Many Positive Developments in Soybean Rust Research Presented at the 2007 National Soybean Rust Symposium

December 17, 2007

Since the arrival of the soybean rust pathogen in North America just three years ago, there has been an unprecedented amount of collaboration among producers, industry, scientists, USDA ,extension staff, crop advisors, and soybean check-off organizations to understand Asian soybean rust and to mitigate it's impact on soybean production in the United States. The positive results of this effort was clear as leaders in soybean rust research and extension shared their experiences and research results on December 13-14 at the National Soybean Rust Symposium in Louisville, Kentucky.

USDA PIPE: Soybean Rust Maps

Notable achievements of this nationwide, grassroots effort are many. In these three years we now have a real-time tracking system for soybean rust (USDA PIPE ), and the sentinel plot system, spore tracking system, and climate-based epidemiological models that feed into it. More than 475 people were involved in the sentinel plot system in 2007, and 13,412 observations were uploaded to the PIPE website. This represents an enormous amount of cooperative work. Many of you reading this page were probably involved - thank you for your efforts! To know when and where rust is found, and if environmental conditions are present that will favor it, is key information for deciding when to spray and when to wait. If one considers how many acres might have been sprayed unnecessarily if none of this information was available, you can see the huge environmental benefit as well as money saved from this information network.

According to the USDA PIPE website, soybean rust was found in 19 states and 301 counties (thought to be a conservative estimate) in 2007. Although disease pressure was high in some southern areas, notably Alabama, Louisiana, and Georgia, rust epidemics were generally held in check by widespread freezing temperatures in the south in April that reduced the amount of spores, and also by the 2007 drought. The northernmost find in the U.S. was in Hancock County, Iowa in October, and by November it was also detected for the first time in Ontario, Canada.

As for soybean rust control, the reports on fungicide trials held in two U.S. and four South American locations in 2007 were positive. Most fungicides tested were reported to be "very effective" in controlling Asian soybean rust. Only when disease pressure was very high did some products perform better than others. The researchers confirmed that good coverage into the mid- and lower canopy is critical, as well as the timing of the fungicide application — especially if soybeans are still in early growth stages.

The risk of fungicide resistance was also discussed, and there was some good news in this area as well. Although fungi can and do develop resistance to the triazole and strobilurin fungicides used for soybean rust, there are factors about the U.S. situation that helps lower the risk. One is that Phakopsora pachyrhizi, the soybean rust pathogen, does not overwinter in most soybean production areas of the U.S., and another is that only one or two, if any, sprays are needed each season.

The red-brown or "RB" type of lesion formed by some soybean varieties in response to infection by P. pachyrhizi may be a type of resistant reaction because fewer spores are produced in these lesions compared to the light tan type of lesions.

Ultimately, as with all rust diseases, it will be the employment of resistant soybean varieties that will provide the most effective and stable long-term control of Asian soybean rust, and progress continues to be made toward this goal. Soybean breeders and plant pathologists are now in their third year of field-testing resistant soybean germplasm in the U.S. This year they evaluated 703 soybean lines in seven different locations and reported a clear sorting of resistant reactions compared to susceptible soybean varieties. Several genes for resistance have been identified and named. The next step is to make the selections, and to cross the resistant germplasm with "elite parents"—- soybean varieties with the desired agronomic characteristics.

Asian soybean rust was found in 19 states and 301 counties (thought to be a conservative estimate) in 2007. Although disease pressure was high in some southern areas, notably Alabama, Louisiana, and Georgia, it was generally held in check by widespread freezing temperatures in the south in April that reduced the amount of spores, and also by the 2007 drought.

The northernmost find in the U.S. was in Hancock County, Iowa in October, and by November it was also detected for the first time in Ontario, Canada. Heavy rainfall in the early summer in Texas, and enhanced southerly winds ("low-level jet winds") that lifted spores up and into the northern states was the explanation by climatologists as to how the spores reached these northern locations. Fortunately, that happened late in the season when the soybean crop would no longer be affected by rust. However, this very route is typical for the spread of wheat rust in the U.S. and can happen much earlier in the season in any given year, which is what soybean rust forecasters are watching for carefully.

This third annual National Rust Symposium was sponsored by the American Phytopathological Society with financial assistance from the North Central Soybean Research Program, the United Soybean Board and the Iowa Soybean Association. Abstracts and PowerPoint presentations can be found in pdf format on the Plant Management Network website.

Information on these pages is from the cooperative effort of researchers throughout the North Central states. Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: NCSRP Plant Health Initiative at www.planthealth.info. Please do not use copyrighted photos without permission.