Brown Spot (Soybean)

Septoria Leaf Spot

Septoria glycines

Soybean leaf infected by Brown Spot
Soybean leaf infected by Brown Spot. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
Brown spot is a common leaf disease and is easily confused with soybean rust due to the similarity in symptoms. Yield loss is generally low, reaching 5 to 8 percent in severe cases when plants are defoliated by the disease.
Brown spot is caused by a fungus that survives in crop or weed residues as well as seeds from infected plants. It prefers conditions of extended warmth and moisture.
Velvetleaf weed known to be a host for brown spot
Velvetleaf weed known to be a host for brown spot. Photo courtesty of F.D. Richards. {{CCBY-SA2.0}}
Initial symptoms of brown spot occur on the lower leaves of the plant and move upward as the plant grows. It begins as many small brown spots that often enlarge, merging together to form large and irregularly-shaped brown areas. The infected leaf usually begins to turn yellow and eventually falls off completely, The symptoms are most prominent during pod fill and maturation.
Soybean plant infected with brown spot
Soybean plant infected with brown spot. Photo courtesy of Daren Mueller. Licensed under CC BY 3.0.
Damage from brown spot is almost never severe, rarely causing yield loss greater than 8 percent. Damage tends to be greater in times of cooler weather, whereas in hotter climates, it will halt progression. Yield loss is caused by defoliation of mature plants.
Tillage is necessary for reducing the amount of crop residue available that may be hosting the fungus.
If infection is severe enough, foliar fungicides can be used to gain control of the fungus. With infections that happen later in the season during pod fill, strobilurin or triazole class fungicides may be used to avoid premature defoliation. Application of fungicides is not highly recommended since economic returns often are not comparable due to such a low yield loss rate from brown spot infection. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.
Planting continuous soybeans creates an environment very conducive to the growth and production of this fungus. Rotation with non-legumes is a good way to avoid infection. There are few, if any, resistant varieties. Planting varieties that mature more quickly can be a good way to avoid risk of disease.