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Soybean pest expected soon


November 30, 1999
By John Dietz

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Be afraid. Be very afraid. And prepared. Soybean cyst nematode or SCN, has not yet been confirmed in Manitoba as of mid-August 2011. It will be bad news when it does become established, but the good side is that it also can be managed with minimum impact if caught early.

SCN was first identified in the southeast corner of North Dakota in 2003, according to Sam Markell, North Dakota State University extension plant pathologist, Fargo. In 2007, it was confirmed in Cass County, near Fargo. Since then, it has been found in four more counties west, but not north of Cass country, between 2009 and 2010. It will become the worst soybean disease in North Dakota as it continues to spread.

SCN also has been confirmed in northwest Minnesota’s Red Lake County, about 75 miles south of Manitoba.

“I don’t know when, but you guys will get it,” says Markell. “It’s not a fast mover. Once you’ve got it, you manage it with resistance and rotation.”

Soybean cyst nematodes are microscopic worms that feed on soybean roots. Once established, the parasites can cause yield losses of 15 to 30 percent before symptoms are noticed. There is no practical nematicide for field-scale application, although experimental nematicide seed treatments are being evaluated.

In a recent publication, SCN Management Guide, soybean cyst experts make these points:
SCN is the leading cause of soybean yield loss in North America.

SCN symptoms are not unique, nor diagnostic. They may look like symptoms due to many other causes.

SCN is not always visible on roots of infected plants, so scout more than once in the season.
SCN can cause substantial yield loss without visible symptoms.

The soybean parasite has roots in China, Japan and Korea. In the western hemisphere, it was first found in North Carolina in 1954. It now is found in all major soybean production areas worldwide, including Ontario.

Producers can suffer declining yields for several years without knowing they have SCN, says Markell. The earlier they identify the problem, the easier it is to manage.

On its own, the nematode is “lucky” to move an inch over its life cycle. However, anything that moves soil can carry the cyst. That includes equipment, water, wind and seed.

In spring 2011, the Red River flooded thousands of acres along its course in North Dakota. It is possible that some cysts were picked up and carried across the border before being deposited in Manitoba farmland. 

When conditions became dry and dusty in August, cysts could have blown northward with North Dakota dust. When agronomists returned from field trips in southeast North Dakota, the truck could have carried cysts and dropped them off at the entrance to a Manitoba field. When a bag of fresh soybean seed from Ontario comes into Manitoba this winter, any soil in the bag also could carry cysts.

The SCN cyst is “just about” microscopic. With good eyesight, it can be seen. It looks like a white aphid attached to a root, or the period at the end of this sentence.  

 “The female nematode will attach to a root, stick her head inside, and start feeding,” Markell says. “Males fertilize the female. She becomes full of eggs and swells many-fold in size, until she dies. The body will encyst, become hard and tough, and will have a couple hundred eggs inside. The cyst form is very stable.”

After the first winter, many cysts rupture. About half the eggs germinate, go through a series of molts, and emerge as juvenile nematodes. If they find a living soybean root within an inch of “home” they start a new cycle; if they don’t, they die.

However, the remaining cysts can stay viable in the soil for many years. They will gradually rupture and look for roots to host another cycle.

“If you’re rotating soybeans with something like corn or wheat, something that’s not a host, the population that came out of the cyst will die. The remaining cysts are pretty stable. They will very often stay in that soil until it has soybeans again.

“You can’t rotate out of SCN,” Markell says. “You can rotate to reduce the level, but if you have it, you will always have it, basically.”

Scout and test
SCN is often referred to as a “silent yield robber” since above-ground visual symptoms often do not appear until SCN populations have built up. Unfortunately, by this time a grower has already lost yield for a number of years. 

Field scouting and soil testing are the primary methods for finding your first SCN infestation. 
The very first above-ground symptom in a crop is likely to be noticed in July or early August, says, Allen Xue, Agriculture and Agri-Food Canada plant pathologist, Ottawa. The edges of lower leaves on soybeans may turn yellow.

“Sometimes it’s confused with nutrient deficiency such as potash or other diseases,” Xue says. “A patch could be only a couple metres wide; sometimes it’s 50 metres wide.”

Testing for nematodes is a different procedure, says Markell. Collect soil cores from the root mass beside suspected plants, protect the samples in plastic bags out of direct sunlight, and get them to a lab for a bioassay within a day or two. The lab will be looking for cysts, eggs and living, microscopic juvenile nematodes, not for fertility.

The ideal time of year to collect samples to determine field population levels is as close as possible to soybean harvest, before or after. If you are interested in knowing if you have SCN, you can do this at any time in the season by digging up suspect plants and examining the roots for cysts. However, the absence of observing cysts does not necessarily mean you don’t have SCN. Cysts can be tough to see, and the population in a field is highly variable.

Resistance
If the field has never been checked before for SCN, the first places to look are field entrances and low areas that have been flooded. Other likely areas include fence lines, areas with high pH and areas where the last soybean yield was lower than expected.

A single positive result is a strong indicator, though not necessarily a proof.

“The CFIA has been doing surveys on fields in Manitoba for a couple years,” says Andrew Saramaga, president, Manitoba Pulse Growers Association. “One sample came back positive.

We re-surveyed and it came back negative, so we’re not entirely convinced it was a real positive. We’re proceeding on the idea that we don’t have SCN infection just yet.”

The MPGA is considering a producer awareness program for SCN, Saramaga says. North Dakota has an informational campaign that begins at the end of August.  

Over the decades, nematode resistant soybean varieties have been developed for the Dakotas and for Ontario. Along with rotations, they have been used very effectively. Iowa growers, for instance, have been producing good soybean yields with infected fields for at least 20 years.

However, some nematode populations have been able to reproduce very successfully on resistant varieties. The interaction now requires growers to choose varieties with resistance to specific nematode types.

“Until you know what race you have, it’s hard to prepare for,” Saramaga says. “Once we start having infections or hearing about infection closer to Manitoba, we’ll start ramping up awareness.” 

For additional information, visit www.planthealth.info/pdf_docs/SCNGuide_5thEd.pdf .