ONE GROWER PUBLISHING, LLC
MAY 2017
A ‘game changer’ Mild weather could prompt redbanded stink bug buildup, surprising growers
■ Target spot outbreak in trial sheds light on disease’s complexity
■ Scientists delve deeper into genetics behind PPO-resistant pigweed
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No simple answer Unexpected target spot outbreak in frogeye leaf spot trial sheds light on complexity of disease management.
By Tom Allen and Trent Irby
4 key take-aways Even though yield was harvested from each plot, yield data were confounded by multiple diseases and the resulting severe defoliation from target spot. Based on these 2016 trials, several key statements can be made. First, regardless of fungicide active ingredient, target spot severity increased over time. Fungicide products applied either contained a stand-alone strobilurin (QoI) or tri-
TOM ALLEN, MISSISSIPPI STATE UNIVERSITY
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ungicide efficacy trials originally designed to focus on frogeye leaf spot also yielded data on two other soybean diseases — target spot and Septoria brown spot. During 2016, two separate fungicide efficacy trials were conducted in Starkville, Miss. Trials, that also included a grad student, were conducted with a frogeye leaf spot-susceptible variety (Armor DK 4744) to look at fungicide efficacy on frogeye leaf spot as well as observed differences in phytotoxicity caused by the selected fungicides. Trials were sprayed with several different foliar fungicides at 15 gallons per acre by ground and spray applications included an adjuvant (0.25% v/v NIS). Plots were sprayed at R4, and disease was rated pre- and two, four and six weeks post-application. Even though frogeye leaf spot was the targeted disease, severe Septoria brown spot as well as target spot were observed throughout the treated plots. Diseases were rated using a general 0 to 9 scale. However, rating for Septoria brown spot as well as target spot requires altering the scale to consider the disease within the crop canopy (from bottom to top). To rate for Septoria brown spot and target spot, rather than using the scale to consider the presence of each disease in the top of the crop canopy, the canopy was split into thirds and rated using a modified 0 to 9 scale.
Target spot of soybean can produce symptoms with subtle concentric rings as well as yellow margins around the lesions.
azole (DMI); were applied as a commercially available pre-mix (QoI + DMI); and in one case a three-way pre-mix fungicide (QoI + DMI + MBC (thiophanate-methyl)). Based on the occurrence of target spot at the three ratings post-application, fungicide products did not reduce disease severity within the crop canopy. As a result of the 2016 target spot infestation, severe defoliation was observed between four and six weeks post-treatment regardless of fungicide product. Second, some fungicide products substantially reduced defoliation caused by target spot; however, this did not translate into increased yield. Third, the greatest difference between the two trials had to do with canopy closure
within the plot area. Trial 2 did not have nearly as much canopy closure, but target spot was still more severe than normal as a result of the prevailing environment that occurred during 2016. Fourth, the response of Septoria brown spot following a fungicide application was similar to target spot regardless of fungicide product. Dr. Tom Allen is an Extension plant pathologist based at Mississippi State University’s Delta Research and Extension Center in Stoneville. He may be reached at tallen@ exension.msstate.edu. Dr. Trent Irby is Extension soybean specialist on MSU’s main campus in Starkville. He may be reached at tirby@pss.msstate.edu
Things to keep in mind for managing soybean target spot 1. In general, soybean target spot is a disease that commonly occurs in the lower canopy. 2. Environment is the most important aspect as to whether target spot will occur to the same degree in 2017. 3. Even though environment is the main ingredient for disease to occur, some soybean varieties are extremely susceptible to target spot. 4. Spraying a fungicide automatically and not observing target spot after the application does not mean that the fungicide “controlled” the disease. 5. Keep in mind that a mixed mode of action fungicide application may be more beneficial in the Mississippi soybean production system for 2017 and the future. 6. Soybean foliar diseases occur as a complex, and rarely is only one disease the target of a fungicide application. For more on target spot, visit http://bit.ly/2oFzcXd
On the cover: Based on a mild winter, redbanded stink bugs are expected to be a problem throughout much of the Southern soybean belt this season. Photo by Russ Ottens, University of Georgia; bugwood.org Twitter: @SoybeanSouth
MAY 2017 SOYBEAN SOUTH
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A ‘game changer’ A mild winter and warm spring have concerned Extension specialists, who say redbanded stink bug numbers could build up rapidly. By Vicky Boyd Editor
