Knowing More about the Soybean Rust Fungicide Options can help Producers better Manage the Disease.

By Loren J. Giesler, Extension Plant Pathologist
Thomas J. Weissling, Adjunct Research Professor

    * Resistance Management Issues

If Nebraska soybean producers find themselves needing to apply a fungicide for soybean rust this year, it will be important to choose an effective product based on the stage of disease development in the specific field. Timing of the first application should depend on when soybean rust is detected in nearby areas of the U.S. Sentinel plots will be place throughout U.S. soybean producing states to serve as early indicators of potential problems. It will be important for all producers and crop managers to be aware of where soybean rust is being detected to pursue a treatment that provides the maximum return on investment for any fungicides being applied.

Based on experiences in other parts of the world, it is critical that a fungicide application be made prior to significant rust development in the field. The critical window of protection for the soybean crop will be from flowering (growth stage R1) through full seed (growth stage R6). Some researchers in South America suggest that at 20 percent disease severity, the leaf damage is already too great for a fungicide to help. Currently, there is no efficacy data from the United States so all recommendations are based on fungicide trials in South America. Fungicides available for Nebraska soybean fields are listed in Table I.

One fungicide group often discussed for soybean rust management is the triazoles. Triazoles include products with myclobutanil, propiconazole, tebuconazole and tetraconazole. These products provide rapid systemic activity with the ability to kill the rust fungus once it infects plant tissues. A triazole or triazole-strobilurin mix should be used if soybean rust is present in the field.

The second group of fungicides is the strobilurins which include azoxystrobin, pyraclostrobin and trifloxystrobin. This group has some systemic activity, but the strobilurins are not as rapidly absorbed as the triazoles. This group is also not as systemic as the triazoles and does not appear to be as effective in killing the fungus once it is established in the plant. Strobilurins should be used only in preventative application or in combination with a triazole if rust is present in the field.

The final product is the contact fungicide chlorothalonil, which is strictly a protectant and has activity only on the leaf surface when the rust spore is germinating. Since this product is not systemic, the residual activity can be affected by environmental conditions (specifically rain or irrigation). Chlorothalonil is generally considered to have a shorter residual activity and will need to be reapplied more frequently than the systemic products. More research is needed with this product to support the suggested longer residual activity claim being made by some.
Resistance Management Issues

While resistance to fungicides has not been detected in the soybean rust fungus to date, it is in all our best interests to follow resistance management strategies. Given the airborne nature of this disease, when resistance develops it will be a problem for everyone. Resistance to strobilurins and triazoles has been found in many other fungi and impacts disease management worldwide. Strobilurins are active on a single site in the mitochondria of a fungus and pose the greatest risk of resistance development in the soybean rust fungus. Because? they have activity on a single site, they are not affected by the rate of product used. Therefore, resistance development will not be favored if lower rates are used for the strobilurins.

Resistance to triazoles will be favored if reduced rates are used. The triazoles are active at a different site of the fungus, in the cell membrane. To avoid potential resistance problems do not use rates lower than recommended for triazole products.

Chlorothalonil is a multi-site active product, and thus, poses little risk of resistance development in fungi. There are no reports of resistance in the literature. In the future we may see the chlorothalonil products used in resistance management strategies.

Links

 Current Fungicides for Nebraska Producers

 

U.S. Environmental Protection Agency
http://www.epa.gov/oppfead1/cb/csb_page/updates/soybean_rust.htm

CDMS
http://www.cdms.net/manuf/default.asp



Return to Soybean Rust Homepage