Asian soybean rust rode wind and wet weather across Arkansas and Mississippi in mid-September and into west Tennessee farm fields in October, looking for a victim for its deadly spores. Fortunately, most of the region's soybean crop had already left town.
According to the University of Tennessee, Asian soybean rust was found in west Tennessee on soybean leaf samples collected from a field plot in Gibson County, Oct. 3, at the University of Tennessee Milan Research and Education Center.
UT Extension staff monitored for rust in soybean sentinel plots and spore traps across the state all summer and had not found any rust lesions on soybean leaves until the October discovery. The disease samples — a few rust pustules — were found on soybean leaves and visually identified with a microscope at the UT Extension lab at Jackson.
The samples were then tested by Melvin Newman, Extension plant pathologist in Jackson. The leaves are undergoing final confirmation that the pustules are Asian soybean rust by polymerase chain reaction testing in Knoxville.
Newman speculates that the lack of rain in west Tennessee during critical times in the growing season prevented soybean rust from developing earlier. “Several rain fronts passed by west Tennessee, but they developed and caused rain west of the Mississippi River. This may be why soybean rust was found in eastern Arkansas in late September while none was found in Tennessee until last week.”
The disease came too late to cause any damage to crops, according to UT Extension plant pathologist Beth Long. “Most soybean plants in Tennessee have been harvested and very little green leaf tissue remains in the field.”
The same holds true for soybeans in Arkansas and Mississippi. In Louisiana, the disease came earlier than ever before, but growers kept it under control.
“The big story was that the disease occurred 53 days earlier than it did in 2006. Because of that we had a lot more observations in the field,” said Louisiana Extension soybean specialist David Lanclos.
Ironically, the disease had one positive impact — the threat of rust caused other diseases to be scouted for more efficiently, he said. “This is probably why we're setting yield records in Louisiana for two years in a row.”
Ground zero for rust in Louisiana occurred around Cheneyville, La., according to Lanclos, “where we had the initial spore shower. Overall, we've had rust in the majority of our fields, especially on our later-planted soybeans. But from a loss perspective there was nothing major.”
A big concern for Lanclos is the effect of current high wheat prices on the state's double-cropped soybean acres in 2008. Louisiana is expected to plant about 400,000 acres of wheat this fall in response to high prices, up from 220,000 acres in 2006.
After harvest, 90 percent of these wheat acres will likely be planted in soybeans, and these crops are likely to be exposed to more incidences of rust. “Research coming out of LSU indicates that we need to plant an early-maturing soybean, a late Group 4 or early Group 5,” Lanclos said. “This would limit our exposure to Asian soybean rust as we go through the season.”
Lanclos cautioned growers against a false sense of security if they did not follow a recommendation to spray a field, and those beans subsequently did not develop the disease.
“Just because we made a recommendation to spray doesn't mean you've had a spore shower in your area. Our recommendation means it's in the area, and it's enough of a concern to spray. But it doesn't guarantee that each field is going to have rust. Next year, there could be rust in those fields.”
While west Tennessee's soybean crop dodged a bullet with soybean rust, it dodged quite a bit of rainfall, too. Soybeans generally rank among the state's top crops, earning farmers approximately $278 million in cash receipts in 2006. Due to the drought, yields are down from 39 bushels per harvested acre in 2006 to estimates of just 24 bushels per acre this year.
The Tennessee Agricultural Statistics Service reported in September that production for 2007 is expected to fall by 57 percent from last year's 44 million bushels to just 25.2 million bushels.
At this time there are no commercial soybean varieties resistant to soybean rust. Fungicide applications can reduce yield losses from rust and other late-season diseases, depending on the plants' developmental stage, the time during the growing season when soybean rust is detected and weather conditions.
Asian soybean rust is caused by the fungal species Phakopsora pachyrhizi and is known to infect kudzu and many other legume species. It has the potential to significantly reduce soybean yields but can be managed with the use of fungicides if detected early.
In addition to the west Tennessee site, Asian soybean rust has been found in 15 states in 169 counties; in 18 counties in Alabama (13 soybean), 33 counties in Arkansas (all soybean), 16 counties in Florida (eight soybean), 14 counties in Georgia (10 soybean), one county in Illinois (soybean), one in Iowa, four counties in Kansas (soybean), two counties in Kentucky (soybean), 15 parishes in Louisiana (14 soybean), 15 counties in Mississippi (12 soybean), four counties in Missouri (soybean); two counties in Nebraska (soybean); 11 counties in Oklahoma (all soybean), seven counties in South Carolina (soybean) and 26 counties in Texas (25 soybean).
More information about soybean rust and recommended measures for controlling the disease are available through the UT Extension Web site: http://UTcrops.com. First click on “soybean” then follow the link labeled “diseases and nematodes.” Growers can also visit the USDA soybean rust Web site: http://www.sbrusa.net/  and view the map showing positive locations.