Stay vigilant, and scout for soybean rust symptoms in peas and beans

By Marni Katz

Although edible beans and peas appear to have a much less chance of becoming infected with soybean rust than do soybeans, growers still should learn about the disease and its control, plant pathologists say.

“It’s still unclear as to how much of a risk there is,” says Greg Shaner, a Purdue University plant pathologist in West Lafayette, Ind. “But it’s enough of a risk that growers should be diligent.”

The lack of concern among vegetable legume growers has left at least one Extension researcher perplexed.

“Still, if I were to offer advice to growers, it would be to be vigilant,” says Dan Egel, an Extension plant pathologist with Purdue University in Vincennes.

Scout your fields and be careful. Become familiar with what symptoms look like on soybeans. And if you have questions, find out the answer before the season gets too far along.”

More than just a soybean problem

Asian soybean rust, which poses a major threat to the United States’ 74 million acres of soybeans, has yet to appear in a commercial crop of edible legumes. But the fungus responsible for the disease—Phakopsora pachyrhizi—is known to have more than 30 different legume hosts, including snap, lima and wax beans, and several pea species.

Soybean rust became an issue after first appearing in southern U.S. soybean fields in late 2004. Scientists have since confirmed the disease on soybeans and kudzu—a fast-growing perennial southern weed that is a member of the legume family—in nine states.

Wind easily can spread spores for up to 100 miles, and the more virulent rust strains can defoliate up to 95 percent of infected plants.

Scientists theorize that the strong winds associated with two hurricanes during summer 2004 carried the spores from Central America into the southern states. The disease has created a stir among U.S. soybean growers because of the speed at which it can infect and kill fields.

“Experience in Brazil has shown that if fungicides are used in a timely manner, we can get good control,” Shaner says. “For soybean growers here, it’s a new expense and new technology because they are not used to spraying fungicides. But most vegetable growers do use fungicides for a lot of other things, so they are more used to dealing with these issues than soybean growers.”

More than 10 fungicides have received Environmental Protection Agency Section 18 emergency-use exemptions to control soybean rust. Many of the fungicides already are used on the related common rust, Uromyces appendiculatus, in legume vegetable crops. They include propiconazole (Tilt, Bumper and PropiMax), pyraclostrobin (Headline) and tebuconazole (Folicur).

Mixed test results

While snap beans, dry beans, lima beans and cowpeas are among the 30 edible legumes listed as hosts to soybean rust, pathologists question which are susceptible within the United States.

Snap peas, while listed as a host crop, did not develop symptoms or test positive for soybean rust even when interplanted with soybeans that developed significant disease symptoms in a fungicide test, says University of Georgia Extension vegetable pathologist David Langston in Tifton.

A similar trial that involved side-by-side rows of soybeans and snap beans yielded comparable results. The soybeans developed rust symptoms whereas the snap beans did not, suggesting that they may not be as susceptible as previously reported, Langston says.

“Based on that and what I’ve heard from other people, I think snap beans really aren’t’ that susceptible to soybean rust,” he says. “I had heard that before, but based on what I saw in my test, I think it’s very real.”

But a 2005 trial conducted by University of Florida plant pathologist Jim Marois showed a collection of legumes, including snap beans, kidney beans and runner beans, could naturally be infected with soybean rust. Marois, who is based at the North Florida Research and Education Center in Quincy, is quick to point out that scientists have never found the disease in commercial bean fields.

Although he admits scientists have much to learn about how soybean rust will behave in the United States, Marois says the disease does not appear to be a significant threat to vegetable legumes as long as they can be treated with registered fungicides.

“I think what we’re going to find is that other bean crops will not be affected by it very much,” he says. “A lot of the time, they are already being sprayed with fungicides to control common bean rust. I would expect anything you are doing to control common rust will control soybean rust.”

Don’t let down your guard

Nevertheless, Shaner stresses that edible legume growers should be diligent in scouting for soybean rust symptoms, particularly if they are near a region where rust has been found on soybeans. 

“When you are scouting the field, don’t look at the top leaves,” he says. “Symptoms will be further down in the canopy before they appear on upper leaves, so you have to look there.”

Soybean rust symptoms take seven to nine days to appear after infection. The fungus infects the leaf internally and produces spores below the leaf epidermis, creating dark spots. As more spores are produced, pustules rupture like tiny volcanoes and spores escape, Shaner says.

Because few people in the United States have seen soybean rust symptoms, Egel says it is important to have suspect samples tested by a laboratory. Most states have plant diagnostic clinics through Cooperative Extension offices that often will test samples free of charge.