Soybean rust (Phakopsora pachyrhizi)

Soybean rust: biology

Asian soybean rust is an invasive fungal disease caused by Phakopsora pachyrhizi, a virulent pathogen of soybeans. The disease life cycle begins when infected spores are blown onto healthy soybean plants. Long periods of leaf wetness are needed for spore germination, as well as temperatures between 15°C and 30°C and a high relative humidity. Under prime conditions, these spores mature six to seven days after infection of healthy soybean plants. Yield loss is very likely once rust is found in the mid-crop canopy. This disease has a short life cycle, which increases the likelihood of many generations being produced within a short period of time.

Infected plants are quickly defoliated, resulting in reduced pod set and fill. The results are reduced yield and seed quality. Left untreated, soybean rust can cause yield losses of 60-100% due to premature defoliation. Plants can become defoliated in as little as 9-14 days. However, preventative fungicide programs have been proven effective in managing the disease and protecting yields.

One of the challenges of dealing with soybean rust is that the disease is hard to identify. The early stages of the disease can be confused with other common foliar diseases such as septoria brown spot, bacterial pustule, downy mildew, frogeye leaf spot and bacterial blight. Early symptoms appear as yellowish mosaic discolouration on the upper surfaces of older leaves. As the disease progresses, infected leaves turn yellow, and brown or reddish pustules appear, generally on the bottom surface of the leaf.