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Untitled Document

Scouting Tips
Soybean Growth Stages
Soybean Rust Glossary
How Close is Rust?
Lessons Learned
Yield Impact
Product Solutions
Application Timing
Scouting for soybean rust is an extremely difficult task.  Rust pustules are extremely small and can be seen only with a 20X or greater magnification glass.  In addition, rust initially infects the lower canopy first, so it’s easy to miss when walking through fields.

Soybean Rust
Early symptoms appear as very small brown flecks and may be associated with yellowish mosaic discoloration on the upper surfaces of older leaves.  As the disease progresses, the “flecks” may appear under the 20x hand lens as raised brown pustules resembling small “volcanoes.”  It is raised and has a pore on top, rather than a fissure. Soybean rust pustules also lack the yellowish halo associated with bacterial pustule.

Soybean Rust Symptoms
Soybean Rust
  • Starts in lower canopy.
  • Yellow mosaic on older leaves.
  • Brown, spotted appearance.
  • Leaves eventually turn completely yellow.
  • Brown or reddish pustules appear.
  • Generally much smaller yellow halo than what is seen with bacterial pustule, if halo is present at all.
  • Pore, not fissure, on top of pustule.
  • Spores issue from pore.
  • Pustules are raised like little volcanoes.

Distinguishing Soybean Rust from the Imposters
On top of the initial challenges of simply finding soybean rust on your crop, rust also resembles a number of other diseases. Bacterial pustule and Septoria look very much like rust.

Bacterial Pustule
  • Bacterial pustule infection begins in the upper canopy.
  • Infection begins as small, light green lesions.
  • Bacterial pustule is a fissure rather than the circular opening seen in soybean rust.
  • Fissure is surrounded by a yellow halo.

Bacterial Pustule   Bacterial Pustule
Bacterial Pustule
Photos courtesy of Glen Hartman, National Soybean Research Lab, Urbana, IL.

  • Infection has no raised pustules.
  • Infection does begin in the lower canopy.
  • Small, irregularly shaped brown spots or lesions.
  • The lesions may enlarge and merge to form necrotic blotches.
  • Infection can occur as early as the V2 growth stage.

Photo courtesy of Gail Ruhl, Purdue Plant & Pest Diagnostic Lab, Purdue, IL.

  Halo Lesions Canopy Location Infection Timing
Rust No Reddish brown Volcano-like pustules Lower Reproductive Stages
Bacterial Pustule Yes Fissure – water-soaked lesions containing mucilaginous bacteria Upper Mid-July, dependent on wet weather 
Septoria No Flat, irregular small brown spots Upper As early as V2 is your source for ag news and information on crop pest identification, commodity and market prices, agricultural news & issues, and access to crop production management tools such as crop yield & planting calculators and herbicide, fungicide, and weed control information.