Threat of Asian Soybean Rust Continues to be Monitored

3/18/2008

It’s been four years since the arrival of the soybean rust pathogen in the US, and as the planting season nears, attention will turn back to tracking the threat of soybean rust with the aid of USDA’s watchful eye. In order to monitor the threat, USDA created a real-time tracking system called the USDA Plant Information Platform for Extension and Education (PIPE). The plan includes a sentinel plot system, a 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 more than 13,412 observations were uploaded to the PIPE Web site, according to an article from USAgnet. Regarding 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, the article stated. Only when disease pressure was very high did some products perform better than others. 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.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. First, Phakopsora pachyrhizi, the soybean rust pathogen, does not overwinter in most soybean production areas of the U.S. Secondly, only one or two sprays – if any – are needed each season.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. Soybean breeders and plant pathologists are now in their fourth year of field-testing resistant soybean germ plasm in the United States. In 2007, 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.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 during April that reduced the amount of spores, and also by the 2007 drought. The northernmost find in the United States was in Hancock County, Iowa, in October. By November it was also detected for the first time in Ontario, Canada.