Farm Progress

Soybean rust: Too little, too late

Roy Roberson 2

November 5, 2008

3 Min Read

Asian soybean rust again moved out of the deep South and into the Carolinas in early October, but as in past years it was too late and at too little infestation to do much damage to the region's soybean crop.

Veteran North Carolina Soybean Specialist Jim Dunphy says the vast majority of soybeans in the Tar Heel state were beyond Growth Stage 5. Once soybeans reach this stage and begin pod fill there is little risk of economic loss from Asian soybean rust.

The first find of Asian soybean rust in the state was confirmed on Oct. 1, 2008 near Laurinburg, N.C., on a sample collected from a sentinel plot by Cooperative Extension Agent, David Morrison. Following that initial find, a mobile survey was conducted.

The mobile survey detected rust in three additional counties in the southern part of the state. However, Dunphy notes that subsequent testing in a number of counties in early October failed to detect rust.

These initial rust findings put the fungus closer than 100 miles of most of the state's soybeans, except the northeast and northwest parts of the state. As expected, rust slowly moved forward to central and northern areas of the state.

“The majority of soybeans in North Carolina were well beyond the stage where rust would be expected to cause yield loss by the time the fungus got to North Carolina,” Dunphy says.

Some areas with rust within 100 miles possibly benefitted from the application of fungicides if late maturing soybeans were planted after small grains, and soybean seeds had not yet reached full size. Pathologists in North and South Carolina and Virginia agree the percentage of beans at this stage was small.

“Spore deposition from the south is still likely to be light, so it will likely be mid- to late-October before defoliation is visible from the road as a result of rust, Dunphy contends.

In South Carolina, Clemson Plant Pathologist and veteran rust tracker John Mueller says most of his state reported light infestations of rust in early October, but as in North Carolina, few beans were at risk from the disease.

By early October soybean rust had spread to nine counties across the Palmetto State and at least four counties in North Carolina.

“The spread to the Tar Heel State suggests that rust could be in any county in South Carolina, but its incidence is very low,” says Mueller, who is also director of the Clemson University Edisto Research and Education Center.

“I am sure the low incidence of rust in South Carolina is due in great part to the drought conditions we have had in the state for much of the summer, as well as the extensive fungicide spraying that has occurred,” Mueller adds.

Like Dunphy in North Carolina, Mueller contends the late season outbreak is unlikely to cause significant damage to crops. Most fields have been sprayed at least once, so any damage will be minimal, he said.

In Virginia, Virginia Tech Soybean Specialist David Holshouser says, rust is likely to make it into Virginia before soybeans are harvested, but not before virtually all the state's bean crop is beyond the point of being at risk to yield loss from rust.

Soybean rust — brown in appearance on the plant's leaves — is a disease that causes premature defoliation, early maturation and severe loss of yield through reduction in the number of pods and seeds, and decreased seed weight.

Soybeans in some areas of the upper Southeast, especially in drought prone areas of South Carolina appear to have defoliated somewhat earlier than expected, but this appears to be due to cercospora leaf spot and bacterial blight, not from Asian soybean rust.

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