Symptoms

Symptoms
Yellow areas appear translucent
Yellow areas appear translucent
Early symptoms begin on the lower leaves deep in the canopy as small lesions that increase in size and change from gray to tan or reddish brown usually on the lower leaf surfaces. Mature lesions consist of small pustules surrounded by a slightly discolored deadened area with masses of spores on the lower leaf surface. Lesions are scattered within yellow areas that appear see-through (translucent) if the affected leaves are held up to the sun. Once pod set begins on soybean, infection can spread rapidly to the middle and upper leaves of the plant.
Advanced disease produces spots usually clustered alongside veins.
Advanced disease produces spots usually clustered alongside veins.


After infection, pimple-like structures or pustules can be seen after about 10-14 days. The pustules produce masses of spores and spore production may continue for weeks. Spores are easily spread by the wind. Soybean plants are susceptible to soybean rust at any stage of development, but symptoms are most common during and after flowering. The disease usually starts within the low to mid canopy and moves up the plant.

Pustules (pimple-like structures) found underside of the leaf.
Pustules (pimple-like structures) found underside of the leaf.
Development
The development of soybean rust is favored by prolonged periods of leaf wetness (6–12 hours) and moderate temperatures of 60 to 85°F. Extended periods of cool, cloudy, wet weather and/or high humidity during the growing season would favor soybean rust epidemics. Weather conditions in Iowa will not always favor widespread or severe rust development, even when spores are present. Hot dry conditions will slow down or even stop disease development. However, dense canopies will provide an ideal microclimate that encourages disease development.

Look-Alike Diseases
For more information on look-alike diseases, click here