Soybean Rust - Symptoms, Disease Development and Management Considerations

Soybean rust is a disease caused by the fungus Phakopsora pachyrhizi and has been identified in Australia, Africa, Asia, South America and most recently in North America.

Soybean rust is an airborne fungal disease that causes lesions on the leaves, stem and pods, premature defoliation, fewer pods, lighter seeds and poor seed quality. Infection is most common on the underside of the lower leaves usually during the plant flowering stage. Lesions appear as irregularly shaped spots that develop spore-containing pustules. These lesions, usually confined to the veins or close to the veins, gradually increase in size and turn brown or reddish as the disease progresses. Soybean rust symptoms are similar to those of other common soybean diseases such as bacterial pustule, bacterial blight or septoria brown spot.

Within lesions on rust-infected soybean plants are pustule-like structures called uredinia. From these structures, masses of microscopic urediospores are released and transported by air currents to other soybean plants. Spores are capable of traveling long distances with prevailing weather systems. Climate and environmental factors are critical to the spread and severity of the disease. Long periods of leaf wetness are needed for spore germination, as well as temperatures of 59 to 86 °F. and a high relative humidity of 75-80%.

Urediospores are able to penetrate plant cells directly, not needing stomatal openings or wounds in the leaf, which allows infection to occur quickly. Rust pustules are produced 10 days after infection, and spores are released after three weeks. The host range of soybean rust is quite broad, with over 30 legume hosts. These plentiful hosts can serve as overwintering reservoirs for the pathogen, as well as build-up of inoculum. Rust spores typically cannot survive cold winters, but would likely over-winter in the southern U.S. and spread north with weather systems in the spring. The pathogen is not spread by seed.

• Fungicides are currently the main method of control against soybean rust.
• In affected areas like Brazil, growers with soybean rust problems are spraying their fields with fungicides two or more times during the growing season.
• Potential future varieties with genetic resistance or tolerance could be used as an alternative to or in conjunction with fungicide applications for control of soybean rust.
• Pioneer is leveraging its global research network to conduct varietal screening for soybean rust resistance against various isolates of the disease.