CHAPTER 4: Disease Progress and Factors That Affect Soybean Rust Development

Phakopsora pachyrhizi is adapted to a wide range of temperatures (15 to >30 degrees C) and can cause yield losses in all regions in which leaf wetness occurs for more than 6 hours. The first lesions often are visible within 4–5 days after infection under favorable conditions and the first fruiting bodies (uredinia) and urediniospores within 6–7 days. Under severe inoculum pressure, it is almost impossible to prevent some yield loss. Severe soybean rust causes early defoliation, reduced pod set, and reduced number and size of seeds. Thus, rust affects all yield components. The extent of loss depends on the growth stage during which the plants are infected, the inoculum pressure, and the ensuing environmental conditions. Yield losses may be as high as 100%. The earlier the infection occurs, the greater and faster the defoliation and consequently the higher the yield loss and the lower the grain quality. In severe cases, when defoliation occurs during vegetative, flowering, early pod set, or the beginning of pod fill, rust can cause enough yield loss that harvesting may not be economically justified.