When looking out across soybean fields, it is daunting to think about managing billions of tiny nematodes.
What can one farmer do against so many?
Science and agronomy hold the keys to smartly managing soybean cyst nematode (SCN) populations. A new Soybean Cyst Nematode Coalition (modeled after the first SCN Coalition of 1997) is encouraging farmers to test their soils for SCN type as well as population.
The big concern is SCN is becoming resistant to the PI 88788 source of resistance that is used in more than 90 percent of resistant varieties.
For 2019, the recommended management strategies from the SCN Coalition include the following:
- Scout for SCN
- Know your SCN number
- Know your SCN type as there are field specific populations
- Practice crop rotation
- Plant varieties appropriate for managing your specific SCN type
- Rotate soybean varieties
- Consider using a nematode seed treatment
In Minnesota, educators estimate that SCN is resistant to 17 percent of PI 88788 varieties. That number goes up to 25 percent in South Dakota and 67 percent in Wisconsin, said George Bird, a professor at Michigan State University serving on the SCN Coalition.
Bird said the Coalition has a goal of keeping PI 88788 resistant SCN at low reproduction numbers.
“From an economic or yield standpoint, when the nematode gets aggressive and reproduces on varieties derived from 88788, yields will be reduced,” said Bird. “It’s estimated that this reduction can be an average of about 14 bushels per acre.”
There are at least 109 known sources of soybean resistance to SCN, and only seven have been commercialized, he said.
He added that farmers also have at least eight commercially available SCN control products available. These include a number of different active ingredients and modes of action, so the SCN Coalition is recommending growers work closely with their seed providers to find the most appropriate products.
The seed treatments prevent SCN infections at early soybean growth stages, he said. The soybean plant may still be attacked by SCN later on, so resistant varieties need to be used.
“If you are going to use a seed treatment, have some research data and maybe even some from your own farm,” he said. “While I can give general information about SCN management, farmers have to do the research about their particular farm sites and fields.”
That includes understanding the types of SCN in each field and what resistance the SCN have to either PI 88788, Peking or both. Bird said he’s aware of about a dozen fields in Michigan where SCN populations can overcome both sources of resistance.
“The thing that is important is to know that in your field populations, each these individual nematode is not the same,” said Rick Masonbrink, associate scientist in the Genome Informatics Facility at Iowa State University. He spoke during a recent webinar put on by Seed World.
When farmers use one or two methods for killing off SCN, eventually the SCN left in the field are the ones that have natural resistance to those methods.
Masonbrink and collaborators have sequenced the genome of SCN and are currently working on an improved version with nine chromosomes. Masonbrink has been working on predicting genes within those chromosomes and expects to have the information available within the next year.
One of the challenges to assembly is that various SCN do not all have the same genome sequence.
“We just can’t sequence a single individual yet,” he said. “The technology isn’t there.”
There is one SCN inbred population that is currently used for the SCN genome, but every population is different, he said. Different populations of SCN develop resistance in different ways.
“We need more sequencing for bunches of different populations, so we can start to identify what genomic features or genes are causing virulence for the SCN to overcome resistance and reproduce,” he said. “If we can get multiple populations, then we can see how these populations are changing.”
He pointed out that both soybean resistance and SCN resistance are sort of like a dog chasing its tail. The soybean develops resistance and the SCN finds a way to overcome it, so the soybean develops another form of resistance.
Masonbrink wants to find a way to give soybeans a hand up to break the dog- chasing-its-tail cycle. There could be a genetic answer involving the number of copies of a gene. It’s also possible that figuring out the SCN genome may help researchers understand what causes various SCN to hatch.
“You can stimulate (hatching) with zinc, but we don’t know what causes them to hatch,” he said. The SCN can stay viable in the soil for up to 10 years without hatching – just waiting for the right host.
The genome could also hold answers on what draws the SCN to the soybean plant, as well as what are the chemicals that make the soybean root receptive to the SCN.
Scientists have identified 121 different SCN “effectors” that make the soybean plant susceptible to these nematodes – and there are many more. They’ve learned there are effectors that allow nematodes to travel through the soybean roots, and there are modifiers that manipulate the soybean’s DNA to work for the SCN. The SCN can also effect carbohydrate metabolism in the soybean plant so that the SCN can assimilate the host’s carbohydrates. There’s immune suppression that allow the SCN to prevent the host cell from triggering defenses too.
Looking at and identifying these effectors and modifiers is a big target for scientists. Once they determine what the SCN genes do, they hope to figure out how SCN harms soybean plants.
“When we figure out what’s being manipulated in soybeans, we can figure out how to shut it off,” he said. “We can’t engineer SCN yet, but we can engineer soybeans to make these things that would either identify when SCN is attacking or to shut it off, prevent SCN from hatching, be prevented from being attracted. It’s still in the future, but it’s coming.”