Stop Soybean Yield Robbers

Emily Unglesbee, Paul Queck and Pamela Smith

Soybean pests are sneaky. Just when you think you’ve got the answer in the bag or the bottle, the troublemakers are back to shave top-end yield.

The 2018 season ushered in a parade of problems, but three grabbed headlines--the pervasive soybean cyst nematode (SCN), the feisty disease known as frogeye leaf spot and a galling new insect that still has researchers scratching their heads.

KNOW YOUR SCN NUMBERS

By Emily Unglesbee

Brian Nunemaker has a shopping wish list for soybean varieties. Like most growers, he desires the whole package of protective traits. But one characteristic has risen to the top of the shopping cart: soybean cyst nematode (SCN) resistance.

About 10 years ago, the south-central Michigan farmer got a shock when the soils he and Ron Hostetler farm, near Bronson, were tested for the microscopic pests.

High populations of SCN are classified by Michigan State University as more than 10,000 eggs per 100 cubic centimeters, roughly half a cup of soil. The numbers for Oak Prairie Farms soils were closer to 20,000 eggs.

The farmers quickly switched to soybean varieties that use a type of SCN resistance known as PI 88788. The results were immediate, Nunemaker recalls. He remembers looking out over a field of soybeans while working high up on a pivot the following year.

“I could tell to the foot where we ran out of cyst-resistant beans,” he says. “They looked like a completely different variety--they were not as tall. They didn’t look good.”

Sounds like a happy-ever-after ending, right?

Not quite. Just five years ago, Nunemaker noticed his bean yields slipping again. Another SCN test confirmed the populations in his field were able to reproduce and thrive on the PI 88788 soybean varieties.

Nunemaker has since switched to varieties with Peking resistance. There are fewer of them in the marketplace, but it’s well worth the effort for growers facing resistant SCN populations, he says.

“We saw a minimum of 5 bushels per acre difference after we switched,” he says.

INDUSTRYWIDE PROBLEM. Since its arrival in North America in 1954, SCN has been confirmed in 31 states and two Canadian provinces. The nematode is widely believed by scientists to be the biggest cause of yield loss in the soybean industry. One 2017 study estimates that SCN robbed growers of 617 million bushels between 2010 and 2014--more than twice any other soybean disease considered.

These yield losses are likely rising thanks to SCN virulence--the pest’s ability to reproduce on an SCN-resistant variety’s roots. This phenomenon has increased dramatically over the past two decades in soybean-heavy states.

“The problem is especially bad in north-central states like Illinois, Iowa and Missouri, which were hot spots for PI 88788 planting,” says Melissa Mitchum, a plant pathologist at the University of Missouri.

Scientists who study this pest have launched a project called the SCN Coalition in recent years. It’s the second of its kind--the first SCN Coalition began in 1997 and helped raise awareness about this pest when it was just beginning to drag down yields in the Soybean Belt.

That coalition helped the industry move to SCN-resistant soybean varieties, but companies favored a single source: PI 88788. That type of resistance came to dominate the market and still accounts for around 98% of all soybean varieties, Mitchum says.

The relaunched SCN Coalition is now tasked with raising farmer awareness of SCN virulence on PI 88788 and helping industry, academics and growers to address it, Mitchum says. See more details and a list of labs that can test soil at www.thescncoalition.com.

“Part of the mission is to introduce new types of resistance, stack them and decide the best rotation strategy moving forward,” she says.

KNOW YOUR ENEMY. With all these big, scary numbers on SCN, why aren’t farmers everywhere rushing to their fields to manage it?

The pest is just too darn subtle, Mitchum explains.

When an adult nematode infects a root, the female forms a cyst full of eggs on the root. That cyst is an amazing survival structure, Mitchum says. Within it, eggs can endure for years, even decades, in the soil, unfazed by the cold temperatures or lack of host plants as they wait for soybean roots to show up and trigger a hatch.

At low to moderate levels, SCN won’t make your soybeans look sick under good growing conditions, says Greg Tylka, a nematologist with Iowa State University. They will just yield less, yellow a little faster and be more vulnerable to other diseases, such as SDS (sudden death syndrome).

When you can actually see a problem, it’s pretty late in the game, explains Pat Duncanson, a south-central Minnesota farmer who has been managing SCN for more than two decades.

