Climate change

How is disease affecting soybean yields?

Soybean is one of the most important sources of vegetable protein globally. Valued as both a human and animal food source, soybeans are used to produce both soy meal and oil. The largest producers of soybean are the United States, Argentina, and my home of Brazil.

Protecting our soybean crops is of utmost importance to our farmers. The land area planted with soybean in Brazil has tripled during the last 25 years. Simultaneously to the increase in soybean acres planted, pests and diseases started to threaten the sustainability of the crop in Brazil.

irregular circles on a microscope slide
A slide plate showing the presence of Asian soybean rust spores. Traps collecting the spores help inform farmers about disease management. Credit: Michel Rocha da Silva

Asian soybean rust is the most important disease in soybean over Brazil. This rust, caused by a fungus called Phakopsora pachyrhizi, was first found in 2001 in the western part of Brazil. It can cause yield losses of up to 90% without controls. Typical symptoms of soybean rust are small tan‑colored lesions on the leaf surface and leaf chlorosis. This leads to premature defoliation while the plant is producing beans. Without leaves, the plant cannot participate in photosynthesis and provide energy for quality seed production.

Even if the fungus is present, it may not spread over the field unless the environmental conditions are favorable. Optimal environmental conditions for soybean rust infection include a temperature of around 50 to 82°F and dew period major than 6 hours.

Asian soybean rust reproduces by making spores. In 1985, Seiji Igarashi developed a specialized spore trap for early detection of this fungus in fields. The spore trap has been used to help farmers to define when it is necessary to apply fungicide based on all these factors.

A spore trap situated above a soybean field. The trap collects spores – if present – and helps farmers better manage Asian soybean rust disease in their fields. Credit: Michel Rocha da Silva

The spore trap is composed by a cylinder and a fan that forces the air through the cylinder. It is installed over the soybean canopy, inside the field. Weekly, a plate with a transparent glue is collected from the spore trap and analyzed for soybean rust identification. If spores are found and the environmental conditions were favorable for infection, the agronomist recommends that the farmer apply fungicide in the soybean field.

On-farm experiments compare the actual fungicide management vs the spore trap-based fungicide management. Some research fields are showing that we may be able to reduce fungicide application up to 50% using the spore traps. This is better for the environment and reduces costs for the soybean farmers.

Leaf with long dark spots
Rusts are a major disease affecting many crops. Credit: Chathurika Wijewardana

Asian soybean rust brought a lot of changes in Brazil’s soybean production system, but agronomists and farmers rose to the challenge. Besides developing the spore traps, farmers have restricted their planting dates. This helps with timing of environmental conditions to a major epidemic of the rust. They have implemented soybean‑free periods of 60 to 90 days, so the rust has nothing to grow on. Breeders have also developed soybeans resistant to Asian soybean rust.

The use of spore traps has been increasing during the last five years. The traps are low cost and reduce the need for treatments. Finally, there is much scientific information able to help farmers to improve the sustainability of soybean production. In addition, they have increased yield of this popular protein source. They have also increased their income, and/or reduced production costs.

Answered by

Michel Rocha da Silva, Federal University of Santa Maria – Brazil

About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.

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