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mild winter across much of the Southern soybean belt has raised concerns of a bad year for the redbanded stinkbug, a voracious feeder that some have called a “game changer.” “The deal with stinkbugs is the more generations they are able to go through in a season, the more of a problem they’ll be,” says Gus Lorenz, a University of Arkansas Extension entomologist based in Lonoke. “With the warm weather we’re having and the warm spring that we’ve had, I suspect (RBSB) will build up to pretty good numbers in late April and early May planted beans.” The relative newcomer from South America has become so troublesome that some university entomologists have even reduced treatment thresholds for RBSB compared to those for green, Southern green and brown stinkbugs. “This is kind of a game changer for us when it comes to stinkbugs because these cause more yield losses than our regular stinkbugs,” Lorenz says. An aggressive feeder A native of Brazil and Caribbean basin countries, redbanded stink bugs were first found in Florida in the early 1990s. They began expanding north and west and were found in Louisiana and Texas in
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SOYBEAN SOUTH MAY 2017
2004, in Arkansas in 2005 and in Missouri in 2010. In Louisiana, for example, the newcomer has displaced native stink bugs. Work in 2011 by Joshua Temple, a Louisiana State University graduate student at the time, found that 65-70 percent of all stink bugs in Group IV soybeans were RBSB. In Group V soybeans, RBSB comprised about 60 percent of the overall stink bug population. Redbanded stink bugs are pod feeders, using their straw-like stylet to penetrate the pod and suck juices from the developing seed. At the same time, they inject more enzymes than other stink bugs, increasing damage to nearby plant tissue. If the feeding occurs early enough in pod fill, seeds shrivel, causing “flatpodding.” Feeding that occurs later may reduce individual seed size and, therefore, the overall weight of the harvest. RBSB infestations also have been linked to delayed maturity, a malady of unknown cause. Cold sensitivity RBSB only feed on legumes, but that provides a large buffet, ranging from clovers to soybeans to hemp sesbania. Unlike many other insects, RBSB don’t enter winter diapause, a state of winter dormancy similar to hibernation, says Jeff Davis, an associate professor of entomology with the Louisiana State University AgCenter in Baton Rouge. Instead, it continues to feed and reproduce. Because of its neotropical homeland, RBSB are cold sensitive. Temperatures of 23 degrees Fahrenheit for five to six hours will likely reduce populations by 10-30 percent, Davis says. SOYBEANSOUTH.COM
LSU AGCENTER
LSU AgCenter entomologist Jeff Davis sweeps a soybean field for redbanded stinkbugs.
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Prevent Subthreshold Insect Levels From Stealing Soybean Yields Advertorial
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starts looking pretty hard from R2 to R4. “Some growers prefer adding a pyrethroid instead of diubenzuron to their fungicide, depending on the bug mix,â€? he says. “It’s “It’s about the same cost, but we only get a few days activity from a pyrethroid. So we’re knocking everything out—beneďŹ cials along with insect pests—and 10 days later we’re right back where we started. With diubenzuron, we get long residual control, three to ďŹ ve weeks, and the insect numbers insect pest numbers never jump up. p. YYou’ll Yoou’ll still ďŹ nd bugs, but your inse stay at or lower than the untreated ďŹ eld across the turnrow turnrow. w. “We Wee plant Group Vs ďŹ rst on good, furrow-irrigated ated ground. W We’ve We had a lot of Vs that normally get a late spraying for some type of it’ soybean loopers or bollworms.. W worm, whether it’s We never have to spray diubenzuron-treated, early planted Group Vs for loopers. And that product is put out a month before we start seeing loopers. “In my scouting reports, for example, I might be running 5 of these insects in 25 sweeps and running 4 of these insects in 100 sweeps. Nothing is at threshold, but I recommend adding didi ubenzuron to your fungicide because you’re already going across the ďŹ eld. I believe the prophylactic treatment will save you a later spraying You spraying. Yoou can walk out there three to four weeks later, later and ďŹ nd didn’t because those untreated ďŹ elds are the ones where we have to go back to spray for loopers 9 out of 10 times.â€?
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Successful redbanded stink bug control Here are a few RBSB management tips provided by Jeff Davis, an associate professor of entomology with the LSU AgCenter in Baton Rouge. 1. Reduce overwintering hosts. 2. Don’t be afraid to take a chance on trap crops. 3. Plant stink bug-tolerant varieties. 4. Rotate insecticide chemistries. 5. Continue to scout until harvest.
