Systems Approach Holds Hope for Soybean Cyst Nematode Control

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If there is a Holy Grail in soybean pest control, a likely contender is soybean cyst nematode (SCN) control. Generally regarded as the most important soybean pathogen, SCN can be found in 14 countries and most soybean production areas of the U.S. For more than two decades, planting resistant varieties has been the main protection against the pest. Today that is changing as new research tools come on line, new products are introduced and, most of all, the pest evolves. Perhaps the most important element in current efforts is the systems approach being used within companies, public research institutions and across the industry.

Bruce Schnicker "What hasn't changed is always thinking about getting a better product faster into the grower’s hands," said Bruce Schnicker, global plant health lead for breeding, Monsanto. "The new opportunity is using all tools available to deliver those solutions. One of the great things for me as a plant breeder is to collaborate with scientists in other fields, including chemists on seed treatments, molecular scientists, as well as internal pathologists and researchers from public institutions as well."

Joseph Byrum, global head of soybean seed R&D, Syngenta, noted that while the primary approach in seeds is selection for tolerance in germplasm, there is significant emphasis on molecular breeding to address resistant traits across the germplasm. Like Schnicker, Byrum celebrates the multiple assets available at his company to find SCN solutions, from germplasm to seed care to biotech.

The first draft of the SCN gene sequence was completed by Monsanto and its since acquired partner Divergence in 2008. The knowledge is paying off, but slowly, suggested Schnicker.

"Learning how the pest interacts with the soybean plant, how it derives nutrients and the genes involved in that provide potential resistance targets going forward," he said. "The challenge is that it takes a lot of scientists to marshal the information and drive product development. Part of the value of sequencing is it opens up all these opportunities from a breeding perspective to identify traits that may already exist in the breeding population to give us another avenue to provide control."

BIOTECH ADVANCES

Dan Tomso, site manager, Bayer CropScience Innovation Center, described biotech advances in SCN control as "super exciting." Tomso joined Bayer as part of its acquisition of the biotech firm Athenix and its patented gene portfolio that includes nematicidal genes. Development with the bacterial genomics-based program has been a long process, but results are promising.

Dan Tomso "We got our nematode screening built up in the mid 2000s, started with modeling, then transformation, and once we joined Bayer, moved it into the greenhouse and the field," recalled Tomso. "With our lead gene, we are seeing a really big impact on cyst formation in soybeans with many fewer cysts. We are now into year two, and if it runs the typical developmental timeline, we hope to introduce a trait solution sometime after 2020."

Tomso described developing this initial trait as a learning experience, opening up new areas for how to control nematodes and other pests. "I think we'll have follow-up products coming on its heels, not only for SCN and other nematodes, but for about 20 other pests as well," said Tomso.

VARIETAL RESISTANCE

While biotech solutions remain somewhere in early stages of various pipelines, varietal resistance remains the core of SCN control. However, seed treatments are showing promising results, according to Palle Pedersen, seed care technology manager, Syngenta. The company launched Avicta Complete Beans, its nematicide/insecticide/fungicide seed treatment in the southern U.S. in 2011 and was also available in the Midwest in 2012.

Stunted soybean plants because of soybean cyst nematodes (one shown highly magnified, below right) can be mistaken for other environmental stresses. "By using multiple modes of actions that are working from day one, the grower has the best chance to protect his genetic yield potential from nematodes," he said.

A systems approach to SCN resistance is front and center at Iowa State University as well. Funded by the Iowa Soybean Association, nematologist-plant pathologist Greg Tylka and a research team that includes a biochemist, an entomologist, a mycologist and a soybean breeder are focused on better understanding soybean pests and how they interact. Their ultimate goal is creating soybean lines with resistance to one or more soybean pathogens. Although Tylka is largely field research oriented, the team includes lab and greenhouse expertise.

PHOTO COURTESY OF UNIVERSITY OF MISSOURI "A lot of what is done in the lab or greenhouse doesn't relate to what farmers can do this year or next," explained Tylka. "The more basic research is interaction, exploring how the nematode affects plants when other pests and diseases like aphids and sudden death syndrome are present. Our most relevant work is done at the field level plots."

One such trial was to establish possible interaction between layers of resistance within a single variety, such as SCN-resistant genes and aphid-resistant genes. The good news was no interaction could be found. At the same time, Tylka reported the team is starting to see hints that aphids may lessen SCN varietal resistance. The team is trying to understand the problem biochemically. He noted that it is hard enough to work with one pathogen at a time, much less two or more.

"We are looking to answer questions of increased susceptibility and does it happen in all susceptible and resistant varieties," said Tylka. "We detect increases in some chemicals and reductions in others involved in natural protection. With aphids, for example, does the interaction take place in all soybean germplasm or just some?"

LOW LINOLEIC SOYBEANS

One area of investigation was low linoleic soybeans. While a value-added product for growers and processors, the concern was what impact the lower levels of linoleic acids were having on pest resistance. Tylka noted that linoleic acids are involved in natural defensive pathways.

"The answer was that reduced levels don't make plants more vulnerable to either aphids or SCN," said Tylka. "The low linoleic plants still have the capability to form defensive compounds. However, that got us started looking at other defensive chemicals in soybean plants. There the picture became more complicated."

That isn't the only place that an answer has raised more questions. Tylka pointed to the vital role resistant varieties play today as the only thing that allows most farmers to raise the crop in the face of SCN. Of greatest concern is the predominant reliance on a single source for that resistance. Out of 807 resistant soybean products offered for sale in Iowa in 2011, all but 21 relied on PI 88788. At the same time, SCN appears to be evolving its ability to reproduce in fields with PI 88788 genes present. While this should affect yield, it isn't.

"We aren't seeing the big drops in yield we expected," said Tylka. "That was unexpected. We are trying to understand why, but it is a very arduous process."

Tylka looks ahead expectantly to the introduction of traits such as Ramos described. He noted the ease with which growers integrate traits into existing management strategies. In the meantime, with continued support from the soybean checkoff, he and the Iowa State team will continue exploring questions, looking for answers and developing new resistant soybean lines with their systems approach. Until and likely after traits are introduced, varietal resistance and seed treatments will remain important tools in SCN control.


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