In their ongoing effort to track the movement of Asian soybean rust, the University of Missouri and the Missouri Department of Agriculture have reported the collection of a single fungal spore that resembles the spores of soybean rust.

"Although only tentative identification is possible, we did collect one 'rust-like' spore in Boone County," said Laura Sweets (photo), Extension plant pathologist with the MU Commercial Agriculture Program. "However, soybean rust spores aren't unique enough to be distinguished from other fungal spores."

To date, there have been no reports or confirmation of soybean rust on soybeans or alternate hosts such as kudzu in Missouri, nor have any spores been collected from a trap in Ste. Genevieve County, she said.

Similar reports of "rust-like" spores have occurred in Louisiana, Kentucky, Tennessee, and most recently, Illinois.

"Ten spores were reported from a spore trap in Louisiana in June, but soybean rust has not yet been confirmed on soybean plants in that state," Sweets said. "In mid-July, both Kentucky and Tennessee also had reports of possible soybean rust spores from spore traps, and soybean rust has not yet been found in soybeans in either of these states. 'Rust-like' spores also have been reported in Illinois, and scouting there is ongoing.

"Definitive identification requires greater numbers of spores than have been found in any of the spore traps in the United States this season," she added.

The spore was identified on a slide Sweets submitted to the University of Arkansas for visual examination the week of July 25. A slide from the same trap submitted Aug. 3 did not have spores resembling soybean rust.

Sweets said that for most of Missouri, the hot and dry weather has not favored the development of soybean rust. At this point, she recommended that soybean producers continue scouting or monitoring for soybean rust, especially in low-lying areas of a field; shaded or protected areas in a field; areas along tree lines, waterways, creeks or streams; and areas where dew accumulation tends to be heavy.

"In those few areas of the state where precipitation has been more normal and conditions have been more favorable for the development of foliage diseases such as rust, we would recommend increasing the frequency and intensity of scouting," she said.

"Because of the low incidence of soybean rust in the southern United States, the unfavorable weather conditions in most of Missouri, and the soybean crop's advance stage of growth, the risk of soybean rust development in Missouri remains very low, so we do not recommend spraying for soybean rust at this time," Sweets said.

MU Extension regional agronomists are continuing to monitor sentinel fields in 28 counties across the state. "They'll continue until the crop reaches at least the R6 stage," Sweets said. "The next spore trap slides will be sent for examination on Aug. 10."

Asian soybean rust is a yield-reducing foliar disease that first reached the continental United States in November 2004. When left untreated, rust has caused up to 80 percent yield loss in infected soybean fields in South America. At least 31 legumes may serve as host species for the disease.

More information about soybean rust is available online at http://agebb.missouri.edu/mgt/soyrust/.

by Jason L. Jenkins
Source: Laura Sweets (573) 884-7307