Asian soybean rust

Lesions tend to appear in mid to late summer. Early symptoms appear as chlorotic and brown or brick red spots on the upper leaf surface on the lower leaves in the canopy. It is easier to see the spots when holding the leaf up to a light source so that the leaf is backlit. Developing lesions can be confused with symptoms caused by other foliar diseases, such as bacterial pustule, bacterial blight, downy mildew, cercospora leaf blight, frogeye leaf spot and Septoria brown spot. It may also be confused with abiotic factors such as burning herbicide damage. The key diagnostic features of soybean rust are the cone-shaped pustules that form mostly on the undersides of the leaves and the dusty, light-tannish colored spores that erupt from the pustules. Once the disease invades an area, there is only a 7-day period for effective rescue treatment. After one week plants are highly infested and yields are negatively affected. When large necrotic areas are present, it is an indicator that the disease is in an advanced stage and that the fungus is no likely to be controlled by fungicides. Among the most common symptoms are premature defoliation, poor pod filling, and smaller seeds. The premature defoliation and reduction in days to maturity cause plants to have lower seed weight and fewer pods therefore fewer seeds. Due to the leaf perforation of the pustules plants tend to lose a lot of water therefore are commonly moisture stressed. There are two types of reactions, tan and red-brown. The tan reactions are considered to be fully susceptible where as red-brown reactions are apparently partially resistant because there is not as much spore production as there is on the tan reactions.