02, 2022 Articles Erik Stokstad
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Source: Science

Potato cyst nematodes are a clever pest. These microscopic worms wriggle through the soil, homing in the roots of young potato plants and cutting harvests by up to 70%. They are challenging to get rid of, too: The eggs are protected inside the mother’s body, which toughens after death into a cyst that can survive in the soil for years.

Now, researchers have shown a simple pouch made of paper created from banana tree fibers disrupts the hatching of cyst nematodes and prevents them from finding the potato roots. The new technique has boosted yields fivefold in trials with small-scale farmers in Kenya, where the pest has recently invaded, and could dramatically reduce the need for pesticides. The strategy may benefit other crops as well.

“It’s an important piece of work,” says Graham Thiele, a research director at the International Potato Center who was not involved with the study. But, “There’s still quite a lot of work to take it from a nice finding to a real-life solution for farmers in East Africa,” he cautions.

Soil nematodes are a problem for many kinds of crops. For potatoes, the golden cyst nematode (Globodera rostochiensis) is a worldwide threat. Plants with infected, damaged roots have yellowish, wilting leaves. Their potatoes are smaller and often covered with lesions, so they can’t be sold. In temperate countries, worms can be controlled by alternating potatoes with other crops, spraying the soil with pesticides, and planting varieties bred to resist infection.

These approaches aren’t yet feasible in many developing countries, in part because pesticides are expensive and resistant varieties of potatoes aren’t available for tropical climates. In addition, small-scale farmers, who can make decent money selling potatoes, are often reluctant to rotate their planting with less valuable crops.

In Kenya, the potato cyst nematode has expanded its range and thrived. “The nematode densities are just so astonishingly high,” says Danny Coyne, a nematode expert at the International Institute of Tropical Agriculture. This is leading to an additional problem of biodiversity loss: Potato farmers are cutting down forests to create new fields free of the nematodes.

The idea that banana paper could help farmers rid their soil of nematodes was hatched more than 10 years ago. Researchers at North Carolina State University (NC State) were looking for a way to help farmers in developing countries safely deliver small doses of pesticides. They experimented with various materials. What works best, they found, is paper made from banana plants. Their tubular, porous fibers slowly release pesticides in the soil for several weeks before breaking down. By then, the plant has developed enough so that even if it does get infected, it already has a healthy root system.

In a field trial, researchers added abamectin, a pesticide that kills nematodes, to the paper. They also planted potatoes in banana paper without abamectin as a control. To their surprise, those plants grew nearly as well as the ones with pesticides. Coyne mentioned this puzzling result to a colleague, a chemical ecologist named Baldwyn Torto who studies the interactions between pests and plants at the International Centre of Insect Physiology and Ecology. “This is fascinating indeed,” Torto recalls thinking.

Together with Juliet Ochola, now a graduate student at NC State, Torto devised several experiments to figure out what was going on. The duo discovered the banana paper holds onto key compounds released from the roots of young potato plants, some of which attract soil microbes that benefit the plant. Nematodes have also evolved to notice these compounds. Some, such as alpha-chaconine, are a signal for nematode eggs to hatch. “If a lot of them hatch at the same time, they’re able to bust open the cysts,” Ochola says. After hatching, the young nematodes sense the compounds and use them to seek out the tender potato roots.

Banana fibers absorbed 94% of the compounds, Ochola and colleagues found. When they exposed nematode eggs to the exudates using the paper, the hatching rate decreased by 85% compared with not using the paper, the team reports today in Nature Sustainability. Other experiments suggested the nematodes that do hatch are far less likely to be able to find potato roots enclosed in the paper.

In nematode-infested fields in Kenya, Coyne and colleagues showed planting potatoes wrapped in plain banana paper tripled the harvest compared with planting without the paper. A tiny dose of abamectin in the paper—just five-thousandths of what would normally be sprayed on the soil—boosted the harvest by another 50%. Presumably, any nematodes that happened to come across the potatoes were then killed by the abamectin. “We’ve got a win here,” Coyne says.

Now, researchers are figuring out how to bring the wrap-and-plant paper to potato farmers in East Africa. Banana plantations in Kenya and nearby countries could supply the fibers, which are now discarded as a waste product. Paper manufacturers could then make the pouches. The biggest challenge, Coyne suspects, will be convincing farmers to buy the paper for the first time.

Once the farmers try the pouches, they’ll find them easy to use, the researchers say. “It’s just wrap and plant,” Ochola says. Simple, yes, but wrapping a lot of potatoes will still be laborious, notes Isabel Conceição, a nematode expert at the University of Coimbra. If a machine is developed to wrap the potatoes, she says, it’s possible the approach might also be feasible on larger farms that use mechanical planters.

Meanwhile, Coyne and his colleagues say they have encouraging results from trials with other tuber crops, such as yam and sweet potato. He also hopes many kinds of vegetables, planted as seeds or seedlings, could be protected from soil pests and pathogens with small pots or trays made from banana fiber, impregnated with various pesticides or biocontrol agents.

The appeal is natural: Banana paper is a biodegradable product, recycled from waste, and it could help protect both farmers and the environment. “We are reducing the amount of pesticides by so much,” Ochola says. “To me, I feel like that’s amazing.”