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Soybean rust making late push on wet weather

Soybean rust is making a late push into west-central Mississippi and southeast Arkansas, but Extension leaders in those states don’t think the outbreak is serious enough yet to warrant blanket sprays.

On Sept. 9, soybean rust was confirmed in Holmes, Humphreys, Sharkey, Warren and Washington counties in Mississippi and in Ashley, Chicot, Drew and Desha counties in Arkansas.

“Our plant pathologists have scoured the south Delta counties and have found rust in five counties,” said Mississippi Extension soybean specialist Trey Koger, who noted that most of the soybeans in those counties are very mature and the level of infestation very low.

Koger said that a large percentage of soybeans in the north part of the state “are still vulnerable, but a lot of them are at R5 and quickly advancing toward maturity. A lot of them have also been sprayed with a fungicide, and the fungicide is still protecting them.”

“At this time, the southeast part of the state is at more risk than the rest of the state and should be the primary area considering applying a fungicide,” said Scott Monfort, University of Arkansas Extension plant pathologist.

“This could change in the next few days as we continue to scout the other regions of the state and/or if Hurricane Ike decides to visit Arkansas.”

According to Monfort, all the infected fields discovered in Arkansas “had very low levels of active soybean rust at an estimated 1 percent incidence (1 out of 100 leaves showing symptoms) and seem to be developing slowly.

“For the most part, soybean rust will not be a problem in a majority of our acreage due to its late arrival. Unfortunately, we do estimate that we still have about 200,000 acres that could be impacted by soybean rust if cloudy and rainy conditions persist over the next two weeks.

“The extremely late soybeans are the ones we worried about. But a lot of early-June planted beans are probably already at R5, and probably are not going to be affected.”

Monfort said a big danger from hurricanes is not always their capability for spreading the disease, but from the moisture and cloudy days they bring. “If these conditions persist, we might see the disease (start to flourish).”

Monfort recommends that producers scout soybeans and send in samples if they have suspicious fields. Before applying a fungicide, he recommends you consider a few things first:

1. Are you within a couple of counties from the confirmed locations?

a. Yes — you are a higher risk

b. No — you have a little more time to make a decision

2. Have you scouted your soybeans?

3. What growth stage are your soybeans?

a. R1 to early R5 are at more risk

b. Mid-R5 to R8 — you will more than likely outrun yield impacts

4. Yield potential of your soybean crop?

a. SBR control may not be economical if yield potential of less than 25 bushels per acre (depends on what you have in your crop to date)

5. What fungicide/fungicide combination do I use if I need to make an application?

a. Soybean fields at R1 to R4 should be sprayed with a combination of a strobilurin and a triazole fungicide.

b. If soybeans are currently between late R4 to R6, an application of a triazole alone should be applied.

c. Soybean fields currently at or beyond R-6 are no longer vulnerable and should not be sprayed.

“We will continue to scout kudzu patches, soybean sentinel plots, and soybean fields throughout the state and will update everyone as soon as we have new information,” Monfort said. “I would like to ask county agents to send in samples to the diagnostic lab at Lonoke. I would also encourage consultants and growers to send in samples (at least 100 leaves) if you think you have soybean rust.

The Soybean Rust Hotline is continually updated with information about the latest rust finds that number is (866) 641-1847. If you have any questions or concerns contact Scott Monfort (870) 659-0648, Jeremy Ross (501) 944-0621, or Amy Carroll (870) 258-2509.


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