Potato cyst nematode is a dangerous pest. These microscopic worms live in the soil, penetrating the roots of young potato plants and reducing yields by 70%. They are also difficult to get rid of: the eggs are protected inside the body of the female, which turns into a cyst after death, it can persist in the soil for years.
The latest research has shown that a simple paper bag made from banana fibers prevents cyst nematodes from hatching and from finding potato roots. The new method increased yields by five times in trials on small farms in Kenya.
“This is an important part of the job,” says Graeme Thiele, director of research at the International Potato Center. But “there is still a lot of work to be done to turn this from a good find into a real solution for farmers in East Africa,” he warns.
Soil nematodes are a problem for many types of crops. For potatoes, the golden nematode (Globodera rostochiensis) is a worldwide threat. Plants with infected, damaged roots have yellowish, wilting leaves. These potatoes are smaller and often badly damaged, making them impossible to sell. In temperate countries, the nematode can be controlled by rotating potatoes with other crops, spraying the soil with pesticides, and planting varieties that are resistant to the infection.
These approaches are not yet applicable in many developing countries, in part because pesticides are expensive and resistant potato varieties are not available in tropical climates. In addition, smallholder farmers, who can make good money selling potatoes, are often reluctant to alternate planting potatoes with less valuable crops.
In Kenya, the potato cyst nematode has expanded its range and is thriving. “Nematode densities are incredibly high,” says Danny Coyne, a nematode expert at the International Institute of Tropical Agriculture. This leads to an additional problem of biodiversity loss, with potato farmers clearing forests to create new nematode-free fields.
The idea that banana paper could help farmers rid their soil of nematodes dates back more than 10 years. Researchers at North Carolina State University were looking for a way to help farmers in developing countries safely deliver small doses of pesticides. They experimented with different materials and found that paper made from bananas worked best. Its tubular, porous fibers slowly release pesticides into the soil for several weeks before breaking down. By that time, the plant has developed enough that even if it becomes infected, it will already have a healthy root system.
In field trials, researchers added abamectin, a pesticide that kills nematodes, to paper. They also planted potatoes in banana paper without abamectin as a control. To their surprise, these plants did almost as well as plants in pesticide-treated paper. Coyne shared this puzzling result with a colleague, an environmental chemist named Baldwin Torto, who studies pest-plant interactions at the International Center for Insect Physiology and Ecology.
Together with Juliet Ochola, now a graduate student at North Carolina State University, Torto ran some experiments to find out what was going on. They found that banana paper contains key compounds secreted from the roots of young potato plants, some of which attract soil microbes that benefit the plant. Nematodes have also learned to notice these compounds. Some, such as alpha-chaconin, are the signal for nematode eggs to hatch. “If many of them hatch at the same time, they will be able to open the cysts,” says Ochola. After hatching, the young nematodes sense the connections and use them to find tender potato roots.
Banana fibers absorb 94% of the compounds, Ochola and colleagues found. When they exposed the nematode eggs to exudate with paper, the hatch rate was reduced by 85% compared to controls, the team reports in the journal Nature Sustainability. Other experiments have shown that hatched nematodes are much less likely to find paper-covered potato roots.
In nematode-infested fields in Kenya, Coyne and colleagues showed that planting potatoes wrapped in plain banana paper tripled the yield compared to planting without paper. A tiny dose of abamectin in paper—only five-thousandths of what is normally sprayed on the soil—increased the yield by another 50%. Presumably, any nematodes that land on potatoes are killed by abamectin.
Now researchers are figuring out how to get paper to wrap and grow potatoes to farmers in East Africa. Banana plantations in Kenya and nearby countries could supply fiber, which is now being thrown away as waste. The paper makers could then make bags. According to Coyne, the biggest challenge will be convincing farmers to buy the paper itself.
Once farmers try the bags, the researchers say they will find they are easy to use. “Just wrap and plant,” Ochola says.
But packing large quantities of potatoes would still be labor intensive, notes Isabelle Conceicao, a nematode expert at the University of Coimbra. If a potato wrapping machine is developed, she said, perhaps the approach could be applied to larger farms using mechanical planters.