Beginning in early April, Lorenz and Nick Seiter, an Extension entomologist in Southeast Arkansas, conducted sweep net surveys of crimson clover planted along roadways to gauge overwintering survival rates. Those figures could be a prelude to RBSB pressure later in the season. They also will be monitoring early planted beans in Southern Arkansas, most of which are indeterminate varieties. “That’s the time we really want to see what’s going on,” Lorenz says. “We suspect those early planted fields are attracting every stink bug in the county into those fields.” Scout early and scout often RBSB can be found in soybeans as early as R1. As a result, Davis recommends beginning to scout then and continuing to harvest. RBSB also prefer the lower parts of the plant canopy on which to feed and lay eggs. To obtain an accurate picture of insect pop-
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SOYBEAN SOUTH MAY 2017
TEXAS AGRILIFE
“We haven’t gotten that cold—it’s been very warm weather,” he said in early April. In states to the north, RBSB has not been the year-in, year-out pest it has in Louisiana. About six years ago, the insect started creeping into Arkansas, moving north into the Bootheel of Missouri and into Tennessee, Lorenz says. Just as they began building to threshold levels, several cold winters knocked down populations, and Lorenz says they hadn’t caused widespread economic damage for four years. He blames the mild 2015-16 winter for allowing RBSB numbers to build up to the point they reached economic thresholds in 2016. “They seemed to be a problem in Arkansas, particularly the southern half of the state last year,” he says. “We saw them in pretty decent numbers up through Marion, and they hit treatment levels about Pine Bluff. And with the mild winter we had (2016-17), we’re afraid of seeing them a little further north than that this year.”
Redbanded stinkbugs are more aggressive feeders than green, Southern green or brown stink bugs. And they inject larger quantities of enzymes as they feed than the other stink bugs.
ulations, Lorenz says scouts must dig deep with their sweep nets rather than just skimming the top of plants. Over the years, university researchers in several states have found treatment thresholds for other stink bugs were too high for RBSB and economic damage still resulted. LSU decreased its RSBS thresholds to 4 per 25 sweeps in 2009. The University of Arkansas has followed, and Mississippi State University reduced its to 4 per 25 sweeps beginning this season. Lorenz says he hopes to revisit Arkansas’ thresholds this season with a caged stink bug study to determine whether the 4 per 25 sweeps is justified. “We want to validate the thresholds we’re using because we don’t have any data in Arkansas to verify we need to spray at 4 per 25,” he says. And because RBSB feed later into pod development than other stink bugs, many universities recommend applying insecticides through R7 should scouting dictate it. Davis says LSU AgCenter soybean crop budgets include three stink bug treatments per season. Three classes of chemistries are registered for stink bug control: organophosphates, pyrethroids and neonicotinoids. Davis’ research has shown some RBSB populations have grown more tolerant to the OP acephate, prompting him to reiterate sound resistance management practices. “We really want our growers to rotate those, but we don’t want all three modes of action in the same mixture,” Davis says. And therein lies the challenge, says Harold Lambert, owner of Lambert Agricultural Consulting in Innis, La. “We’re trying to avoid excessive damage
to the crop and at the same time preserve the utility of the chemistries,” he says. Every recommendation Lambert makes is based on meeting or exceeding treatment thresholds. In the Group IVs he scouts, he says he’ll typically recommend one to two stink bug sprays. But with the Group Vs, he may have to recommend as many as four applications, although three is the norm. If the infestation is early and the field doesn’t have a tall, dense canopy, Lambert says he has good success with a tankmix of imidacloprid, lamda cyhalothrin and methylated seed oil in 20 gallons of water per acre. The materials are applied without air induction nozzles and with a sprayer speed of no more than 10 mph to obtain good spray coverage. “The proper spray application practices are essential for RBSB control regardless of insecticide choice,” he says. Otherwise, Lambert’s first recommendation may be bifenthrin by itself or a bifenthrin-neonic tankmix. The second one may be a bifenthrin-acephate tankmix, and the third one—if needed—may be acephate alone. Most of the soybeans grown in Louisiana require a desiccant before harvest. Depending on timing, Lambert says he may recommend a tankmix of acephate and a desiccant, such as paraquat. Growers also need to be mindful of the maturity stage of their neighbors’ fields, Davis says. If neighbors apply a desiccant, RBSB will likely migrate to nearby greener fields, a situation Lambert refers to as the “snowball effect.” “Those surviving stink bugs just roll to the next field,” he says. SOYBEANSOUTH.COM
It’s complicated Testing Arkansas and Tennessee samples Tranel and his colleagues decided to sequence the PPX2 gene in plants from Tennessee and Arkansas to see if they could find additional mutations. Sure enough, they found not one, but two, located on the R98 region of the gene. “Almost all of the PPO-resistant plants we tested had either the glycine 210 deletion or one of the two new R98 mutations. None of the mutations were found in the sensitive plants we tested,” Tranel says. Furthermore, some of the resistant plants had both the glycine 210 deletion and one of the new R98 mutations. Tranel says it is too early to say what that could mean for those plants. In fact, there is a lot left to learn about
BAYER
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Chris Tinius, Bayer global soybean breeding director, cuts the ribbon on the firm’s new Marion, Ark., Soybean Breeding and Trait Development Station.
