Brown spot is caused by the fungus Septoria glycines and may also be called Septoria leaf spot. It is the most common foliar disease of soybean. This disease can cause premature defoliation that contributes to yield losses when susceptible cultivars are planted and conditions favor disease development.
Symptoms start as dark brown, irregular spots on both upper and lower leaf surfaces. The lesions gradually darken to blackish brown and often develop a yellow halo around the leaf spot. Adjacent lesions frequently merge to form irregularly shaped blotches. Leaves become yellow to rusty brown. As the spots progress, leaf yellowing will expand far beyond the necrotic region. Defoliation of the lower and middle canopy can occur in severe cases. irregular shaped, brown lesions with undefined margins may form on the stem as well.
Brow spot can be confused with Bacterial blight (Pseudomonas syringae pv. glycinea). Bacterial blight lesions tend to remain small and multiply across the leaf surface with the chlorosis spreading only a short distance from the spots, and no fungal signs are present in the lesions.