Brown spot is the most common foliar disease of soybean. Disease develops soon after planting and is usually present throughout the growing season. Yield losses depend on how far up the canopy the disease progresses during grain fill. Diseased plants are usually widespread within a field. 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 and dark brown, and are found on both leaf surfaces. Adjacent lesions can grown 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. Lesions on stems, petioles and pods are not as common, but appear as brown, irregularly shaped spots ranging from small specks to 1/2 inch in diameter. 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.
The best time to scout for this disease is V2 through R6. This disease is found in most fields in Iowa. Starts in the lower canopy.
Variety selection: There are no known sources of resistance, but differences in susceptibility occur among soybean varieties.
Crop rotation and tillage: The host range of Septoria glycines includes other legume species and common weeds such as velvent leaf. Rotation to non-host crops such as alfalfa, corn and small grains and incorporation of infested crop residue into the soil will reduce the survival of Septoria glycines. If tillage is an opinion, use conservation tillage practices to maintain soil quality.
Fungicides: Foliar fungicides labeled for 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.
Photo by Daren Mueller