The wet weather across Indiana has been favorable for the development of foliar soybean diseases. Two diseases that are appearing in soybean fields are the fungal disease Septoria brown spot, (caused by Septoria glycines, and commonly referred to as brown spot), and bacterial blight. These diseases have similar symptoms, but it is important to accurately identify the disease issue in a field before applying a fungicide for disease management. In my recent field visits across the state, bacterial blight is the primary disease in the upper canopy, particularly in the northern part of the state, and brown spot is still confined to the lower canopy in soybeans across the state.
How do we distinguish between these two diseases? Leaves infected by Pseudomonas sp. bacteria have brown angular lesions that are surrounded by a yellow ring or halo, and may have a water-soaked appearance. As lesions age, they turn dark brown and fall out of the leaf tissue, giving leaves a tattered appearance (Figure 1). Bacteria survive on soybean residue and in seed, and enter plants through stomates and wounds caused by equipment or other mechanical damage, or from weather events such as heavy rains, wind, and hail. Long periods of leaf wetness and cool weather favor infection. Hot, dry weather will limit disease development. Yield loss may occur if disease is severe and plants defoliate. However, most fields in Indiana exhibiting symptoms of bacterial blight are only lightly to moderately affected by the disease and we would not expect to see yield loss due to this disease in these fields.
Symptoms of brown spot are typically observed in the lower canopy first, and are characterized by brown to black spots on upper and lower leaf surfaces (Figure 2). Lesions may or may not have the yellow halo of bacterial blight lesions, but leaves with lesions can turn yellow due to senescence (Figure 3). Research indicates yield reduction from this disease will be minimal if it stays confined to the lower 2/3 of the canopy.
Figure 2. Early symptoms of brown spot.
Preventative management options for both diseases include crop rotation, tillage, and planting less susceptible varieties. These methods can lower the risk of disease developing in the subsequent soybean crop. Fungicides will not manage bacterial blight. Fungicide applications for brown spot are rarely warranted and may not be consistently profitable.
Figure 3. Late symptoms of brown spot on soybean.