This is soybean rust disease. Soybean rust is a foliar disease caused by an obligate biotrophic fungus, Phakopsora pachyrhizi. The pathogen can infect many legume species. Soybean rust results in reduced photosynthesis and defoliation with up to 80% yield reductions in severe infestation. Soybean rust symptoms are commonly found on the underside of the leaf and first appear as small, water-soaked lesions that gradually turn into lesions 2-5mm in diameter. Within the lesions, spore-containing structures (uredinia) are visible with the aid of a dissecting microscope or hand lens. These will produce copious amounts of airborne spores that will go on to infect other soybean plants. Leaves will start to yellow, or become chlorotic, as the disease progresses. Severe infections cause premature defoliation and early maturation.
Soybean rust causes a reduction in photosynthesis and an increase in premature defoliation. This leads to fewer pods and smaller seeds, resulting in lower yields. In general, with every 10% increase in rust severity there may be a 10% reduction in yield. Yield losses range from 10-80%.
Using resistant varieties is the more economical and sustainable means of managing soybean rust. Currently only a few commercial rust-resistant soybean varieties are available that are adapted for Sub-Saharan Africa.
Cultural management methods include: planting early and using early maturity groups to avoid infection during the susceptible period (flowering), sanitizing equipment and fields that have had historically bad outbreaks, and rotating with a non-host, such as maize. Fungicides are the commonly used method to control soybean rust. Triazoles are recommended for single fungicide applications as they prevent new infections.
Triazole+strobulurin mixtures will provide longer residual protection than either fungicide alone. Fungicide applications should only be considered during late vegetative to R6 growth stages and only if environmental conditions are conducive to the pathogen. Spraying is not economically feasible if the disease has progressed to the upper canopy or if plants are at R6 growth stage. Fungicide availability will vary by location so please check with your extension agent for an updated list of available products.