Figure 1. Early symptoms of rust appear on lower leaves deep in the canopy. Photo: University of Wisconsin.
Click on image to view a larger version
Rust infection begins in the low to mid-canopy and moves up the plant. The first symptoms are typically small, gray spots on the undersides of leaves and along leaf veins (see Figure 1) starting around the flowering stage. The spots increase in size over time and change color from gray, to tan, reddish-brown or black.
It is important to recognize that these symptoms are not exclusive to rust. Other diseases of soybean including brown spot, bacterial blight, and particularly downy mildew all have similar symptoms and can easily be confused with soybean rust.
Soybean plants are susceptible to soybean rust at any stage of development, but symptoms are most common during and after flowering.
If symptoms are observed, look for signs (sporulation) of the rust pathogen
Lesions caused by the rust fungus enlarge, becoming tan or reddish-brown with a raised appearance. These pimple-like structures, called pustules, are diagnostic for soybean rust. When rust pustules are mature, they burst and release masses of spores into the air. Pustules contain powdery spores, which are diagnostic for rust in the field (see Figure 2).
As the plant matures and begins to set pods, rust symptoms can spread rapidly to the middle and upper parts of the plant. Lesions are found on petioles, pods, and stems but are most abundant on leaves.
Figure 2. Rust spores as viewed through a hand lens. Click on image to view a larger version.
In the early stages, it is easy to confuse the symptoms of soybean rust with symptoms of other soybean leaf diseases Detecting low levels of soybean rust typically requires incubating samples for 24 to 48 hours and observing them under laboratory conditions by a trained diagnostician.
Images of these foliar diseases can be found in Common Soybean Leaf Diseases and Soybean Rust (pdf) by Alison Robertson and Greg Tylka, Iowa State University
View more images of rust in the Photo Gallery