Septoria brown spot (Septoria glycines) overwinters and sporulates in soybean residue. The disease is spread by wind and by splashing rain that moves spores from the soil surface and residue onto plant tissue. Conditions that keep plant leaves warm (temperatures between 60° F to 85° F) and wet for long periods favor Septoria brown spot while hot, dry weather can often stop the disease cycle. The disease cycle continues on an infected plant during favorable conditions. Septoria brown spot symptoms are generally mild during early growth stages, but can appear on the cotyledons and unifoliate leaves. Spores developed on cotyledons and unifoliate leaves are the inoculum for later infections of trifoliate leaves, stems, pods, and surrounding plants.1
Small, irregular, dark-brown spots occur primarily on leaves. Symptoms occur first on lower leaves during warm, wet conditions and then progress to the upper leaves. Tissue surrounding lesions may be yellow. Late in the growing season, leaves become rusty brown or yellow and drop prematurely. Septoria brown spot symptoms are similar to bacterial blight (Pseudomonas sp.); however, bacterial blight symptoms appear on the upper new leaves while septoria brown spot infects older leaves in the lower canopy.2
Although the disease is common in the Midwest and incidence can be high, it is rare for the disease to cause significant yield loss. The severity of disease at the R6 growth stage indicates the effect on yield. Greater than 25 to 50 percent of premature defoliation can affect soybean productivity or result in smaller seed size.2 A 5 to 8 percent yield loss may be possible in severely diseased fields where there is much defoliation.3
Fungicides from the strobilurin family may prevent disease development if applied prior to symptoms.4 An application of a foliar fungicide is generally recommended at the R3 to R4 growth stage to provide an economic return if only one application is made. One application may protect plants for 2 to 3 weeks.5
Cultural practices to reduce the incidence of disease include selecting a soybean product with some tolerance to the disease. Tillage and rotation to non-legume crops can also help reduce the incidence of septoria brown spot.