January 4, 2005

US, China Share Similar Soybean Rust Risks

The risk of soybean rust in Brazil is far greater than in the US because of its unique tropic and sub-tropic climates, according to X. B. Yang with the Department of Plant Pathology at Iowa State University.

However since China and the US have similar geographical size and latitude range for soybean production, a comparison between those two countries is more meaningful to what might happen in America.

Soybean rust overwinters in southern China, and the southern US is a potential overwintering area. After the pathogen establishes in North America, soybean rust epidemics are expected to depend on the long- distance dispersal of spores northward from the southern areas, similar to how they spread in China.

In China, excessive rainfalls have been associated with soybean rust epidemics, similar to what is reported in Brazil. To have a moderate epidemic - losses up to 30 percent - a minimum of 22 rain days in two months are needed.

In the US, the average rain days in the Midwest are less than 20 days in July and August. But the US has sufficient dew periods and cool temperatures during summer, which are favorable to rust infection. Two schools of opinions are popular if epidemics can occur in a season with plenty of dew but fewer rain days. This risk factor is yet to be understood.

The kudzu situation in the US is unique and remains a great uncertainty in the risk equation, but the plant is a very good host in Asia and South America.

In regions where soybean rust cannot overwinter, kudzu leaves that grow a month before soybean emerges in the spring may serve as a bridging host for spore increase and northward spread.