Disease Identification

Symptoms of soybean rust are subtle and can be confused with several diseases common to Pennsylvania. Diagnostic features of soybean rust include tan to reddish-brown lesions containing fruiting structures that rupture the outer layers of the leaf tissue (Figure 2A). Reproductive structures may be more prevalent on the lower surface of the leaves. Magnification of at least 20X is required to see these features. Help with diagnosis is available at your local cooperative extension office, and samples can be forwarded to the Plant Disease Diagnostic Clinic at Penn State.

How to Check for Soybean Rust

Begin to check fields for soybean rust when the crop begins to flower (R1) and continue until pods containing full-size green seed are detected at one of the four uppermost nodes on the main stem with a fully developed leaf (R6). The following list outlines the scouting procedure.

1. Walk through the entire field in a standard scouting pattern such as a “W” or a “Z” shape.
2. Stop periodically and examine the soybean canopy for disease symptoms. Be sure to open the canopy and check the lower leaves for symptoms of rust.
3. Examine suspect lesions carefully for fungal reproductive structures rupturing through the out layers of the leaf tissue. Magnification of at least 20X will be required to see the key features of soybean rust.
4. Target areas with distinct yellow or brown discoloration in addition to standard scouting pattern. Areas of field shaded by trees or that have dense canopy should also be checked.

Figure 2A. Soybean rust. Tan to reddish-brown lesions (i) with fruiting structures rupturing through the outer layers of leaf tissue (ii). Lesions tend to be angular, are restricted by leaf veins, and diameter ranges from one-sixteenth to one-eighth of an inch. Lesions are often associated with leaf yellowing, and large numbers of lesions may result in premature defoliation. Pods, stems, and leaf petioles may also be infected.