Rust detectives are on the lookout for signs of the soybean disease moving out of the far-south region around the Gulf of Mexico, but word this week of spores trapped in Kentucky and Tennessee are no cause for alarm. The find does not necessarily mean Asian soybean rust has moved north into the heart of soybean country, according to Suzanne Bissonnette, the soybean rust educational state coordinator for University of Illinois Extension.
Don Hershman, University of Kentucky Extension Plant Pathologist, reports that as of July 11, 2005, no soybean rust has been detected in Kentucky. Ten presumed spores of the soybean rust fungus were identified on July 7 from spore traps located near Woodburn, Warren County, Ky. "Keeping this all in perspective, nine other spore traps were negative for spores and no soybean rust has been found on soybean or kudzu in the state," he says.
Beth Long, Extension Specialist for the University of Tennessee Knoxville, reports that on July 7 the university was notified that there were two rust spores found in a spore trap located in Jackson, Tenn. These spores were collected on Tuesday, July 5th at a sentinel plot at the West Tennessee Experiment Station.
"It is impossible to determine for sure if these spores are really soybean rust spores," she says. "But, according to Dr. John Rupe at the University of Arkansas, who is heading up this spore trapping system, the Tennessee spores are similar to soybean rust spores when viewed under the microscope." Ten rust spores were also collected on July 5 in a spore trap located in Bowling Green Kentucky. Rupe says that no other spore traps in Tennessee have shown any spores. There have been no rust infections or rust pustules found anywhere in Tennessee as of this week.
"The word 'rust' refers to a huge family of fungi that infect plants," Bissonnette says. "There are many hundreds of species of rust that infect green plants. It doesn't mean that just because they were sampling for soybean rust spores that those are the only rust spores that would be sucked up into a spore trap."
She notes that the fungi in the rust family have numerous microscopic features that look similar and thus they are organized into the rust family for purposes of identification by a mycologist or plant pathologist.
"If rust spores are found in a spore trap in a state, it only means that fungal spores that look like they are in the rust family are in the trap," Bissonnette says. "Many different kinds of rust spores could be in there."
Bissonnette points out the spores must be identified microscopically by an individual trained in rust taxonomy and properly identified as one of the species of soybean rust.
"Even then, that isn't the end of the identification process," she says. "There are two species of soybean rust, a severe one known as Asian soybean rust and a mild one that is of little or no concern. Laboratory tests, based on proteins on the outside of the rust spores, must be done to differentiate between the two known species of soybean rust."
She advises growers to await confirmation of the presence of Asian soybean rust before spraying the crop with a fungicide.
"It is important to make sure that the information you have is from a reliable source and that there is an indication that the rust species is actually Asian soybean rust and not just one of the innumerable other rust species we have floating around in the air at any given time," Bissonnette says. "You will save yourself time and money and will be a continuing good steward to the land by getting the right information before reacting."