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Light soybean rust pressures in 2014 may not repeat in 2015

COLD TEMPERATURES this past winter and a parasitic wasp helped to suppress kudzu bugs in Alabamarsquos 2014 soybean crop
<p>COLD TEMPERATURES this past winter, and a parasitic wasp, helped to suppress kudzu bugs in Alabama&rsquo;s 2014 soybean crop.</p>
Hard freezes last winter pushed soybean rust out of Alabama and back into central Florida. During the season, it moved slowly back into Alabama, but with hot, dry midsummer weather&nbsp; &ldquo;it just didn&rsquo;t move very much.&rdquo;

Alabama farmers were bracing for the worst this year in terms of soybean pest pressure, after getting a taste in 2013 of the havoc that kudzu bugs and soybean rust can wreak on a crop.

But most growers were lucky — and that’s a good thing, since they planted one of the Southeast’s largest soybean crops, about half a million acres.

But 2015 might be different, warn specialists with the Alabama Cooperative Extension System at Auburn University.

After what seemed like a kudzu bug infestation of plague proportions in 2013, the pest appeared to have been staved off this year, thanks to cold winter temperatures and a parasitic wasp.

“We’ve had a pretty light insect year on soybeans, for the most part, across the state,” says Tim Reed, Auburn University Extension entomologist.

“The cold winter appears to have knocked back the kudzu bug really hard. We didn’t spray that many acres, but we did get infestations along Interstate 65 — and that’s what we expect. They tend to be worse where they were initially spread, and they started along Interstate 65, hitching a ride on cars.”

Mixed pest populations

In late summer, Alabama growers were dealing with mixed populations of caterpillars, kudzu bugs and stink bugs.

“Those are the main pests we were seeing,” says Reed. “It makes for a difficult treatment decision, but you want to look for 20-percent defoliation and caterpillars being present, along with other insects.”

Producers with good yield potential were advised by Reed to go ahead and spray their soybeans.

“It will prevent a resurgence of kudzu bugs,” he says, “but you may have caterpillar problems to come in after you spray.

“We also were fighting soybean loopers at the Fairhope Substation in Baldwin County for about two weeks. We started out with a lot of them, but beneficial insects took them out.

“At Brewton, we had a lot of kudzu bugs at the research station. We sprayed an insecticide over some soybeans, which created a problem with loopers and green clover worms where we were conducting a trial.”

Thankfully, overall insect pressure generally was low on soybeans this year, says Reed. But next year might be different, depending on winter weather conditions and how the parasitoid wasp performs on kudzu bugs.

Slowed by the winter

“The parasitic wasps also were slowed by the severe winter,” he says. “They started off slow, with less than 1 percent parasitized kudzu bug eggs in June.

“By July, that had picked up to about 10 percent. Later, we were seeing about 20 percent to 25 percent parasitization of egg masses in the field.

“What I’ve observed in the plot work I’ve done is that if you start spraying these kudzu bugs early, before you begin picking up immatures, subsequent numbers of immatures will be reduced. Knocking those numbers down in mid- to late June really helps.”

At Brewton this year on June 18, Reed says, he was catching seven adult kudzu bugs per sweep. At Prattville, it was about the same as in 2013, with 16 adult kudzu bugs per sweep across two rows.

“We’ll see if we get a yield reduction. We’re looking at spraying different numbers of times during the year, and at different times, at Brewton and Prattville to see how that affects the yields.”

Threshold levels

Reed says Alabama Extension won’t consider changing the treatment threshold for kudzu bugs unless final yield data from the various test plots indicate a need to do so.

Thresholds for this year included:

1. Prior to first bloom, treat when there is an average of five bugs per plant for the whole field.

2. After first bloom through R6, apply insecticide when sweep net sampling catches 10 adults per sweep or one nymph per sweep.

3. If immatures are easily and repeatedly found on petioles and main stems during visual inspections of canopy, treatment is likely warranted.

Reed cautions that growers shouldn’t bias sampling to only border rows.

Treatment options include bifenthrin (Brigade, Discipline); lambda cyhalothrin (Karate, Silencer); carbaryl (sevin); zeta-cypermethrin (Mustang Max); acephate (Orthene); and gamma-cyhalothrin (Declare).

Light year for soybean rust

Alabama soybean producers saw a mixed bag of weather conditions this year: a wet, cool spring, followed by hot, dry conditions midsummer, says Dennis Delaney, Auburn University Extension agronomist.

What they didn’t see was a lot of soybean rust in their fields.

“We had a lot of frogeye leaf spot early on,” says Delaney. “With enough rain, even resistant varieties will be affected by the disease.

“We recommended that growers with frogeye-susceptible varieties, to moderately-tolerant varieties, should strongly consider applying a tank-mix fungicide combination at the R3 growth stage before the disease appeared in the field.

“Applying a fungicide after frogeye is observed in a field is not an effective method of controlling the disease. We’ve found that a lot of the disease in the Southeast is resistant to strobilurin fungicides. So, we’re recommending that our growers use a mixture of different chemistries to control it.”

A constant check continued throughout the year for the presence of soybean rust disease in the state, he says.

“The hard freeze we had during this past winter pushed the disease back into central Florida. It moved slowly back into Alabama and was found first in Baldwin County near the Gulf Coast and was then found in central Alabama.

“With hot, dry weather midsummer, it just didn’t move very much. We have sentinel plots that we check once each week to stay on top of the disease.”

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