Soybean rust continues to get much attention from many perspectives in Illinois. In one sense this is perplexing because this disease is not in the continental United States and we have no clear knowledge of when it will arrive or how much damage it will cause relative to other present diseases if it does arrive. In another sense, we know that soybean rust is a significant problem in other areas, including South America and Africa, and we must be prepared to deal with it if and when it arrives in the United States.
Although much has been written and reported on soybean rust, I will provide a brief update from an Illinois perspective: How soybean rust could be moved into Illinois, how to scout for and diagnose soybean rust and how it can be managed if it comes.
Soybean rust potentially can come to Illinois by several routes: via storms and wind from South America or Africa, via the land bridge from South America, or via accidental or deliberate human transport (for example, on plant material or contaminated clothing). Much work is in progress to model and understand these potential paths for introduction of soybean rust. Although there are still many unknowns, for various reasons it appears unlikely that soybean rust will enter Illinois and cause widespread damage to the soybean crop in 2004. Still we don't know for sure what will happen, and there will be scouting activity in progress for soybean rust.
Soybean rust can be difficult to detect and diagnose, especially in the early stages of infection. This disease can begin at any time in the growing season when humidity is high and leaves have prolonged wetness. For early detection, look for yellowing, rust lesions, or pustules on the undersides of leaves in the lower canopy before flowering. The lesions often have sharp edges that are bordered by leaf veins, and the pustules in the lesions are pinhead size, tan to gray to brown in color. Several small pustules may be observed in the lesions, whereas only one pustule develops in lesions from bacterial pustule. Lesions can develop on leaves, stems, petioles and pods.
If soybean rust arrives, it will be critical that it be confirmed and diagnosed accurately, and it will be important to identify the species of the soybean rust fungus. Soybean rust is caused by two different rust fungi, which can only be differentiated with detailed laboratory procedures. The two species are Phakopsora meibomiae and Phakopsora pachyrhizi. P. pachyrhizi, the more aggressive of the two species, is the one that raises the greatest concern. Inaccurate rumors will (and already have) spread that soybean rust has been found in the Midwest, but this illustrates the point that a proper plan for sample collection and diagnosis must be followed to confirm the presence of soybean rust. Suspect samples collected in Illinois should be sent to the Plant Clinic at the University of Illinois in Urbana (217-333-0519; www.cropsci.uiuc.edu/research/clinic/clinic.html). After preliminary diagnosis, the first samples will then be sent to a USDA laboratory in Beltsville, Maryland, for verification of soybean rust and identification of the rust species present.
The first line of management for soybean rust will be fungicides, and in the long term we hope the answer will be soybean varieties with resistance to soybean rust. Currently three fungicides are labeled for soybean rust management in Illinois. These are Quadris flowable (active ingredient azoxystrobin), Bravo Weather Stik and Echo 90 DF (both with active ingredient chlorothalonil). Fungicides may have to be applied early in disease development and at least twice to get good control. A Section 18 emergency label application is being prepared to allow additional fungicides to be used for soybean rust control in Illinois. Much work to identify and develop soybean varieties with resistance to soybean rust is continuing. However, the presence of different races of the rust fungus is one of the factors making resistance difficult to develop.
If soybean rust becomes a widespread problem in Illinois, additional research-based information will be available to help understand and manage this disease. Further, an Illinois soybean rust response plan is under development by representatives from multiple public and private organizations to assist in coordination of information and a response to soybean rust.
The following references contain much additional information on soybean rust.
A good in-depth review: www.apsnet.org/online/feature/rust/
Soybean rust update from Iowa, March 2004: www.ipm.iastate.edu/ipm/icm/2004/3-22-2004/soyrust.html
Soybean rust update from Kentucky, March 2004: www.uky.edu/Agriculture/kpn/kpn_04/pi040308.htm
This site also contains links to multiple sources of good information on soybean rust.
Soybean Rust--What Are the Risks for 2004? (Ohio) February 2004: www.corn.osu.edu/story.php?setissueID=25&storyID=62
USDA information on soybean rust and strategic plan to reduce the impact of soybean rust in the United States: www.aphis.usda.gov/ppq/ep/soybean_rust/