“If you wait until SCN problems demonstrate themselves visibly in the field, it’s a horrible problem, and it will take a long time to correct that problem on a farm,” he says.

In addition to their survival skills, this type of nematode has another weapon in its arsenal--genetic diversity.

“Most growers are planting PI 88788, and because there is a lot of genetic heterogeneity in SCN field populations, there are always a few individuals able to grow on those beans,” Mitchum explains. “They infect, reproduce, create more progeny and then reinfect. Next year, they’ll do better than the rest and slowly shift the population toward more individuals able to grow on an SCN-resistant variety.”

Companies have been slow to work with new types of resistance, so finding SCN-resistant varieties beyond PI 88788 is a challenge, Mitchum adds. Peking varieties are increasing in availability, but they are still just a fraction of the soybean seed sold each year.

Public breeders at the University of Missouri are actively breeding soybeans with additional forms of resistance, such as a broad spectrum resistant source called PI 437654 (Hartwig), Mitchum says. “The ideal situation is that companies will pick up that germplasm and start using it in their programs or, at least, license some of these varieties,” she says.

BEYOND THE SEED BAG. Soybean growers have a lot of management to think about, and for years, SCN seemed to have a reliably simple solution, Duncanson says.

“Part of the problem is that we look at our seed guides, and we look under the heading of SCN, and it says it’s resistant, and that’s as far as most people probably go,” he says.

While breeders work to improve genetic resistance, there are other ways to try to combat the pest.

First and foremost, set up your beans up for success, Duncanson says. “SCN is a pest of opportunity,” he says. “If we do a really good job with fertility, with drainage, with trying to have a healthy soil system, we can do an awful lot to mitigate maybe not the prevalence of SCN but the effects of it.”

He also credits longer rotations away from soybeans and the use of seed treatments that target SCN with keeping his SCN numbers low and even undetectable for more than 20 years--even as he used PI 88788 resistance year after year.

Companies may be slow to find and use new genetic sources of resistance, but they have been racing to produce nematode seed treatments recently, Tylka says.

At least eight products on the market are advertised as targeting SCN. While none of them have consistently produced measurable yield bumps or changes in SCN numbers in university field trials yet, from time to time, one will produce a significant result, Tylka says. “It’s hit or miss [in our experiments],” he says. He’s working to understand better why seed-treatment benefits are so variable.

Both Duncanson and Nunemaker also use cereal rye cover crops on some of their acres.

Cover crops haven’t been definitively linked to decreasing SCN numbers, Tylka notes, but Duncanson is interested in the possibility that their soil-health capabilities could limit the pest’s impact over time.

“If we can lower the overall stress in the soybean-production system, we don’t give the cyst as much of a foothold,” he says.

But, of course, the first and most important step is testing your soil.

“If you don’t know your numbers, you can’t take advantage of your options,” Duncanson says.

GET THE SPOT OUT

– By Paul Queck

Frogeye leaf spot (FLS) fungus lives up to its name. Walk into an infested field in mid- to late summer, and the round eyelike spots seem to glare back from the sea of soybeans. All those staring intruders would be almost comical if there weren’t such potential for yield loss.

Unfortunately, FLS made a big splash in 2018, popping up much farther north than normal. DTN disease prediction maps pegged the early signs of FLS risk in central Illinois in mid-June, and, by late July, the entire soybean belt was under red alert.

A few preventative steps may help manage the disease in 2019.

THE RIGHT MIX. “It takes a combination of conditions for FLS to create yield loss,” says Gail Stratman, a regional technical support manager for FMC Corp. “You must have a susceptible host (such as a susceptible soybean variety), the fungus inoculum, which can blow in or be on residue, and then, you have to have the right weather conditions.”

Farmers can’t control the weather, but they can plant resistant soybean varieties. “Susceptibility to frogeye leaf spot will depend on the race of the pathogen and the resistance genes present within the soybean variety,” explains Darcy Telenko, Purdue University Extension plant pathologist.

You can also rotate crops to control the inoculum. “The fungus causing FLS can survive and overwinter in infested soybean residue,” notes Missouri Extension plant pathologist Kaitlyn Bissonnette. Rotating crops and not growing soybeans after soybeans can break the disease cycle.

