Timely scouting of soybean rust symptoms is a core component of successful management. Crops can suffer increasingly severe yield reduction the longer the fungus goes untreated. You can track the annual infection reports and historical maps of soybean rust at Soybean IPM PIPE.
Soybean rust has the potential to reduce yield by as much as 50% when environmental conditions are favorable.¹
Soybean rust (sometimes referred to as Asian soybean rust) is a difficult disease to control. The fungus that causes soybean rust produces spores that easily disperse in the wind, making it nearly impossible to prevent the fungal pathogen outright. Additionally, the fungus can infect over 95 species of plants that serve as alternative host, building inoculum even when fields are not in a soybean rotation.²
When left untreated, soybean rust causes premature defoliation, which inhibits photosynthesis, leading to yield loss.
Soybean rust life cycle
Soybean rust cannot survive outside of a living host, so all soybean rust infections begin at the site of another infected plant. When conditions are warm and humid, the fungus produces spores that disperse in the wind, sometimes traveling great distances, and typically settle on the leaves of maturing soybean plants.
While most “rust” pathogens enter plant tissue through small stomatal openings and penetrate cells after they’ve entered stems or leaves, soybean rust spores penetrate plant cells directly, which may help explain why this fungus can infect such a broad range of hosts.
Once on a host, spore germination and penetration needs 6 to 12 hours of leaf wetness and temperatures between 35 F and 82 F. Lesions typically produce spores within 7 to 14 days.³
Soybean plants can be susceptible to rust at any growth stage, but infections are often observed during critical soybean growth stages. Infected plants often produce fewer pods and seeds, and seeds are often lower weight as well.
Scouting for soybean rust symptoms
To spot the fungus, begin by searching the plants’ lower first leaves. Look for mosaic-like areas beginning to appear at or around the flowering stage. Lesions may appear anywhere on the plant but are most commonly visible on the underside of leaves. Soybean rust symptoms begin as gray-green, tan to dark-brown, or reddish-brown lesions that burst through the plant tissue. These spots or bumps tend to be angular and 2-5 mm in diameter. These lesions most commonly appear on leaves but can also be found on petioles, pods and stems. Every lesion has the potential to launch spores within 7 to 14 days.⁴ Severe weather events like hurricanes are thought to be able to rapidly disperse spores across a wide area.
More mature plants may begin to show signs of damage as pods set, and the infection can spread to the middle and upper leaves of the plant.
Be aware that early signs of soybean rust infection can easily be confused with bacterial pustules, bacterial blight and brown spot. These diseases also cause raised spots on the underside of soybean leaves. Experts require a hand lens or dissecting microscope to distinguish between them.
Experts are available to help you make your decisions. Reach out to your seed retailer, a nearby extension office agent or a seed company professional like your regional BASF representative.
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Endnotes
- Malvick, Dean. “Soybean Rust.” Extension at the University of Minnesota, 2018, extension.umn.edu/soybean-pest-management/soybean-rust. Accessed 9 Mar. 2026.
- Ariatti, Annalisa. “Soybean Rust Life Cycle.” Department of Plant Pathology and Environmental Microbiology, Penn State, plantpath.psu.edu/research/labs/ceal/research/soybean-rust/historial-risk-assessment/sbr-life-cycle. Accessed 9 Mar. 2026.
- Rosado Rivera, Yara, and Lindsey Thiessen. “Asian Soybean Rust.” NC State Extension Publications, 2 Sept. 2020, content.ces.ncsu.edu/asian-soybean-rust. Accessed 9 Mar. 2026.
- Rivera, Yara R., and Lindsey Thiessen. “Asian Soybean Rust.” North Carolina State Extension, 2 Sept. 2020, content.ces.ncsu.edu/asian-soybean-rust. Accessed 9 Mar. 2026.
