Widely Variable Soybean Cyst Nematode Makes On-Farm Testing Key

Think Different
The widespread drought made 2012 a banner year for SCN in Iowa. “In 25 years I’ve never seen as much reproduction as we did last year,” says Greg Tylka, Iowa State University (ISU) Extension nematologist and plant pathologist. “We don’t understand exactly why SCN seems to thrive when it’s dry, but we also know that nematodes grow best in the greenhouse under dry conditions. “
Conversely, the 2013 growing season began in Iowa with record rains, delaying soybean planting. “But by June 2,” Tylka says, “SCN females were reported on soybean roots in sandy soils just 26 days after planting. In a normal spring, we expect to see female nematodes 40 days after planting.

Criminal investigators often think like outlaws in order to catch them. Perhaps farmers should apply that strategy to stop the No. 1 soybean yield thief? Yes, soybean cyst nematode (SCN), a nearly microscopic organism, gets away with billions of soybean profit dollars each year. Thinking like a nematode can help arrest them, too.

What is SCN’s modus operandi? How does it thrive? What characteristics make it difficult to control? How does weather impact infestations? What management practices and growing conditions are conducive to reproduction?

Think differently for each field

Judd Hendrycks of North Mankato, Minn., began his SCN-management program 10 years ago when one field yielded 20% less than others. “SCN egg counts were higher than we wanted to see,” he says.

It’s been tough, the Nicollet County, Minn., farmer admits. “I’ve not seen a lot of change in yield. The resistant varieties have helped cut losses from 20% to 10%. I’ll continue to try new resistant varieties.”

One tough customer

“SCN is the ultimate pathogen,” says Greg Tylka, Iowa State University (ISU) Extension nematologist and plant pathologist. “It causes yield loss directly and indirectly by making other things, like soybean sudden death syndrome (SDS) and soybean brown stem rot (BSR), worse. We don’t know completely how it works, but it is a very consistent relationship,” Tylka says. “SCN has a unique biology that makes it very difficult to control. High reproductive capability, genetic diversity and variable responses to different cultural practices and control methods all contribute to it being the leading cause of soybean yield loss.”

SCN populations build rapidly because females’ very large ovaries enable them to lay several hundred eggs at a time, Tylka says. “Its fairly short lifecycle allows four or five generations to occur in one season.

“SCN females mate with many different males, providing tremendous genetic diversity,” Tylka says. This impedes resistance management because SCN can respond differently to various control methods. The genetic diversity is one of the reasons that SCN has been able to reproduce on soybean varieties with the PI 88788 and Peking sources of SCN resistance.”

And SCN has the unique ability to live in dormancy for 10 or more years without food, Tylka says. “Many farmers have infested fields and do not know it’s there. Infestations can increase undetected for five to 10 years. Soybean yields decrease steadily but the plants don’t look sick.”

Unpredictable response to weather

The widespread drought made 2012 a banner year for SCN in Iowa. “In 25 years I’ve never seen as much reproduction as we did last year,” Tylka says. “We don’t understand exactly why SCN seems to thrive when it’s dry, but we also know that nematodes grow best in the greenhouse under dry conditions. “

Conversely, the 2013 growing season began in Iowa with record rains, delaying soybean planting. “But by June 2,” Tylka says, “SCN females were reported on soybean roots in sandy soils just 26 days after planting. In a normal spring, we expect to see female nematodes 40 days after planting.

“Sandy soils almost always experience more SCN damage because they drain quickly and don’t hold nutrients very well. Poor soil moisture and nutrient deficiencies stress the soybean plants, making them more vulnerable to damage from SCN feeding.”

“SCN egg numbers also tend to be greater in higher-pH soils,” Tylka says. “Experiments in Iowa, Michigan, Wisconsin and Minnesota have shown that as pH increased across fields from 5.5 to 8, SCN populations increased too.”

Life after SCN

“An SCN infestation is not a death sentence,” Tylka says. “Finding it doesn’t mean you have to stop growing soybeans, you just have to manage it. “

Prevention is critical for long-term SCN control. “If you can catch it when populations are relatively small, it is much easier to keep low numbers low than to bring high numbers down.

“If you discover SCN in a field, start managing it with resistant varieties, crop rotation and seed treatments,” Tylka says.

“If results of an HG type (formerly called the SCN race test) reveal that you have SCN with increased ability to reproduce on soybeans with the PI 88788 or Peking sources of resistance, it’s advisable to plant soybeans with a different source of resistance,” Tylka says. “Unfortunately there are not too many available. Out of 771 varieties in the 2013 Iowa State SCN resistant-variety list, 755 of them had PI 88788 resistance.”

New nematicides and seed treatments, including Avicta, Votivo and N-Hibit, have come to market recently to help bolster SCN management programs early in the growing season.

