Tennessee looking to south to assess risk of soybean rust

The discovery of Asian soybean rust in a sentinel plot in Alexandria, La., and recent southwesterly winds have Mid-South soybean producers asking themselves — does something wicked this way come?

There is cause for concern, according to Melvin Newman, a plant pathologist with the University of Tennessee Extension Service, Jackson, Tenn., primarily because the Mid-South soybean crop is at a vulnerable stage of development and the disease is relatively close.

“A lot of our beans are in the R-1 flowering stage or slightly thereafter,” said Newman, with the University of Tennessee Extension Service, Jackson, Tenn., speaking on soybean diseases during the Milan No-Till Field Day in July. “The weather is humid, it's warm, moisture is on the way and for the last couple of days, the wind has been coming from that direction (Alexandria). It's not a stretch of the imagination for us to get some spores in here.

“Of course, we have to get more proof than just spores flying through the air. The spores could be dead. When we see an actual infection on soybeans or kudzu, then we'll know, and that's when we'll put out the word to spray.”

In the meantime, soybean producers should begin preparations for spraying should an infection occur. Newman provided these tips:

Learn how to identify the disease

Asian soybean rust is similar in appearance to the disease Septoria brown spot, Newman said. “It's very confusing. Brown spot is usually on the lower leaves, just like rust. Soybean rust is always on the underneath side of the leaf. You'll see little, raised pustules on the soybean on the underneath side with rust. Brown spot is a little darker and flatter.”

The perfect time to spray is when you find the first pustule. “Once it gets rolling and you get a lot of it in the canopy, it's going to be a lot more difficult to control. This is why we have sentinel plots all around the country — to let you know where rust starts, so you can spray early.”

The rust pustule can be seen under a microscope and resembles a volcano with a hole in the top, according to Newman. “The hole is where these tiny spores are. The spores are ejected into the air and can blow hundreds of miles. In a strong wind, they can travel just like dust and pollen. If they land on kudzu or soybeans, they're going to infect those plants.”

How to sample for soybean rust

Start scouting at the R-1 stage of development, or first flower on the main stem. Look at the leaves on the inside of the canopy, down low, in the areas that are the most shaded and damp. Once it gets started, it can spread over the entire field rapidly.

Pull 100 leaves that you suspect might have rust. Put the leaves in a plastic bag with a dry paper towel, then double bag it. Carry the sample to your county agent. In Tennessee, county agents have microscopes and are trained to spot the disease.

Store the sample in a refrigerator until you deliver it to the agent. If you ship the sample, double-bag the sample and seal it in a cardboard box.

Know when to spray

Spray from R-1 to R-5, first flower on the main stem to when pods in the upper part of the plant are filling out. “After R-5, you don't spray anymore for diseases because you're probably going to be off-label and you're probably not going to do much good.”

If there is no threat of rust, many west Tennessee soybean producers spray a fungicide anyway at R-3 for diseases such as frogeye leaf spot. R-3 is when there is at least one quarter-inch pod in the upper four nodes of the plant. Continue to watch for rust after that.

Get good coverage

The spray has to penetrate the canopy. “If you don't get the spray down to the bottom leaves, you're not going to get good control.”

Smaller droplets are better, 250 microns to 300 microns. “You can get that with a flat fan or even a hollow cone nozzle. Be careful. You're going to get some drift with a droplet this small. A lot of you may want to put glyphosate in with the fungicide. That's okay, but remember, you may get drift onto another crop and get some damage if there is glyphosate in the tank with the fungicide.

“If you don't get good coverage on the lower leaves, the disease will continue to produce spores and reinfect the new growth that does not get any spray.”

Use the right fungicide

Fungicides for rust fall into two categories, protectant and curative. But fungicides are of little use once soybean rust has become even moderately established in a field.

“Triazoles do a little better job,” Newman said. “They dig in a little deeper into that pustule. But once that pustule forms, you're not going to kill it with any fungicide. That's why you have to get it on early. No matter if it's a triazole or a strobilurin it's not going the fungus that's already established.

“The strobilurin fungicides will control rust too. But it's imperative that you get them on before the rust gets there. They will prevent rust, but they won't cure it. If rust is coming and you put out a combination, you get the best of both worlds.”

Stay on top of disease movement

Check the Web site, www.sbrusa.net, which is updated daily on the current locations of Asian soybean rust. The University of Tennessee also has a Web site, www.utcrops.com, which has information of the disease.

About 75 percent of the soybeans in west Tennessee are currently susceptible to the disease, according to Newman. “When the word goes out that we should be spraying, we need to spray in a hurry. The rain and wind may not be conducive to that. So farmers need to be ready go.

“We don't know whether or not we're going to get rust. But our soybeans are in the right stage. You need to watch. You need to be vigilant.”

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