Author: XB Yang, Department of Plant Pathology, Iowa State University
Fungicides are the only tool U.S. soybean producers will have to fight Asian soybean rust for the next several crop seasons. Therefore it will be important that producers and their advisors be able to correctly assess the risk of soybean rust as the season progresses. Since soybean rust is expected to spread from south to north during a growing season, growers in northern states will be able to continually assess the situation and adjust their decision making process.
The southern corn leaf blight epidemic recorded by William Moore in 1970 is the best example available for us to assess the likely spread of soybean rust. There are several similarities between the two diseases. Corn and soybean grow in same geographic areas in the same growing season. There was no resistance in corn to southern leaf blight and soybeans have no resistance to Asian soybean rust. The epidemic was recorded over the season and maps of it progress can be seen below. The red areas are infested regions by southern corn leaf blight in that epidemic. The front of red area on each date shows how fast this corn disease spread in that season.
There are three key factors in determining the risk of Asian soybean rust in the northern United States: 1) the occurrence of soybean rust during the spring in the United States Gulf region. This determines the amount of spores available to blow northward; 2) the July-August climate conditions in the northern United States, and 3) northward movement of soybean rust spores. Producers can assess risk of seasonal epidemics by using the following steps. Assessments made before March will be of little value for accurately predicting the spread of the disease in the 2005 crop season.
In March, monitor information on outbreaks of Asian soybean rust in Florida and southern Texas. This would be an early indication that rust spores would likely move to the Gulf Coast states and north. Also monitor rust outbreaks in the Caribbean Islands and Mexico.
April, May and June will be critical months. We will need to watch for soybean rust occurring in Louisiana, Mississippi and Alabama. These are potential regions that would act as a rust pathway to the north. Texas and Georgia also are regions to watch, but with less of an impact on the northern region. If outbreaks occur on soybean plants or kudzu in April and May, the spores are likely to reach northern soybean regions as early as in July. Rust outbreak in June in the south is important but not as great as that in May or earlier.
April and May are also important time periods to check the Climate Prediction Center at NOAA. July and August are critical months for the north central soybean region. Climate prediction can vary from one-half month to one year. I like the prediction with 3-month lead time. A prediction that includes normal to below normal temperatures and normal to higher moisture levels would be suitable for rust occurrence in Iowa. Climate predictions for July to August and occurrences of rust in the Gulf Coast regions have to be considered in risk assessment for soybean rust.
A soybean rust detection program is in place and establishment of sentinel plots has been proposed to ensure early detection. During a crop season, these programs should provide producers with timely information once rust is detected.
Keep in mind that airborne spores spread faster than the disease moves. A disease may be detected many days after initial infection occurs. The lower a disease level is, the harder it is to find infected leaves. An experienced detector can find Asian soybean rust at low levels. The more trained people scouting means we are more likely to find the disease, even when there is a lower level of infection.. Finding the disease in a timely manner will help us determine when and where to l effectively and economically apply fungicides to control the disease.
Dr. Yang is a professor and soybean pathologist. He has studied soybean rust since 1989.
He thanks Drs. Emerson Del Ponte, Palle Petersen Shimon Pivonia and Claudia Goody for reviewing this article and Barb McBreen and Jean McGuire for editing the article.
