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Late diseases again hit Tennessee soybean crop

While ample rainfall has contributed to a very nice Tennessee soybean crop, late-season scouting shows the moisture's ugly side. Diseases are sprinting through the state's crop and producers are scrambling to protect their assets.

“Diseases are really hot right now,” says Melvin Newman, a University of Tennessee plant pathologist stationed in Jackson. “Frogeye leafspot and anthracnose are giving us fits. This is the third year in a row we've had bad problems with disease in our beans. Last year, we lost almost a third of our crop (some 10 million bushels) to diseases and nematodes. This year — because of the wet weather — could be even worse.”

No isolated instance

Some fields are already badly damaged with frogeye leafspot. But with Group 5s just now being ready to spray, “I thought it would be a good idea to bring this up. I've got umpteen variety plots around Milan that are disease-ridden. And farmers across the state — this isn't an isolated problems in a couple of counties — are facing the same thing. This disease cycle has begun and, quite honestly, I don't look for it to slow down.”

Over the last couple of weeks, Newman has fielded an endless stream of calls on soybean diseases.

“I get calls from Extension, from producers, from chemical reps, seed reps, consultants — everyone. In the last 10 days, I've gotten more calls on soybeans than I have in all the years I've worked put together. I'm on the phone nearly all day, every day.”

Frogeye leaf spot has already damaged many susceptible early-maturing varieties that were not protected with a fungicide spray, says Newman. Other diseases, such as anthracnose, will likely also be very damaging this year. But symptoms won't show up until leaves begin to fall.

Control options

Newman says the best method for control is planting varieties that are resistant to the most destructive diseases — but there are no varieties resistant to all diseases and nematodes.

“Producers must know their disease potential and choose varieties that fit their conditions. In addition, in years when conditions are favorable for disease, producers have the option to spray with a foliar fungicide to control most of the diseases. Although well-researched and shown to increase yields, care must be taken to ensure proper application for maximum coverage and timing for best results.”

Applications, says Newman, must be made just before leaf and stem diseases infect soybean plants. Once disease symptoms are visible, it's usually too late for foliar fungicide application.

“However, one or two well-timed applications can significantly reduce the damage from disease and improve yields as much as 5 bushels to 15 bushels per acre, depending on the situation and yield potential. Soybean fields with a yield potential of less than 30 bushels per acre do not profit very well from a foliar fungicide. Late-maturing soybeans following wheat may be at risk of an early frost, and yield increases may not be realized.”

Usually, when soybean plants reach the R-3 to R-5 stage of growth (pods are about 0.25 inch to 0.5 inch in length), it's time to spray a recommended foliar fungicide. Seed producers and those wanting maximum yield may wish to make the two applications two to three weeks apart. However, for most producers the first application at growth stage R-3 is the one most profitable.

Coverage with the spray material is extremely important, says Newman. “Plant parts that do not get covered with fungicide will not be protected from disease. It's highly recommended that producers apply the fungicide in at least 20 gallons of water per acre with ground spray equipment or 5 gallons per acre with an airplane.

“For best results, a fine mist using hollow-cone nozzles under high pressure is preferred for fungicide applications. A spreader-sticker added to the spray mix can improve penetration and longevity of the fungicide under rainy conditions.”

(Editor's note: information for this article was taken from a report issued by Newman to Tennessee Extension personnel. Disease ratings for most varieties can be found on a new IPM Web site ( or at local Extension offices. A foliar fungicide point system is also available to assist producers with their spraying decisions.)


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