UW
Soybean Plant Health

Soybean Rust - Risk Assessment

Asian Soybean Rust

There are three key factors in determining the risk of soybean rust movement into northern soybean production regions:

1) the occurrence of soybean rust during the spring and early summer in the Gulf coast areas. This determines the amount of spores available to blow northward;

2) the July-August climate conditions, which establish where in the U.S. conditions are favorable for soybean rust development, and

3) the northward movement of soybean rust spores in weather systems and by "green-bridging".

The soybean rust pathogen has not been shown to be moved with soybean seed.  Thus, growers should not be reluctant to purchase seed grown in regions where soybean rust developed the previous growing season.

Wisconsin sentinel plots

Monitoring rust in Wisconsin
Until rust-resistant varieties are in place, a major decision will be whether the potential for rust merits the application of a fungicide. The USDA/APHIS, state agencies, and land-grant universities have prepared a national monitoring network to provide timely information on rust spore movement from south to north. This information will be combined with weather-based models that predict aerial transport of rust spores and potential disease severity, so that local spray advisories can be generated and updated regularly.

Wisconsin is part of the soybean rust sentinel plot network with approximately 20 plots in the state. Plots are monitored by county extension agents, Agricultural Research Station personnel, and UW-Madison staff. As of June 19, 2007, plans for monitoring for soybean rust are in place with sentinel plots established Buffalo, Chippewa, Clark, Columbia, Dane, Dodge, Grant, Green Lake, Jefferson, La Crosse, Marinette, Monroe, Outagamie, Pierce, Polk, Rock, Sheboygan, and Wood Counties.

Plots were monitored on a routine basis and the sampling protocol is designed to detect soybean rust at an incidence of less than 5%. The results of the monitoring are published on the USDA Pest Information Platform for Extension and Education (PIPE) website.

Weather conditions that favor soybean rust
Soybean rust will develop across a broad range of temperatures common to Wisconsin, but moisture conditions required for spore germination and infection are more restrictive and precise. 

The key to moisture conditions is the duration of continuous moisture on the leaf surface. Rust spores require 8 hours or more of continuous leaf wetness made possible by rain or relative humidity of >75 to 80%. Temperatures above 86°F may stop or slow disease development, especially if moisture conditions are limiting. New rust spores are produced 10 days after infection and are continually released from lesions if weather conditions are favorable...read more about the rust disease cycle»

Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: University of Wisconsin-Madison, Departments of Agronomy, Entomology, and Plant Pathology at  www.plantpath.wisc.edu/soyhealth.