Clemson University Plant Pathologist John Mueller confirmed Aug. 20 that samples taken from a field in Calhoun County, near St. Mathews, S.C., were positive for Asian soybean rust.
Asian soybean rust was identified in samples collected Aug. 17, from a field of Maturity Group V soybeans. The plants were in a sentinel plot and were at growth stage R-5 (full size pods containing developing beans).
Out of the 25 leaves examined only one exhibited rust and that lesion was less than one-fourth inch in diameter. The lesion contained three very young pustules that had just begun sporulating.
Immediately adjacent to this sentinel plot of Group V soybeans was a sentinel plot of Group VI soybeans. No rust was detected on the 25 leaves examined from that plot. The Group V plot exhibited very lush growth, 40-plus inches tall in 38 inch rows.
Mueller says, “to keep this find in perspective, growers should be aware that last week we examined almost 1,400 leaves from 43 soybean fields or sentinel plots in South Carolina. One leaf exhibited the smallest detectable level of rust (1 lesion or clump of pustules),” Mueller concludes.
Mueller’s research team began on Aug. 22 to further determine the real extent of the infestation. They will be collecting extensive samples from the monitoring plots, neighboring kudzu, and soybean fields throughout Calhoun County, which is in central South Carolina.
Slightly cooler temperatures and a fairly good chance of thunderstorms are predicted for the area and this weather could be conducive to the development of rust and other pod, stem and leaf diseases especially within a field, according to Mueller.
LSU AgCenter scientists confirmed finding of Asian soybean rust on soybeans in Rapides Parish on July 21. Though this was the first finding of the disease on soybeans in Louisiana for 2006, it had been found about a month ago on kudzu, a plant that can host the disease, in Iberia and Lafayette parishes. Subsequent findings on soybeans were reported Aug. 11 in Natchitoches Parish, Aug. 15 in Avoyelles and Tensas parishes and Aug. 18 in St. Mary’s Parish.
Most of the beans in the infected parishes are Group V maturity and some are beyond the R5 growth stage level. However, beans in growth stage R1-R5 and have high yield potential should be sprayed, according to LSU Plant Pathologist, David Lancous.
In neighboring Mississippi, rust was found on beans in the southwest corner of the state, but no further infections have been detected since Aug. 1. Hot, dry conditions in the area, are not conducive to development of the disease. A combination of drought, early defoliation, and beans being beyond the R5 growth state are not conducive to widespread rust problems further north in Mississippi.
Unconfirmed reports of rust finds on the University of Georgia campus would put active rust closer to heavy bean production areas in North Carolina and South Carolina, but as of Aug. 22, these reports are not documented, making the mid-August find in Tifton, Ga., and the recent find in Calhoun County, S.C., the closest finds to production areas in the Carolinas and Virginia.