UW
Soybean Plant Health

Scouting for Soybean Rust - Hints for Early Identification

Asian Soybean Rust

Accurate and timely diagnosis of soybean rust is critical to achieve control of soybean rust, especially if fungicides are involved in the management plan. Soybean growers and crop advisors should plan to scout frequently for rust.

Areas to scout

• Early-planted fields
• Early-maturing varieties
• Low-lying or protected fields with prolonged dew periods
• Fields with early canopy closure

Rust symptoms and signs start in the lower canopy, usually after the R1 growth stage. Click on image to view larger version.

Figure 1. Early symptoms of rust on a lower leaf along the leaf veins. Click on image to view larger version.

Figure 2. Rust is most likely to appear on the areas of the leaves that stay wetter longer: the bottom of a leaflet and close to major veins.

Figure 3. Backlighting helps to discern early symptoms of rust. Click on image to view larger version.

Figure 4. The spots caused by the rust fungus increase in size over time and change color from gray, to tan, reddish-brown or black. These symptoms are not exclusive to soybean rust, however! Click on image to view larger version.

Figure 5. Using a hand lens is important to detect signs of soybean rust. Read more on how to use a hand lens. Click on image to view larger version.

Figure 6. Actively sporulating rust lesions. Click on image to view larger version.

Check the lower leaves in the lower canopy
Leaves from the lower canopy will show symptoms first. Environmental conditions in the lower canopy favor spore germination and older leaves have likely been exposed to rust spores longer. Select leaves from the main stem only. Leaves from the lateral branches are less mature, even if picked from the same height.

Inspect leaves on site to avoid disseminating spores. Use a 10X or 20X hand lens to view symptoms. Rust is most likely to appear on the areas of the leaves that stay wetter longer: the bottom of a leaflet and close to major veins (Figures 1 and 2).

Read How to Use a Hand Lens»

Backlighting may enhance the viewing of early symptoms (Figure 3).

Look for early symptoms
Initial symptoms of soybean rust include small, gray spots, particularly on the undersides of leaves and along leaf veins (Figure 1). These can be visible on both sides of the leaf, and also on petioles, stems and pods.

The spots increase in size over time and change color from gray, to tan, reddish-brown or black (Figure 4). It is important to recognize that these symptoms are not exclusive to rust. Other diseases of soybean including brown spot, bacterial blight, and particularly downy mildew can easily be confused with soybean rust.
Which one is rust? View look-alike diseases»

If symptoms are observed, the next step is to verify the cause.

The next step is to look for signs (sporulation) of the pathogen
If symptoms are observed, that is a signal to look for signs of the pathogen. In the case of rust, look for evidence of sporulation. Tan lesions mature to form pimple-like structures called pustules on the lower leaf surface. Active pustules contain the powdery tan spores of the rust fungus

In the early stages of infection the emerging pustules look like miniature volcanoes topped with a pore (below). There is no yellow halo surrounding the pustule. Later, the pustules "burst" releasing masses of spores.

Use a hand lens to inspect the lower sides of symptomatic leaves for pustules that contain spores (Figures 5 and 6). These signs are diagnostic of rust.

It is absolutely necessary to observe sporulation in order to diagnose rust. Symptoms are a signal to look for signs, and signs are the only definitive means to diagnose soybean rust. Symptoms are not.

Advanced symptoms and signs
Initially, chlorosis (yellowing) is not apparant, but it develops over time as lesions age and become more abundant.

As plant canopies close and pods begin to set, the soybean rust fungus can spread rapidly from the lower to upper foliage of the plants.

Reddish-brown lesions might be observed. This is an inactive type of lesion, mainly composed of dead tissue and few pustules (Figure 7). The presence of reddish-brown lesions indicates that climatic conditions are not favorable for rust development. Or they may indicate that the soybean variety has some degree of partial resistance to rust, or the plant is too mature to support rust sporulation. Reddish-brown lesions may also occur when a fungicide is having a suppressive effect. Presence of reddish-brown lesions may mean a fungicide is not warranted at that time.

Figure 7. Reddish-brown lesions are an inactive type of lesion, mainly composed of dead tissue and few pustules.

Printable online diagnostic guides
Soybean Rust - Wisconsin Pest Alert (pdf)
Identifying Soybean Rust - ID card (pdf) with excellent photos, including "look-alike" diseases, prepared by the USDA, U.S. land grant universities, and the Ontario Ministry of Agriculture and Food
Soybean Rust - United Soybean Board (pdf file)
Common Soybean Leaf Diseases and Asian Soybean Rust (pdf)
Alison Robertson and Greg Tylka, Iowa State University

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.