UW Soybean Plant Health
Septoria leaf spot, also called brown spot, is a common soybean disease that occurs most years on the lower leaves of soybeans. It is caused by the fungus Septoria glycines, and is recognized by small, irregularly shaped brown spots or lesions on soybean leaves (Figure 1). These spots may enlarge and merge to form necrotic blotches (Figure 2).
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Figure 1. Leaf spots caused by the fungus Septoria glycines . Photo credit: Brian Hudelson, UW Plant Disease Clinic. Click on photo to view a larger version. |
Under favorable weather conditions (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 is usually halted during hot, dry weather.
Agronomic Effect of Septoria Leaf Spot
Septoria leaf spot does not usually affect soybean productivity in Wisconsin. In some years the disease can cause premature defoliation, which can be confused with normal maturity. Yield loss occurs most often in high yield environments and is related to timing and rate of defoliation. Nationally, yield loss estimates are in the 8 to 15% range and occur when 25-50% of the canopy prematurely defoliates. Disease severity at the R6 growth stage is predictive of yield. Severe brown spot results in smaller seed size.
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Figure 2. Leaf spots caused by Septoria glycines may merge and form large necrotic blotches. Photo credit: Ohio State University |
Look for irregular, dark brown spots on both upper and lower leaf surfaces (Figure 1 and 2). Adjacent lesions frequently merge to form irregularly shaped blotches (Figure 2). Leaves become rusty brown.
Symptoms of Septoria leaf spot can also develop on stems and pods of plants approaching maturity. Stem and pod lesions have indefinite margins, are dark in appearance and range in size from flecks to lesions several inches in length, but they are not distinct enough to be diagnostic. Seed are infected but symptoms are not conspicuous.
Management of Septoria Leaf Spot
Soybean Variety
The onset of Septoria leaf spot symptoms is influenced by the relative maturity of the soybean variety, and symptoms appear earlier in the season on an early-maturing variety. 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
Septoria leaf spot 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.
Fungicide
Application of fungicides to soybean foliage from bloom to pod fill has effectively reduced the severity of Septoria leaf spot and subsequently increased grain yield. Thiophanate-methyl is registered in the USA as a fungicide to control Septoria leaf spot (brown spot) of soybean. Fungicides, if applied at growth stages R3 and R6, effectively slow the rate of disease development
Tillage
For fields with very high levels of Septoria leaf spot, plow under soybean straw to promote rapid decay.
References
Lee, G.B., and G.L. Hartman. 1996. Reactions of Glycines species and other legumes to Septoria glycines. Plant Disease 80:90-94.
Sinclair, J.B., and G.L. Hartman. 1999. Brown Spot. In G.L. Hartman, J.B. Sinclair, and J.C. Rupe (editors) Compendium of Soybean Diseases. 4th ed. American Phytopathological Society Press. St. Paul, MN.
Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: University of Wisconsin-Madison, Departments of Agronomy, Entomology, and Plant Pathology at www.plantpath.wisc.edu/soyhealth.
Last update 3/07