Able to travel great distances via wind-borne spores, soybean rust has spread around the globe to wreak havoc with soybean crops throughout the Southern Hemisphere. Also known as Australasian rust, this fugal infection can defoliate soybean fields rapidly, often resulting in severe and sometimes total yield loss.

Scientists believe it is just a matter of time before soybean rust arrives on our shores. Possible infection scenarios range from unusual weather patterns, importation of infected soybean materials or even agroterrorism.

Commercially available soybean varieties with resistance to rust are at least two years away. So when, and if, the dread disease arrives, farmers will need to rely on rapid detection and treatment with fungicides. But widespread infection would quickly deplete the supply of fungicides approved by EPA for use on soybeans. Some of these products might do only a partial job of controlling the disease, so there is concern that rust could easily develop resistance.

Exemptions for fungicides

The threat has prompted chemical companies, university agronomists and the American Soybean Association to seek emergency Section 18 exemptions for several different fungicides currently used on other crops. At an ASA conference in January, company representatives from BASF Corporation, Bayer CropScience, Dow AgroSciences, Sipcam Agro USA and Syngenta Crop Protection explained how some of these fungicides have been used with good results in Brazil.

The Minnesota and South Dakota Departments of Agriculture have outlined potential infection scenarios and possible treatment options with currently approved and Section 18 fungicides.

Quadris, an approved systemic strobilurin with broad-spectrum activity against many fungal pathogens, has shown that it can control rust. However, researchers are wary that this chemistry is particularly prone to fungal resistance if overused.

Bravo, Echo and other chlorothalonil fungicides are approved and widely available at relatively low cost and can control rust. But the product must be applied often to provide protection and must not be applied after six weeks before harvest.

That leaves a host of Section 18 products, mostly triazoles, to do the job. These include brand names such as Tilt, Propimax, Bumper, Folicur, Laredo, Stratego, Eminent, Pristine and Headline.

Treatment

In his report, South Dakota State Extension plant pathologist Martin Draper suggests how these products might best be used in different situations:

  • If soybean rust is established on site, treat with a Section 18 triazole product. If a second application is needed, treat with azoxystrobin if the disease is at a minimal level; otherwise treat with a Section 18 product. If a third application is needed, treat with chlorothalonil.

  • If soybean rust is expected, but not yet present, treat with azoxystrobin or pyraclostrobin. If a second treatment is needed, treat with a Section 18 triazole product. If a third application is needed, threat with chlorothalonil or a Section 18 product.

  • If soybean rust develops after an initial preventative treatment, treat with azoxystrobin or pyraclostrobin. If a second treatment is needed, treat with a Section 18 triazole. If a third treatment is needed, treat with chlorothalonil or a Section 18 product.

If that sounds like a lot of spraying and extra expense, it is. But Draper says crop rotation won't help.

Like wheat leaf rust, soybean rust could remain airborne throughout large sections of soybean-growing areas, spreading from south to north on seasonal wind currents and persisting on alternate host plants.