June 22, 2004

Soybean Rust Not Likely To Impact US State Iowa This Season

Iowa and other prime U.S. soybean-growing states face very little risk of a soybean rust epidemic this year, according to a soybean rust researcher.

Iowa State University plant pathologist X.B. Yang, who has studied soybean rust since 1989, said in a story on the Iowa Farm Bureau Web site, it is impossible to predict the disease's arrival in the U.S. First, there must be confirmed reports of rust north of the equator, he said.

A report from the University of Illinois last week said the U.S. was facing an increased threat of a soybean rust infestation this growing season. The report, though, was based on unconfirmed rumors of the fungus having already spread north of the equator into Venezuela and Brazil's Roraima state.

When the disease moves north of the equator, Yang said, prevailing winds are more likely to carry rust spores over the land bridge from South America to North America.

The U.S. Department of Agriculture is planning to dispatch a research team to South America in June to scout for soybean rust in the areas where the most recent rumors originated, Yang said. If rust is actually confirmed in those regions, the disease still probably would not spread into the Midwest in time to cause major damage this year's crop, he said. Any U.S. rust infestation is likely to show up first in the South where it will have the greatest impact, Yang said.

The fungus flourishes at temperatures in the 70s and 80s Fahrenheit with adequate moisture, and poses the most risk to soybean plants around the flowering stage while under the right weather conditions. Potential yield damage is relatively insignificant after the seed-fill stage.

Historically, it takes over a month for an airborne crop disease to spread from the southern U.S. to Iowa, Yang said in the story. Although unlikely, if rust enters the southern U.S. this season, Iowa farmers will have sufficient time to prepare fungicides for controlling rust.

Rust spores generally travel only about 20 miles per day. The spread of rust requires the presence of a large number of soybean plants or other hosts, such as kudzu, as well as weather-related factors, such as wind currents and moist conditions that can bring spores down to the ground, the story said. Although the fungus cannot survive winter weather in the northern U.S., it can survive on kudzu plants along U.S. southern coastal areas. Experts say the spores will then spread into the interior states during the growing season, but will emerge at the same places every year.

In Brazil this year alone, the disease has caused an estimated $1 billion in damage while estimated yield losses can range from 5% to 80% under the right conditions. Two fungicides named Bravo and Quadris are currently labeled for soybean rust control. Several other treatments are expected to receive emergency exemptions from the Environmental Protection Agency in the event rust is confirmed in the continental U.S.