Asian Soybean Rust - Scouting

Backlighting may enhance the viewing of early symptoms

Timing

In northern growing areas begin scouting in mid-July and continue through mid-August. Soybean producers in southern growing areas will need to begin scouting before their counterparts in northern areas because of where Asian soybean rust is expected to overwinter.

Areas to scout

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

How to check a field for rust

1. Walk through the entire field in a standard Z or W scouting pattern, checking plants as you go.
2. Take a hand lens with you.
3. Look deep into the low to mid-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.
4. Inspect leaves on site if possible 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
5. Backlighting may enhance the viewing of early symptoms
6. Initial symptoms of soybean rust include small, gray spots, particularly on the undersides of leaves and along leaf veins.
7. Older lesions are may be larger and change color from gray, to tan, reddish-brown or black. It is important to recognize that these symptoms are not exclusive to rust. Other diseases of soybean including brown spot, bacterial pustule, bacterial blight, Cercospora and frog eye leaf spots, and particularly downy mildew can easily be confused with soybean rust.
8. If symptoms are observed, that is a signal to look for signs of the pathogen. It is absolutely necessary to observe sporulation in order to diagnose rust. Rust lesions mature to form pimple-like structures called pustules on the lower leaf surface. Use a hand lens to inspect leaves for active pustules containing the powdery tan spores of the rust fungus.

Adapted from: Scouting for Soybean Rust - Hints for Early Identification
Craig Grau, University of Wisconsin
Common Soybean Leaf Diseases and Asian Soybean Rust (pdf)
Alison Robertson and Greg Tylka, Iowa State University

Instruction for Sample Collection

If rust is suspected, work with your Certified Crop Advisor, your local extension agent, or certified professional agronomist to collect a sample for confirmation. Check here for contact information on the university-based plant disease clinic in your state.

Place leaf, stem or pod samples in a self-locking plastic bag and store under cool conditions. It would be helpful if leaves can be placed between paper towels or pieces of paper to keep them flat. Care should be taken to ensure the outside of the bags are not contaminated by the sample.

Samples that must be kept under ambient conditions should be sealed in a paper bag to prevent mold growth. Once they can be refrigerated, the paper bag can be place in a self-locking plastic bag.

Include the following information with the sample:

• date
• county
• exact location of the field
• sample location within the field
• host plant
• collector's name and phone number.

Source: USDA-APHIS

Information on these pages is from the cooperative effort of researchers throughout the North Central states. Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: NCSRP Plant Health Initiative at www.planthealth.info. Please do not use copyrighted photos without permission.