Issue No. 14, Article 9/June 29, 2007
Soybean Rust Scouting: Now Is the Time
We need to be alert because soybean rust is actively progressing in Louisiana, Texas, and Mexico. These areas are of particular concern to us in the Midwest. Our recent rains have the soybean crop growing well, and as the crop enters late vegetative growth stages, it is now the time to start scouting at least weekly for rust in your fields. As you are aware, unprecedented effort has been extended these past three years to drive home the point that scouting your soybeans is going to be essential to manage soybean rust. To supplement your efforts and to serve as an early warning system, we have 40 soybean sentinel monitoring plots in the state.
Having spent a fair amount of time on the road discussing soybean rust management and plans in Illinois, I know that many of you still need to make a commitment to scouting yourself or to hiring someone to scout your soybeans. Doubtless many of you are suffering from rust information overload, so let's get down to the basics of how to scout for rust in soybeans.
First, you will need a 20X hand lens, some zip-lock plastic bags, some paper towels, and an indelible marker. To use a 20X lens, place it at your eye and bring the leaf you are examining up to the lens (this is the opposite of how you use a 10X lens). To procure a lens, try doing a Web search for "20X hand lens"--several geological survey sites should come up. Or try Edmund Scientific, or get one through University of Illinois Pubs Plus or another vendor of your choice. I also recommend that you get a good scouting guide, because you will see a whole lot of things in the field aside from rust--the University of Illinois Field Crop Scouting Manual is available to guide your efforts. Additional publications I recommend are our handy soybean disease pocket guide (Pocket Guide to Soybean Diseases in the Midwestern U.S.) and, specifically for soybean rust, the severity-key Soybean Rust Assessment Tool. In addition to ordering them online, you can call 800-345-6087 to order.
It is important for you to be able to recognize the symptoms of soybean rust, which doesn't look like your average common field corn rust; the color and location on the plant are different. There are two types of lesions you may see on the bottom of a leaf as the disease begins. First is a pustule that, using the 20X hand lens, looks like a little volcano; it has tan spores that are produced from the top of the pustule. The second lesion may be reddish brown with no spores evident; it is also on the underside of the leaf. Soybean rust shows up on the bottom of the plant first, not on top. Its symptoms are fairly similar to those of two common diseases we see in Illinois. The reddish lesion may mimic Septoria brown spot, and the pustules may mimic those of bacterial pustule. Careful observation of these types of symptoms will be very important for early detection and identification of the disease in Illinois this year.
Begin your weekly scouting as soybeans enter the late vegetative stages of development. Once a week, in one contiguous field, scout five locations in a zigzag pattern, observing 20 plants in each location. Soybean rust starts on the bottom of a plant, so look carefully at the bottom of plants, on the underside of the leaves.
If you see something that looks suspect for soybean rust, follow this simple protocol. Collect 20 leaflets with suspect symptoms. Leaflets should be flat, dry, and placed between dry paper towels. Package the leaflets in two layers of zip-lock plastic bag, labeled clearly with a permanent marker providing the date of collection, location of the field (county, township, section, and nearest intersection), and location within the field. GPS information is helpful if available.
Through your local Extension unit office, submit samples to the University of Illinois Distance Diagnostics through Digital Imaging system (DDDI; www.dddi.org/uiuc). (You can find the office near you at web.extension.uiuc.edu/state/findoffice.html.) Ask them to use the special Soybean Rust Form for sample submission so that your sample will enter the fast track for observation. The results of your prescreening should be available within a few hours. If the DDDI prescreening appears suspect, your samples will be submitted to the U of I Plant Clinic. The Illinois Soybean Association (ISA) will pay your plant clinic fee if you use the DDDI prescreening, but an overnight mailing fee will be charged to you prior to the first annual detection in the state. (Following confirmation of soybean rust in your area, scout soybean fields using the same protocol and sample submission procedures. However, overnight delivery will not be required.)
You may of course submit samples directly to the University of Illinois Plant Clinic (plantclinic.cropsci.uiuc.edu) for a $12.50 fee, but we are using the county Extension units as points of entry to try to minimize the number of false samples and allow the process of detection and identification to run as efficiently as possible. Please note that we cannot process soybean rust samples from outside of Illinois. Samples collected in other states have to be diagnosed in the state where they were collected, and they cannot be transported across state lines.
In suspect areas, increase your scouting frequency to every three days. To get the most up-to-date commentary on the spread of rust in the soybean-growing states and in Illinois, you will want to monitor the national USDA soybean rust Web site. You can also find information localized to Illinois.--Suzanne Bissonnette