Septoria brown spot of soybean
Septoria brown spot is caused by the fungus Septoria glycines. It is the most common foliar disease of soybean. Disease develops soon after planting and is usually present throughout the growing season. Symptoms are typically mild during vegetative growth stages of the crop and progress upward from lower leaves during grain fill. Infected young plants have purple lesions on the unifoliate leaves. Lesions on later leaves are small, irregularly shaped, dark brown, and are found on both leaf surfaces. Adjacent lesions can grow together and form larger, irregularly shaped blotches. Infected leaves quickly turn yellow and drop. Disease starts in the lower canopy and, if favorable conditions continue, will progress to the upper canopy. Yield losses depend on how far up the canopy the disease progresses during grain fill. Septoria brown spot lesions on soybean leaves. Image: C. Grau
Septoria brown spot lesions. Image: A. Robertson
Septoria brown spot disease cycle.
The fungus survives on infected leaf and stem residue. Warm, wet weather favors disease development. Disease usually stops developing during hot, dry weather but may become active again near maturity or when conditions are more favorable.
There are no known sources of resistance, but differences in susceptibility occur among soybean varieties. The host range of the pathogen includes some other legume species and common weeds such as velvetleaf. Rotation to non-host crops such as alfalfa, corn, and small grains and incorporation of infested crop residue into the soil will reduce survival of S. glycines. Foliar fungicides labeled for Septoria brown spot control are available. Applications made during R3 through R5 soybean growth stages may slow the rate of disease development into the middle and upper canopy and protect yield.