Figure 1. Brown spot developing on soybeans in a soybean on soybean field rotation. The picture on the right is rotated soybean across the road.
Soybean fields scouted last week showed brown spot (also known as Septoria leaf spot) starting to develop. This disease was more pronounced in soybean on soybean fields compared to rotated soybean (Figure 1). Brown spot developing this early in soybeans may lead to premature leaf drop, hence significantly reducing soybean yield.
Symptoms & Source of Inoculum
Brown spot symptoms start as small dark brown lesions. These can expand to form large irregular blotches (Figure 1, left picture). Continued expansion of these lesions leads to leaf yellowing and eventually premature leaf drop. Brown spot symptoms may be confused with herbicide injury symptoms. However, brown spot symptoms are found on the lower leaves often starting with the unifoliate leaves, whereas herbicide injury symptoms will be on all treated leaves.
Brown spot is caused by a fungal pathogen, Septoria glycines. This pathogen survives on soybean residue and is spread to plants mainly through rain splash but spores can also be blown from neighboring previous soybean fields. Infection is favored by warm moist weather, especially when plant surface remains wet for more than 6 hours.
Crop rotation with a non-host crop such as corn, wheat, or oats is effective in managing brown spot. The brown spot pathogen does not survive for more than one year on crop residue; therefore, crop rotation away from soybeans will greatly reduce the inoculum level. No resistance is available against brown spot but cultivars may vary in susceptibility. Fungicides labeled for the brown spot pathogen are available. However, timing is important in order to obtain benefits from fungicide application. For non-rotated fields showing a moderate level of brown spot infection on the first trifoliate leaf early in the season, a fungicide at R3 (beginning pod) may be beneficial. For rotated fields with a low level of brown spot observed in the lower leaves and when the soybean is at R1 (flower initiation), a fungicide may not be necessary. The North Central Soybean Disease Working group maintains a list of fungicides and their efficacy on soybean fungal diseases.