Pathogen(s) causing disease:

Phakopsora pachyrhizi

Symptoms:

Early symptoms appear as chlorosis and brown flecking on the lower leaves in the canopy. The key diagnostic features of soybean rust are the cone-shaped pustules that form mostly on the undersides of the leaves and the dusty, light-tannish colored spores that erupt from the pustules. Tiny spots can appear at least 4 days after infection on the upper leaf surface and volcano-shaped pustules can be seen with a high-powered hand lens or microscope after at least 10 days on the lower leaf surface.

Conditions:

Rust is spread by windblown spores and has caused significant crop losses in many soybean-growing regions of the world. When pathogens are present, extended periods of cool, wet weather or high humidity favor disease development.

Management:

Deciding when you should spray fungicides is the most critical decision you will make in controlling rust. When weather conditions are right, rust can move rapidly within and between fields. Applying a fungicide before rust becomes prevalent in a field is necessary if yield losses are to be avoided. The rule of thumb is to spray when less than 5% of the leaves have three to five pustules present. If spraying is delayed then yield losses can be substantial. Most fungicides will control rust for only two to three weeks. So, an early spray may not be the wisest decision. Spraying prior to flowering is probably not cost effective and may lead to the need for a second spray.

Sources:

NC State
ISU