ILSOYADVISOR POST

Disease Management: Soybean Cyst Nematode Threat Calls for New Management Options

Published in the June 2015 edition of No-Till Farmer…www.no-tillfarmer.com

No-tillers may already have this parasitic soybean pest, but rotating crops, using resistant varieties and adding seed treatments can help them win the battle.

There are two kinds of soybean farmers in the Midwest, Greg Tylka says: Those who are concerned about soybean cyst nematodes (SCN), and those who should be.

The Iowa State University plant pathologist showed attendees at the Iowa Soybean Assn. Research Conference just how much SCN has spread in the Midwest over the last several decades.

As of 2014, 30 states from the East Coast up into the Great Plains have been affected by SCN. Random sampling found 3 out of every 4 fields in Iowa have SCN, and it’s present in 4 out of every 5 fields in Illinois. 

Tylka explained why this parasite is difficult to terminate, and what no-tillers can do to limit its presence.

Rapid Reproduction

Nematodes are colorless, microscopic worms that start out as eggs that feed on soybean roots, Tylka says. When they’re mature enough they’ll mate outside of the root, and the female will produce eggs.

“This female will be mated by many different males, and a male will mate with many different females,” Tylka notes. “What that means is the offspring aren’t identical, genetically. And that gives the nematode great ability to adapt to different conditions.”

The female will produce about 50 eggs outside of her body, he adds, and then she’ll fill up internally with another couple hundred eggs. Eventually the female nematode dies and turns into a cyst – a dark brown, tough, leathery shell full of eggs – and the process starts over.

This life cycle occurs about every 24 days in the middle of the summer once the soil has warmed up, Tylka says.

“Depending on the growing conditions, we can have three to six generations per year,” he adds. “Each female produces a couple hundred eggs. Numbers build up pretty quickly.”

Rotation Isn't Enough

To make matters worse, those populations can last for years. Tylka says not all SCN eggs hatch, even during soybean years, and unhatched nematode eggs can survive for more than a decade without soybeans. Scientists believe that 30-40% of all SCN eggs are dormant.

"As that happens year after year, it's just a steady progression of nematode parasitism," he says.

Because of this survival tactic, crop rotation alone can only do so much.

"Corn is a useful management tool, but it can't eliminate SCN," Tylka says. "Imagine you grew soybeans and you ended up with a level of 17,000 eggs for 100 cubic centimeters (cc) of soil (equal to a little less than a half-cup of soil). One year of corn is going to drop it by as much as 50%.

"But then a second year of corn is gong to decrease the numbers less, and then it'll be even less of a decline in third-year corn, and then the decrease really flattens out."

What causes a nematode to hatch remains unknown, Tylka says. And while SCN likes high-pH soils, there hasn't been enough research to determine what conditions sustain dormant SCN.

Use Resistance

Since skipping soybeans only reduces SCN a certain amount, no-tillers should look at other options for eliminating this pest. 

Tylka's first recommendation: Use a soybean variety resistant to SCN.

"They pay you twice," he says.

The first payment is in yield. Tylka says Iowa State does a variety trial program every year where dozens of resistant soybeans are compared to susceptible varieties. They found resistant soybeans yield about 7 bushels higher than susceptible soybeans.

Because there's no additional cost for seed of the resistant varieties, that yield advantage results in pure profit.

"The second payment is the next time you grow soybeans," Tylka says, noting one study where researchers started out with 1,300 SCN eggs, and at the end of the year the number had increased to 2,100 under the resistant varieties. 

"That's pretty much a sampling error in our world, those two numbers could be identical," he says. "But under the susceptible varieties, those numbers increased 5, 6, 7, 8-fold. 

"How much is that going to translate to bushels per acre the next time you grow soybeans? There's your second payment, and that one is as valuable, I think, as the short-term yield boost payment."

But Tylka warns that resistant varieties aren't a cure-all. While there are hundreds of resistant varieties to choose from, about 98% have the same source of resistance: a breeding line called PI88788. The few nematodes that can feed on that source will continue to build over time. 

"Most of our nematode populations throughout the Midwest have levels of 20-40% reproduction on PI88788," he says. "The yields are going to continue to decrease as long as we only have one source of resistance."

Treat Seeds

Because there is resistance buildup to PI88788, Tylka thinks more growers will have to turn to seed treatments as an added management option. 

There are five different seed treatments for nematodes available at the moment. Syngenta's Avicta Complete was the first on the market, Tylka says, adding that its active ingredient, abamectin, is a naturally occurring chemical that inhibits nematode nerve transmission. It has effects on all plant-parasitic nematodes, not just SCN.

Plant Health Care's N-Hibit seed treatment contains a harpin protein that's known to stimulate plant defenses, Tylka says. Another options is Votivo from Bayer CropScience. It contains the biological Bacillus firmus, which repels nematodes from entering the roots.

Clariva from Syngenta is a bacterial parasite that infects SCN, and the latest introduction to the market is Bayer's Ilevo. Tylka says Ilevo targets sudden death syndrome (SDS) but was discovered to have efficacy against SCN as well. It works by inhibiting enzyme respiration on both fungi and nematodes.

Laura Barrera is the managing editor of No-Till Farmer and Conservation Tillage Guide magazines. This article originally appeared on No-TillFarmer.com and has been reposted with permission.


Laura Barrera


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