In the early stages, typically at flowering, the microscopic spots are the size of pinpricks and initially appear on the underside of lower plant leaves. The lesions gradually increase in size and become tan or brown clusters of raised bumps on the leaves.
Infected plants take on a yellow hue as the disease progresses, followed by premature defoliation and a reduced number of days to maturity. In turn, fewer pods and seeds are produced, resulting in sometimes substantial yield losses.
Straightforward as it sounds, the disease can be difficult to identify, cautions USDA-ARS scientist Monte Miles.
“It can be confused with brown spot, bacterial pustule or bacterial blight, and there have already been several false identifications,” he says.
Thus, especially in an initial diagnosis of soybean rust in the U.S., DNA testing will be used to confirm the disease, Miles says.
American producers are encouraged to be alert for signs of soybean rust, and many southeastern states already have aggressive scouting programs in place.
Arnold Tschanz, who has studied soybean rust in Taiwan and is a plant pathologist with USDA-APHIS, says the disease does have certain identifying characteristics.
First, look for the telltale brown lesions on the lower portion of the plant. Lesions are most common on the undersides of the leaves, but can also appear on the stems and pods. As the plant matures and sets pods, infection progresses rapidly under the right environmental conditions — moisture, high humidity and heat — to cause high rates of infection in the middle and upper leaves of the plant.
Tschanz reports that, initially, a young soybean rust lesion can look like bacterial pustule, with a small raised blister or lesion. “But as bacterial pustule matures, it breaks open with a linear crack; the soybean rust pustule opens with a circular pore in the top center of the lesion. It can easily be seen with a hand lens,” he says.
Tschanz says soybean rust lesions will also multiply and form together into small clusters. With bacterial pustule, each lesion is a distinct, single spot. Tschanz says another differentiating characteristic is that the raised blister is typically more pronounced in rust than in brown spot.
SPORES: How It Spreads
Soybean rust spreads by spores produced in the rust-colored lesions found on the plant.
When the lesions break open, millions of clear to yellow-brown microscopic spores are released and transported by air currents to other soybean or host plants — such as kudzu, yellow sweet clover and kidney beans.
The spores are able to penetrate plant cells directly and, after infection, plants may not show symptoms for 10 days.
“It is a fast-moving disease,” says Miles, who reports that the spores have infected 90% of Brazil in just two growing seasons. “When it hits, be ready to apply fungicide,” he adds.
Tschanz suggests producers monitor soybeans as well as host legume plants that stay green through the winter and help over-winter the fungus. “Keep an eye on kudzu; it might appear there first,” he says. If you think you see symptoms, contact a county extension agent or university plant diagnostic center immediately.
SOUTH AMERICA'S DEVASTATION
Bob Streit, an agronomist and crop consultant based in Iowa, reports that most of the large crop losses in South America were due to producers deciding to spray fungicides too late. “They seemed to have a delayed reaction because they didn't realize how fast it was going to move,” he says.
While at American Plant Pathology meetings in Charlotte, NC, this summer, Streit visited with Jose Tadashi Yorinori, a USDA soybean rust researcher with the Brazilian Agricultural Research Cooperation. He asked what advice Tadashi would offer crop advisors in helping U.S. producers manage this disease. “Tadashi said, ‘You cannot scare them (farmers) enough about this,’” Streit reports.