Soybean Rust

Phakopsora pachyrhizi

Soybean leaves infected with Soybean Rust
Soybean leaves infected with Soybean Rust. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
Soybean rust appeared in the United States in 2001 and has become a considerable threat to the production of soybeans. It causes leaves to be covered in tiny rust-colored spots, eventually killing the leaf. Losses have been reported to be as high as 80 percent in the most severe cases.
Soybean rust overwinters in infected crop residue. It does not survive in winters with freezing temperatures but is rapidly spread via wind across vast areas. Development can be explosive and cause rapid loss of leaves, usually following periods conducive to high moisture buildup.
Initial symptoms are small, brick-red spots on the lower leaves of the canopy during the flowering stage. These lesions generally begin at the base of the leaves and then continue to spread. The spots are visibly raised on the underside of the leaf surface. As spots cover the leaf area, the entire leaf begins to yellow and eventually falls off. In severe cases, the entire plant is defoliated.
Soybean Rust
Soybean rust spots are raised on the underside of leaves. Photo courtesy of Florida Division of Plant Industry. Licensed under CC BY 3.0.
Soybean rust is especially prevalent in the southern United States, but its windblown spores are rapidly spread north. The spread is dictated by the weather conditions. It quickly reproduces spores (9-10 days), allowing the infection cycle to repeat itself multiple times and spreading further each time. It has been considered as one of the most dangerous diseases for soybeans, causing yield losses up to between 30 and 80 percent.
Tilling will have little effect on soybean rust overall.
Currently the most effective method for dealing with soybean rust is to apply fungicides. It is crucial to select the right fungicide and apply it at the right time. Chlorothalonil is a fungicide that targets the pathogen's mode of action and must be applied directly to the rust, as it will not be taken up by the plant. Strobilurin fungicides can be taken up by the plant and are only to be used in preventative measures. Triazoles are taken up by the plant and kill the fungus currently on the plant, providing protection for up to 4 weeks. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.
Currently, there are no moderately resistant soybean varieties on the market. Planting earlier and in wider spaced rows can cause the soybean rust virus to have a much lesser effect on total yield. Wider spaced rows will enhance the canopy's ability to dry quicker. Small sentinal crops can be planted early near large fields, enabling the earlier-maturing crop to warn the farmer if soybean rust is present.