(The following material was reprinted in part from the Minnesota Soybean Rust Response Plan.)
There are two pages of rust images to help with your detection:
Infection begins on the lower leaves of plants and appears as chlorotic or mosaic-like areas with uredinia observed, usually at or after the plant flowering stage. Lesions may appear on most above-ground plant parts, but they are most common on the underside of the leaves. As the plant matures and sets pods, infection progresses rapidly under the right environmental conditions (moisture, high humidity and temperature) to cause high rates of infection in the middle and upper leaves of the plant. Clouds of spores have been observed within and above canopies of highly infected plant stands.
Plants show two different lesion reactions to infection by soybean rust. Tan lesions consist of small uredinia surrounded by slightly discolored necrotic areas on leaf surfaces. Early stages show an ostiole, or small hole, where urediniospores emerge. As uredinia become larger, they release masses of tan urediniospores that appear as light brown or white raised areas. Uredinial pustules become more numerous with advancing infection and often will coalesce, forming larger pustules that break open and release masses of urediniospores.
The other type of lesion that occurs with soybean rust infection is the reddish-brown lesion. These lesions have larger areas of necrosis that are reddish brown surrounding a limited number of uredinia. A few urediniospores are usually visible on the surface.
Early symptoms of soybean rust are easily confused with bacterial pustule (caused by Xanthomonas campestris pv. phaseoli (Smith) Dye), or bacterial blight (caused by Psuedomonas glycinea Coerper), and brown spot (caused by the fungus Septoria glycines). The diseases also occur often on the underside of soybean leaves causing a raised light brown blister within a lesion. These leaf lesions vary from small specks to large irregular brown areas that form when small lesions coalesce. A hand lens or dissecting microscope is used to distinguish these disease symptoms from ASBR, but the early stages of disease are difficult to distinguish if no spores, conidia or bacteria are evident.Identification of Asian Soybean Rust
The LSU AgCenter and the LSU College of Agriculture