The brown spot pathogen, the fungus Septoria glycines, is common in soybean residue and spreads from the soil to young soybean plants by splashing rain. Infection occurs as early as the V2 growth stage on lower leaves, causing small, irregularly shaped brown spots or lesions (Figure 1).
In warm, wet weather, the disease may move up through the plant. Late in the growing season, infected leaves may turn rusty brown or yellow and drop prematurely. The spread of the fungus usually stops during hot, dry weather.
Sometimes, brown spot can be mistaken as bacterial blight but the two diseases are easy to distinguish because bacterial blight occurs on upper new leaves and brown spot infects the older leaves on the lower parts of plants.
Brown spot does not usually affect soybean productivity unless more than 25-50% of the canopy defoliates prematurely. Disease severity at the R6 growth stage is predictive of yield. Severe brown spot results in smaller seed size.
Management of Brown Spot
Soybean Variety. Symptoms of brown spot appear earlier in the season on early-maturing soybeans. Complete resistance has not been identified in soybean varieties or lines, but varieties do differ in partial or rate-reducing resistance which can be used effectively.
Crop Rotation and Tillage. Septoria leaf blight is more severe in continuously cropped soybean fields. The host range includes most species of Glycine, other legume species, and common weeds such as velvetleaf. For fields with very high levels of Septoria leaf blight, plow under soybean straw to promote decay.
Fungicide. Although rarely needed, fungicides applied at growth stages R3 and R6 effectively slow the rate of disease development. Thiophanate-methyl is registered in the USA as a fungicide to control Septoria leaf blight of soybean.