Reduced tillage systems tend to have more problems since the pathogen overwinters in residue.

POTENTIAL LOSSES. How much soybean yield are you risking by ignoring managing for FLS? “Yield losses can be up to 35 percent if there’s a severe outbreak early or just after flowering,” Telenko says.

FLS thrives in long periods of warm, humid weather with frequent rains and overcast skies. Those were the weather conditions much of the Midwest experienced in early August, with much of the soybean crop in R4 and R5. Even at that stage, Stratman estimated then that FLS-infected soybeans could see yield reduced by as much as 10 to 15 bushels per acre from their potential.

FLS infection begins in newer leaves at the top of the plant. Stratman explains that, as the season progresses, the tops of plants infected with FLS start to turn yellow, then brown, as if the plants were maturing early. “Unfortunately, they are maturing early due to disease progression and not because they are finished growing,” he says.

Soybean pods typically fill from the bottom of the plant to the top. “When the leaves at the top of plants start browning and dying from FLS, the plant is going to try to finish what it has,” Stratman says. “The upper part of the plant where the leaves are dying back is probably not going to produce a lot of soybeans. Losing the pod-fill at the top of the plant is the difference between a fair soybean yield and a really good soybean yield.”

FUNGICIDE DECISIONS. Applying a fungicide is a control option for growers. However, by the time you notice lesions, it can be too late to get an economic return from an application. The decision to treat should be made at around R3 for optimum control. That often means applying fungicide protection before FLS symptoms appear in full severity.

“The key is to do regular scouting when soybeans reach the early reproductive stages and treat when you first begin to see FLS lesions appear,” Stratman says. “Catching it early is key to minimizing yield impact. When the field starts looking tough from the road, it is likely too late. You need to get in the field and look closely on a regular basis.”

The decision to apply a fungicide becomes much easier if there are other diseases (besides FLS) and insects that need to be controlled, Telenko observes.

If you apply a fungicide to control FLS, Telenko recommends applying one with multiple classes and for sure not a fungicide with only a QoI, strobilurin (Group 11), fungicide. Isolates of the pathogen causing FLS have been found to be resistant to strobilurin fungicides in several states.

A disease risk calculator that takes preceding crop, tillage program and variety resistance into consideration can be found at cropdisease.cropsciences.illinois.edu/?p=776. DTN customers using the MyDTN.com product can find Crop Disease Risk prediction maps for FLS and three other diseases in the Ag Weather maps section.

MONITOR GALL MIDGE

– By Pamela Smith

Wayne Martin is on a mission. A small maggot called the soybean gall midge infiltrated his soybean fields this summer, and the Iowa farmer isn’t going to rest until he finds some answers.

“They started out in the field edges but moved into the middles of fields and destroyed big sections of the field,” Martin says. The maggots eat away at the vascular tissue under the stem surface at the base of the soybean plant, eventually causing the plant to die.

The pest has been reported before but was initially thought to only feed on diseased plants. This year, it showed a taste for healthy plants and marched across portions of western Iowa, eastern Nebraska and eastern South Dakota, and was found in a few spots in southwestern Minnesota.

Still, little is known about its habits and behavior. Extension entomologists in several states are attempting to rear adults from the maggots captured this year for species identification and to try to devise management strategies.

Gall midge is thought to be a relative of the Hessian fly. Insecticidal controls have been unsuccessful, because the fly doesn’t feed before laying eggs into soybean stems, and the larvae are protected from insecticides inside the stem.

Erin Hodgson, Iowa State University entomologist, says what makes the pest more challenging is the gall midge may have two to three overlapping generations per year. “They appear to be weak fliers, only moving as far as they have to to lay their eggs,” Hodgson says. The pest typically infests the edges of the field first.

There have been little other patterns that are consistent to crop rotation, cover crop, varieties and crop injury.

Martin cooperated with researchers this summer by putting sticky traps in his fields to monitor adult emergence. “My hope is perhaps we’ll find a match with another gall midge species and can ride on some existing research.

“It’s because we have so few answers that it’s important to keep telling the story of this pest. I’ve heard farmers say that they might not grow soybeans next year if we can’t get this figured out,” he says.

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