Soybean Rust Update- August 7, 2006
By Laura Sweets

Development and spread of soybean rust in the southern United States continues to be slow. However, over the last few days soybean rust has been confirmed in additional sites in Florida and Georgia as well as confirmed for the first time this season in Mississippi. According to the USDA Public PIPE Website (www.sbrusa.net), soybean rust has been found on this year’s soybeans in nine different counties in five states (Alabama, Florida, Georgia, Louisiana and Mississippi). The rest of the positive reports for 2006 have been on kudzu. The Mississippi report from the first week in August stated that soybean rust had been found in Jefferson County in southwestern Mississippi on both soybean and kudzu. The rust on soybean plants was found in an area shaded by oak trees and the incidence was very low- only 4-5 leaves which were later removed. The rust infected kudzu was along a roadside and was a heavier level of infection. The Delta and northern soybean production areas of Mississippi have been quite hot and dry so the threat of rust remains low in those areas. Overall, dry to very dry conditions in many regions of the southern United States have slowed soybean rust development and spread.

In Missouri we are continuing to monitor 25 sentinel plots. Most of the sentinel plots have reached the R3-R5 stage of growth. Thus far, soybean rust has not been detected in any samples submitted from sentinel plots. As original sentinel plots reach R5 growth stage, observers are trying to locate fields in earlier stages of growth to monitor. Much of the state of Missouri has also been unusually hot and dry so conditions have not been favorable for the development of soybean rust even is spores had been introduced from the southern United States. The southeastern part of the state has had slightly better precipitation so scouting in that part of the state remains a priority.

In addition to the sentinel plot program, the University of Missouri is again participating in the Syntinel RustTracker Spore Trap Network. This is a soybean rust spore monitoring program using spore traps designed to trap spores and other particles present in air moving through the spore traps. Spores and other particles are trapped in petroleum jelly coated on a microscope slide mounted in the spore trap. The slides are removed from the spore traps on a regular basis and shipped to a central laboratory for microscopic examination. Since the spores of soybean rust aren’t unique enough to be distinguished from other rust spores and even some other fungal species, only tentative identification of spores resembling those of the soybean rust pathogen has been possible with this spore trapping system.

There are two spore traps located in Missouri, one in Boone County and one in Ste. Genevieve County. The slide from Ste. Genevieve County that was microscopically examined on July 25 did have one spore which resembled a soybean rust spore. No other slides have had any spores resembling those of soybean rust. Both traps are located in sentinel plot fields and soybean rust has not been found on soybean plants in the sentinel plots surrounding the spore traps. Plants at both sites are at R4 stage of growth and weather conditions have not been favorable for rust infection nor disease development.

Overall, because of continued low levels of soybean rust inoculum to the south, unfavorable weather conditions in many parts of Missouri and the advanced stage of growth of much of the soybean crop, the risk for soybean rust in Missouri remains low and further management actions are not necessary. Exceptions would be areas of the state which have or are receiving more normal rainfall and late planted or double cropped soybean fields. Those fields should be scouted on a regular basis.

Image taken from www.sbrusa.net

Laura Sweets
573-884-7307