On the underside of a soybean leaf, the pustules look like eruptions of small volcanoes. Out in the field, it can often be confused for a number of look-alike soybean diseases. From the roadside this past fall, even soybean fields confirmed with Asian soybean rust looked healthy.
Simply put, Asian soybean rust is difficult to identify. It lives just above the microscopic level. However, its effect on soybeans in Africa and South America — and its ability to damage the U.S. soybean industry — is a straightforward tale of real and potential destruction.
Experts say early detection and treatment is critical. Growers in South America and South Africa not timely with fungicide applications have suffered major yield losses.
In South Africa, the disease has damaged as much as 80 percent of some soybean crops. In Brazil, where the disease has been resident since 2001, farmers who haven't sprayed fungicides properly have experienced 40 percent to 60 percent yield losses. In some unsprayed fields, losses were 100 percent.
“(Effects of the rust) look very much like bacterial pustules,” says Erik Stromberg, Virginia Tech Extension and professor of plant pathology.
Normally, the word ‘rust’ is associated with brown or red colors. Don't be fooled with Asian soybean rust. “(With this rust) you can get some brown spots, but the spores are clear or white with a grayish cast,” cautions the pathologist.
Stromberg likens the spores to a bunch of white grapes. Spores can look like downy mildew and are often confused for brown spot in soybeans.
“From the underside of the leaf, lesions look like eruptions of small volcanoes,” says Tim Momol, an associate professor of plant pathology at the University of Florida. Momol runs the lab that first diagnosed Asian soybean rust in Florida.
The fungus causes lesions on plants that can lead to premature defoliation and decreased yields. In its early stages, these lesions appear on the underside of the soybean leaf. The fungus starts on the stems near the ground.
Tiny bumps within the lesions contain spores that can spread like wildfire. The bumps, called “unredia,” give the disease leaf its rusty appearance.
The spores are extremely mobile — a puff of wind and they're off to a new home. In fields infected with the rust, farmers in South America have noted clouds of spores floating above the canopy.
Air currents transport the spores to other soybean plants, often over long distances. In Brazil, rust spores traveled more than 3,500 kilometers in two years. Its arrival in the U.S. may have been due to an active 2004 hurricane season.
Even before the disease was confirmed in Louisiana in November, the approach to Asian soybean rust resembled a build up of homeland security as Extension experts, agents, consultants, state departments of agriculture, and USDA prepared for combat. The disease just showed up earlier than expected.
Last August, 200 so-called “first-responders” spent half a day in Virginia soybean fields at two training sessions. “We did this to show our Extension agents, consultants and others in the agricultural industry what Asian soybean rust looks like,” Stromberg says. “We gave them lens and microscopes and set them loose in the fields.
“Although Asian soybean rust needs a live host to reproduce, more than 30 legume species can act as its host. We talked about the biology of the rust and how to identify it. We had already done surveys in the state that confirmed the disease was not here.
“In order to have rust, you have to have a live host. We've already had frost here, but I suspect there might still be green tissue in the Coastal Plain because it was found in South Carolina recently. It just depends on which way the wind is blowing.”
Discovery of the rust in Florida is an example of the quick action of a system anticipating the arrival of such a dreaded disease. Similar scenes played out wherever experts confirmed the rust.
In late November, University of Florida plant pathologist Jim Marois spotted the disease on soybeans. Later, he and Momol looked at rust symptoms on kudzu and then sent a sample to the USDA-APHIS lab in Maryland for confirmation.
“The early detection was made possible by the Southern Plant Diagnostic Network, in collaboration with the National Plant Diagnostic Network, with their training and funding to land grant universities,” Momol says. “The cooperation among the states and federal officials helped us make this early detection.”
APHIS is using morphological and molecular means to identify Asian soybean rust. The molecular method, called the “polymerase chain reaction,” is designed specifically for Asian soybean rust. Researchers use a mixture with a primer to determine whether a sample is infected by the rust. A positive test shows up in a band similar to the blue stripe indicating a positive pregnancy test.
“It's always good to use more than one method to confirm a disease in order to avoid mistakes,” Momol says.
And the work is just beginning. Experts are now on the look out for a second type of spore: teliospores. “It's another type of spore of this pathogen,” Momol says.