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Issue No. 25, Article 6/December 3, 2004

Asian Soybean Rust Confirmation in Louisiana Raises Concern of Potential Impact of This Disease in Illinois

Note: This article was first published on the Bulletin Web site as an update to issue no. 24 (November 5, 2004). It has been modified to reflect the additional states in which soybean rust has been observed.

As widely reported on November 10, 2004, Asian soybean rust was confirmed for the first time in the continental United States. Soybean rust is a fungal disease of soybeans that infects leaves and can cause defoliation and significant yield losses. The soybean rust reported in Louisiana was Asian soybean rust, the aggressive species of rust (Phakopspora pachyrhizi), not the relatively mild American soybean rust (P. meibomiae). Since the discovery in Louisiana, the Asian form of soybean rust has been confirmed as far east as Florida and South Carolina and as far west and north as Arkansas and southeastern Missouri.

As has been said many times, the question has been not whether soybean rust will arrive in the continental United States, but when. Now we know when for the continental U.S., but we still do not know when soybean rust will arrive in Illinois--it may be in 2005 or not for another few years. Nor do we know how much damage it can or will cause in Illinois. The yield losses in Illinois may be significant, but they also may be much less significant than some have suggested. There are many questions that will not be answered before this disease arrives in Illinois. Regardless, the risk of soybean rust's occurring in Illinois in 2005 has now increased substantially with the discovery of the disease in Louisiana.

Here are a few key things to note:

    The Illinois Department of Agriculture, along with a team of representatives from several federal and state agencies, and University of Illinois Extension have developed a plan of action (Illinois Soybean Rust Program) to diagnose and manage Asian soybean rust if it arrives in Illinois. This plan can be found on the Web at http://www. agr.state.il.us/.
  • Soybean rust is not expected to survive over the winter in Illinois or adjoining states. Spores of the pathogen must be blown up from infected plants in the far south to initiate infections in the Illinois soybean crop. Some models suggest that the overwintering sites of soybean rust may be restricted to the gulf coasts of Florida and southern Texas, or in Mexico.
  • The climate over much of Illinois will not always be favorable for widespread and severe soybean rust epidemics.
  • Management of soybean rust will depend in the next few years on judicious use of fungicides. Appropriate fungicides applied properly at the correct time have been shown to control rust in other countries. Applications at the earliest time possible after rust is detected will be most effective. At least 6 to 10 different fungicides should be available for soybean rust management if they are needed in Illinois for the 2005 crop.
  • Highly resistant soybean varieties will probably not be available for a number of years; however, there may be varieties available sooner that have tolerance or partial resistance to soybean rust.
  • Much more information on soybean rust will be presented at conferences and workshops organized by University of Illinois Extension this winter. One to keep in mind is the Illinois Crop Protection Technology Conference, where sessions will cover soybean rust in depth (http://cptc. ipm.uiuc.edu/). Soybean rust will also be covered at the Southern Illinois Crop Management Conference (Effingham, February 8-9), the Central Illinois Crop Management Conference (Jacksonville, February 22­23), and the Northern Illinois Crop Management Conference (Malta, March 1-2). Contact your local University of Illinois Extension office to learn more about educational programs that will cover soybean rust in your area.

What are the risks of soybean rust's arriving in Illinois in 2005 and causing considerable damage in the state? There is no way we can know for sure because there are too many environmental, biological, and other factors involved to make a prediction with any degree of certainty. A report from the USDA-Economic Research Service, however, provides a useful outlook on the risks of soybean rust arriving in the upper Midwest and how much damage it may cause. Economic and Policy Implications of Wind-Borne Entry of Asian Soybean Rust into the United States examines how the economic impacts of soybean rust establishment will depend on the timing, location, spread, and severity of rust infestation and on how soybean and other crop producers, livestock producers, and consumers of agricultural commodities respond to this new pathogen. The risks appear to differ in different parts of Illinois and will not be the same every year.

Dr. X. B. Yang from Iowa State University and others suggest that the incidence and severity of soybean rust in the spring in the southern United States may be an indicator of whether the disease will become a problem in Illinois or Iowa later in the season. Two factors to consider are (1) that, to our knowledge, there have been no widespread and severe plant disease epidemics in their first year of detection after introduction into the United States and (2) that it's likely that before an epidemic will occur in Illinois it will take time for the rust pathogen to increase its population in the South to a sufficient amount in order to be spread north.

The following Web sites offer more information on soybean rust:

USDA

Plant Health Initiative

Soybean Rust: Is the U.S. Soybean Crop at Risk?

American Soybean Association

University of Illinois Department of Crop Sciences

--Dean Malvick

Author:
Dean Malvick

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