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