n Bayer opens new soybean station near Marion, Ark. Bayer officially opened its $6 million Soybean Breeding and Trait Development Station near Marion, Ark., recently, reinforcing the company’s commitment to MidSouth variety development. “When launched in 2014, Credenz soybeans added strength to the production opportunities available to growers,” says Chris Tinius, Bayer global soybean breeding director. “This new facility adds weight to the bar Credenz raised.” The facility, surrounded by 180 acres of Twitter: @SoybeanSouth
Researchers have found genetic mutations may be responsible for PPO herbicide resistance developing in Palmer amaranth, also known as Palmer pigweed.
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research and showcase plots, will focus on developing maturity Group IV and V varieties. About 10 people at Marion will work with a larger global team to conduct research on genetics, chemistry and traits. In addition, Bayer operates three other regional sites for U.S. soybean breeding— Champaign, Ill.; Pikeville, N.C.; and Beaver Crossing, Neb. The Marion station is part of Bayer’s commitment to invest nearly $1 billion in the United States between 2013 and 2016. n BASF cuts ribbon on Beaumont, Texas, dicamba plant expansion BASF Corp. officials cut the ribbon recently on the completed $270 million expansion of the company’s Beaumont, Texas, dicamba manufacturing plant. The three-year project was the largest investment in the history of the BASF crop protection unit, Debbie Dalley, Beaumont site director, said during a live-streamed ceremony. “The investment marks a very important investment by BASF to remain competitive with a very competitive compound in a very competitive industry,” says Markus Heidt, president of BASF’s crop protection business unit.
this resistance mechanism. “We don’t know what level of resistance the new mutations confer relative to glycine 210,” he says. “There are a lot of different PPO-inhibiting herbicides. Glycine 210 causes resistance to all of them, but we don’t know yet if the R98 mutations do.” The team is now growing plants to use in follow-up experiments. Tranel hopes they will be able to determine how common the three mutations are in any given population. “That way,” he says, “when a farmer sends us a resistant plant and it doesn’t come back with the glycine 210 deletion, we will be able to tell him how likely it is that he’s dealing with another one of these mutations.”
S The company has invested an additional $290 million in eight other U.S. production facilities, including ones in Hannibal, Mo., and Sparks, Ga. The Beaumont facility historically produced dicamba, but the expansion will add capacity to manufacture Engenia—the company’s new BAPMA dicamba formulation. It is paired with cotton and soybean varieties genetically modified to withstand in-season over-the-top applications of dicamba. The Beaumont plant also is BASF’s only facility in the United States that manufactures dicamba. In addition, it produces dimethenamid-P, the active ingredient in Outlook herbicide.
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ork by University of Illinois researchers has found the mechanism behind PPO resistance in Palmer amaranth may be more complex than originally thought and may include a handful of genetic mutations. They hope other plant-testing facilities will use their findings to help determine how widespread the genetic differences are. About a year ago, UI researchers discovered Palmer amaranth was resistant to PPO inhibitor herbicides due to a mutation known as the glycine 210 deletion on the PPX2 gene. They were using a quick test originally developed for waterhemp, a cousin of Palmer amaranth, to determine PPO resistance in Palmer pigweed. The test worked frequently, but other times the mutation wasn’t showing
VICKY BOYD
up even though researchers were fairly confident the samples were resistant. “We started to suspect there was another mechanism out there,” UI Urbana-Champaign molecular weed scientist Patrick Tranel said in a news release.
Researchers delve deeper into the genetic mechanisms behind PPO herbicide resistance in Palmer amaranth.
BASF representatives lead visitors on a tour of the recently completed dicamba manufacturing plant expansion in Beaumont, Texas. MAY 2017 SOYBEAN SOUTH
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