Maximizing the profit potential of soybeans starts with deliberate scouting for Asian soybean rust (ASR) and late-season diseases, and a readiness to apply foliar fungicides.

After evaluating the performance of fungicides in soybean trials against ASR in Georgia, Bob Kemerait, University of Georgia Extension specialist and associate professor, says he recorded a 17 bu./A yield difference between a treated and untreated check in one trial and greater differences in other trials.

“We know we can manage soybean rust with the portfolio of fungicides available,” Kemerait notes. “When we treat for the disease, we have to treat early and with a product that hits the pest hard.”

Meanwhile, Dan Poston, Mississippi State University soybean weed scientist and Extension specialist, looked at the potential yield increase when controlling late-season diseases since ASR was not as prevalent in his state.

“On average, we’ve seen an average yield response of about five bushels per acre,” he says.

Control options
In these research trials, both Kemerait and Poston looked at fungicides from different classes of chemistry. For late-season diseases, both agree that products from the strobilurin class of chemistry (Headline, Quadris) offer excellent control, but only provide preventive control against ASR. Meanwhile, products from the triazole class of chemistry (Laredo, Domark) offer both curative and preventive control against ASR, but can be less effective against late-season disease.

Combining products from these classes of chemistry (Headline SBR, Quilt, tank mix of Laredo/Headline) is also an option that offers control of late-season diseases and ASR with a built-in resistance management tool.

However, what product is used becomes less of a priority if the application timing is not perfect. For late-season disease, Poston says he consistently sees the best yield response when a fungicide application is made around early pod development (R3, R4). Kemerait also notes similar findings when a fungicide is applied to control ASR at this reproductive stage, but Poston says he is still determining how early a treatment is needed for the disease, especially on indeterminate soybeans.

“We’ve looked at applications for ASR from R1 to R5 and found that a preventive application at R1 often has to be followed at R3 with another application,” Kemerait notes. “We’ve seen an inconsistent response to an application at R5. Often, one well-timed application with a triazole-based product at R3 is enough.”

Application timing
To help growers prepare for when to apply a product to target ASR, both researchers encourage growers to rely on reports from their state’s sentinel plots available at www.sbrusa.net.

Growers should base their decision to treat for ASR on three things, Kemerait notes. “Are your soybeans at or approaching a reproductive growth stage? Have you seen reports of soybean rust in your region of the state or nearby counties? How much of a gamble are you willing to take with ASR before making a fungicide application?”

In fall 2006, when weather conditions became ideal for ASR – moist – the disease spread up the Mississippi River into the Midwest. With reports of ASR overwintering in the southern United States, growers should take note of the current weather conditions and note air current speeds. As air current speeds pick up, the potential for the spores to infect additional areas increases.

As everyone prepares to write the history on the impact of ASR in 2007, Poston leaves this word of advice: “Don’t get complacent. We’ve had little soybean rust incidence over the past two years, but the affects from the disease are detrimental.”

Bader-Rutter, who represents Dow AgroSciences, contributed information for this article.

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Discovering The Impact On Yield From AS

To monitor the potential yield loss caused by ASR, Mississippi Extension specialist Dan Poston created trials to replicate the defoliation that often results when soybean leaves are infected by the disease.

After planting determinate and indeterminate varieties, Poston manually simulated the potential foliage loss caused from ASR by manually removing a percentage of foliage from each plant.

From the trials, Poston reports that when the foliage was eliminated at R1, the effect on potential yield outcome was more detrimental to determinate soybean varieties than indeterminate varieties. However, at R5, opposite results were shown.

“From what we saw, we’re recommending growers spray at R3 to R4 because the growth habit of the plant at this stage has little impact on the difference in yield loss after removing the foliage,” Poston notes. “We consistently saw a 25 percent yield loss at this stage regardless of growth habit.

“Our results have us evaluating if we may need to protect the foliage on indeterminate soybean varieties longer than we had originally thought, especially in environments with high yield potential.

“It’s important that when we make preventive or curative applications, we do a good job to cover the plant late in the season,” he adds.