Soybean Rust

Symptoms usually begin on the lower leaves as small, angular, tan to grey-green lesions bounded by veins, with corresponding yellowing on the upper surface. On the underside of these lesions, small raised pustules (uredinia) develop and release masses of pale spores. As the disease intensifies, leaves yellow and drop prematurely, often working up the plant from the bottom.

Phakopsora pachyrhizi is an obligate parasite that needs living host tissue and cannot survive on residue. It survives on legume hosts in frost-free areas and produces repeating generations of wind-dispersed urediniospores. These can be carried long distances to initiate infections in new regions, and under warm, moist conditions a new generation of pustules forms within a short interval, allowing rapid build-up.

• Requires living legume hosts and cannot overwinter on dead residue
• Survives on living hosts in frost-free areas between seasons
• Wind-dispersed urediniospores can move long distances into new regions
• Warm, humid weather drives rapid, repeating infection cycles

Soybean is the most economically important host, but the fungus infects a wide range of legumes, including common bean and various wild and cultivated species that can act as reservoirs. Warm temperatures combined with high humidity, frequent rain, and long periods of leaf wetness strongly favour infection and spread.

• Warm temperatures with high humidity and prolonged leaf wetness
• Frequent rainfall or heavy dew during the growing season
• Nearby legume reservoirs and overwintering hosts in frost-free areas
• Dense canopies that keep the lower leaves humid and shaded

Because soybean rust can arrive on the wind and intensify quickly, management relies on regional monitoring and early detection so that timely decisions can be made, combined with resistant or tolerant varieties where they are available.

• Follow regional rust monitoring and forecasting networks
• Scout the lower canopy regularly during warm, humid weather
• Grow resistant or tolerant varieties where recommended
• Avoid excessively dense canopies that hold humidity in the lower leaves
• Prioritise later-planted or later-maturing crops still filling pods