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:

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

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