In 2014, Syngenta plans to commercialize a biological seed treatment for SCN in its new Clariva Complete Beans. (See sidebar) The seed treatment uses a living, natural parasite of SCN– Pasteuria nishizawae --to kill the pest. “We’ve known for a long time that it is effective against SCN,” Tylka says, “but it remains to be seen whether it will colonize in the root system for season-long control.”

“SCN is almost universally the No. 1 soybean disease in the nation,” Tylka says. “But as other ‘sexier’ challenges — like SDS or soybean aphid — have arisen, a lot of people seem to have let down their guard about SCN.”

New seed treatment against SCN coming in 2014

Syngenta hopes to introduce a novel seed treatment for SCN control in 2014. Clariva Complete Beans nematicide/insecticide/fungicide is a combo treatment that adds a new nematicide to the broad-spectrum seed treatment of CruiserMaxx Beans with Vibrance insecticide/fungicide, says Palle Pedersen, seed care technology manager with Syngenta.

“Clariva Complete Beans includes the active ingredient Pasteuria nishizawae, a naturally occurring SCN predator,” Pedersen says. “It provides a direct mode of action because it must infect the nematodes to reproduce. P. nishizawae penetrates nematode juveniles to lay their eggs. This reduces SCN feeding and reproduction, and ultimately kills the pest.

“Spores of the parasite remain in the root zone throughout the season to offer continued activity against SCN,” Pedersen says.

“This has the potential to revolutionize soybean production,” says Seth Naeve, University of Minnesota Soybean Extension specialist. “We don’t know where this will go, but this naturally occurring bacterial that attacks SCN could be huge if it really works. SCN is our No. 1 pest; its impact has been completely underestimated. Breeders have had difficulty building resistance into germplasm, and we don’t have chemical controls,” Naeve says.

During three years of replicated yield trials, Syngenta found that Clariva improves yield by 3-5% on top of current SCN-control techniques and CruiserMaxx,” Pedersen says. “In addition to comprehensive pest protection, Clariva optimizes root health to deliver better plant emergence and stand, nutrient uptake and water usage, stress tolerance and overall soybean performance.

University research

Soil cores and root samples are needed to thoroughly test for all possible nematode species that feed on corn. From 2000 through 2010, 124 samples had soil alone, 17 samples only had roots, and 190 samples had both soil cores and root samples.

On average, only 15 samples were submitted annually for testing for nematodes on corn from 2000 through 2004. Annual sample numbers increased threefold beginning in 2005, but still averaged less than 50 per year through 2010.

Samples were received from only 53 of the 99 Iowa counties, mostly from northern, central and eastern Iowa (Figure 1).

Nematodes found

One or more species of plant-parasitic nematodes that feed on corn were found in 92 percent of the samples analyzed from 2000-2010.

The nematodes most frequently found were spiral (present in 77 percent of samples submitted) and root-lesion nematodes (found in 51 percent of samples submitted).

Most species of plant-parasitic nematodes cause damage to corn only when numbers exceed a damage threshold. Overall, 15 percent of the samples from 2000 through 2010 had nematodes present in numbers exceeding the damage threshold.

No sample had more than one nematode species present at damaging levels.

The nematode most commonly found at damaging levels was the needle nematode (eight percent of all samples submitted). Almost all of the samples with needle nematode were from Muscatine County (Figure 2).

The dagger nematode was second most frequently present at damaging population densities (six percent of all samples submitted).

Although spiral nematode was found in 243 of the 331 samples, only one percent of the samples had numbers exceeding the damage threshold.

Implications of the results

It was not surprising that most samples contained one or more species of nematodes that feed on corn. Most of these nematodes are likely native to Iowa and fed upon native plants before corn was cultivated as a crop. The nematodes are not specific to corn. They are very commonly found at low population densities not thought to be damaging to corn. The high concentration of damaging populations of needle nematodes in Muscatine County is likely because needle nematode is damaging at very low population densities (basically at the detection level of one worm per 100 cc soil) and because of the high prevalence of sandy soils in that area of Iowa. (Needle nematode occurs only in soils with at least 49 percent sand). One should not extrapolate the summarized results to counties from which no nematode samples were submitted for testing. The total number of samples tested for plant-parasitic nematodes that feed on corn from 2000 to 2010 was extremely low considering there are more than 13 million acres of corn grown in the state. There were 77 samples submitted to the ISU Plant and Insect Diagnostic Clinic in 2011 for nematodes that feed on corn. It is not likely that healthy-looking corn is being damaged by plant-parasitic nematodes. So, not every field in the state needs to be sampled for nematodes that feed on corn. But significantly more cornfields showing symptoms of stress should be checked for plant-parasitic nematodes. The ideal sampling times and methods for nematodes that feed on corn were discussed in an earlier article in ICM News. Increased sampling for nematodes that feed on corn will lead to a better understanding of the importance of these native nematode pests in corn production in Iowa. And information from such samples will allow farmers and those who advise them make more informed decisions concerning the use of current and future nematode management products.