Soybean growers in the Carolinas and Virginia will get an early visit from Asian soybean rust, apparently thanks to a storm spun from Hurricane Isaac.

The disease was found last week in Barnwell County, S.C., and this week in Robeson County, N.C.

This is particularly alarming because Asian soybean rust has never been found this early in North Carolina, says long-time North Carolina State University Soybean Specialist Jim Dunphy

Dunphy says soybean rust was detected in soybean in Robeson County, near St. Pauls, N.C., on Sept. 11 and in Johnston County today. This is the earliest soybean rust has been detected in North Carolina. 

Rust was identified on one of four leaves brought to the lab. The earliest detection of soybean rust in North Carolina in previous years was Sept. 15 in 2007, he says.          

Full season soybeans that have just reached full bloom (stage R2) typically have 65 days until they’re safe from rust or frost (stage R7) or closer to 55 days for double crop beans.

If soybeans have small pods in the top of the plants (stage R3), they have 55 and 47 days, respectively, to R7. With full sized pods in the top of the plants (stage R4), they have 45 and 38 days, respectively, until R7. 

From stage R5 (small seeds in the top of the plant) they typically have 35 and 30 days, respectively. From stage R6 (full sized seeds in the top of the plants), they typically have 20 and 17 days, respectively.

Rust will typically take 10-20 days from initial infection to develop to detectable levels. It will take another 7-14 days to spread to other leaves on the same plant, and another 10 days to cause significant defoliation.

Rust prediction models

“The rust prediction models say there was a fair to good chance rust spores were deposited in North Carolina this weekend. If so, we expect to detect rust in about three weeks in sentinel plots, which would be about Oct. 1. It will likely take another two weeks with optimal conditions for rust to increase to damaging levels, Dunphy says.

The soaring price for soybeans is likely to spur most growers to treat either late-planted, late-maturing soybeans or double-crop beans to prevent late-season yield loss to soybean rust. Choosing the right fungicide and applying it at the right time will be critical.

There are four primary choices for fungicide treatment of Asian soybean rust: chloronnitrites of which chlorothalonil is the only product labelled for use; strobilurins, which offer a number of fungicide options; or traizoles, again offering a number of options; or a fungicide with a combination of strobilurins and traizoles as the active ingredients.

Chlorothalonil can be effective, but weathers much quicker than other options, and as a result may need to be re-applied several times to get adequate protection from the disease.

Strobilurin fungicides are modeled after a natural anti-fungal compound produced by certain mushrooms. Strobilurins inhibit mitochondrial respiration in the pathogen. Strobilurins are typically absorbed by the cuticle, and act as protectant fungicides against Asian soybean rust.

Triazoles inhibit sterol production, which disrupts cell membrane function in the disease causing pathogen. Triazoles are absorbed and translocated upward in the plant. While they generally do not prevent infection, the triazoles can kill the fungus in the plant and prevent pustules and spores from forming.

Fungicide products containing both strobilurins and triazoles may be the safest bet for management of Asian soybean rust. These fungicides offer the grower the greatest flexibility of timing of application at a time when many growers are busy harvesting are getting ready to harvest other crops.

rroberson@farmpress.com