Asian soybean rust: A review of 2005

Encyclopedia Article

After all the excitement about Asian soybean rust this time last year, the 2005 growing season turned out to be somewhat disappointing . . . at least from a plant pathologist's point of view! Jokes aside though, the following is a recap of what happened with Asian soybean rust in the United States in 2005 and so far in 2006.

Asian soybean rust was first reported in 2005 in the United States on February 24, on kudzu near Tampa, Florida. It was another two months, April 27, before the second report of the disease was made in Georgia on volunteer soybeans. After the infected plants were closely examined for a short period, they were destroyed. Two more reports of Asian soybean rust followed in June, including the first report of the disease on soybeans in a sentinel plot. In July came the first report of Asian soybean rust in a commercial soybean field in the United States. Very slowly, the disease spread north and slightly westward, and then up along the eastern coast. By the end of the growing season, the disease had reached as far north as Kentucky, as far east as the North Carolina coast, and as far west as eastern Texas--that is 773, 758, and 910 miles, respectively, from the first discovery in 2005 on kudzu in Florida. The discovery of the disease in Texas concerned many in Iowa because of the low level jet stream that travels up from that part of the country.

There are several things that were learned about Asian soybean rust in the United States in 2005.

  • Sentinel plots were critical for initial discovery of Asian soybean rust infections in almost every state. The initial infections were discovered when the disease was present on very few plants (incidence), at very low levels (severity), and always in the bottom of the canopy.
  • It was not possible to accurately identify soybean rust in the field when it occurred at very low levels. Suspect leaves were incubated in humid conditions and then observed carefully with a microscope for conclusive identification.
  • The usefulness of information obtained from spore trapping (with visual observation or molecular identification) for Asian soybean rust remains to be seen. "Rust-like" spores were trapped from fields in which the disease never occurred, and the disease occurred in fields near spore traps from which no "rust-like" spores were recovered.
  • Nearly every occurrence of soybean rust in the United States in 2005 was on soybeans that were in the reproductive phase of growth (flowering or later) and always occurred in the earliest maturity group varieties in plots where multiple maturity group soybean varieties were grown.
  • In several states in which Asian soybean rust occurred in 2005, university experts advised growers to apply fungicides; however, there were also some states in which the disease occurred, but fungicide applications were not recommended because of the timing of the infections.
  • Aerial and ground applications of fungicides in the southern United States were effective at controlling the disease.

Although Asian soybean rust is believed to have arrived in the United States via Hurricane Ivan in 2004, the highly active 2005 hurricane season appeared to have had little influence on disease spread.

For plant pathologists, the past 15 months have been history in the making. In 30 years, a new generation will be looking back at this disease scenario and learning from it, just as we learned from the southern corn leaf blight epidemic. But perhaps more historic than the disease development is the teamwork that developed among soybean pathologists, agronomists, and epidemiologists across the entire U.S. soybean production region. Information regarding soybean rust disease progress in the South was relayed to soybean specialists in the North on a daily basis in the form of e-mails and via teleconferences. In addition, the United States Department of Agriculture maintained a map of the United States (, illustrating where soybean rust had been reported, soybean growth stages, and comments from local university experts. Soybean state specialists provided updates on scouting, disease progress, and management in their respective areas.

The battle against Asian soybean rust has not cooled off since the 2005 soybean season came to an end. Throughout the winter, kudzu has been monitored for Asian soybean rust infection by scientists in the southern states. At the time this article was written, a total of 14 occurrences of Asian soybean rust on kudzu have been reported in Alabama, Florida, and Georgia. Thus, we start 2006 with a higher inoculum level than we did this time last year, and a higher inoculum level increases the risk of this disease throughout the United States.

As we look forward to the 2006 growing season, what can we expect regarding rust in Iowa and the rest of the country? One thing we know is that now is not the time to be complacent. Asian soybean rust is an aggressive disease; the Brazilians and South Africans can attest to that. We need to be prepared to react quickly and economically to protect our soybean crop should the disease occur in Iowa in 2006.

Spores of Asian soybean rust from lesions on a kudzu leaf. (Tom Isakeit)

This article originally appeared on pages 37-38 of the IC-496 (3) -- February 27, 2006 issue.

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