Asian soybean rust is caused by the plant pathogen Phakopsora pachyrhizi. The disease can result in significant yield losses ranging from 10-90% under favourable conditions. Asian soybean rust is a serious disease that threatens sustainable soybean production in North America. Currently, foliar fungicides are the only method of control as resistant seed varieties are not yet available.
Asian Soybean Rust begins as small (2-3 mm) yellow (chlorotic) spots that are irregular in shape. As the disease develops, these lesions turn brown or reddish in color. Bumps on lesions are visible. These bumps are called "uredia." Uredia are usually produced close to the veins and the rust-like appearance is given by the formation of the spores (urediospores). Uredia are more abundant on the underside of the leaf but they can also be found on petioles, pods, stems and the upper leaf surface.
Soybean rust causes premature defoliation, leading to serious yield losses, fewer seeds per pod, a decreased number of filled pods per plant and early maturity. Soybean plants are susceptible to rust at any stage, but the plant is most susceptible to Asian soybean rust from the beginning of flowering (first sign of reproductive growth) to full seed (pods containing seed), as the plant is investing energy in the formation of the reproductive structures.