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The silent yield robber: soybean cyst nematode

Resistant varieties do not sacrifice yield potential.


November 13, 2007
By Top Crop Manager

Topics

16aSoybean cyst nematode (SCN) is now the number one soybean disease problem in
Ontario. Currently it can be found almost anywhere from Toronto to Windsor and
will continue to spread slowly into the Niagara region, eastern Ontario and
Quebec.

If it follows the same pattern in Canada as elsewhere, growers can expect to
find SCN everywhere soybeans are grown. Since SCN has been known to live up
to 20 years, it is a safe bet that once found, it is hard to get rid of it.
All that can be done is to manage it through crop rotation and by selecting
resistant seed varieties.

"You've got to manage soybean cyst nematode before it manages you,"
says Albert Tenuta, field crops pathologist with the Ontario Ministry of Agriculture,
Food and Rural Affairs (OMAFRA). "If it is in the area you might as well
consider that it is infested and initiate management tools; the quicker the
better. Stick with them too, do not start to change your management strategies.
Don't bunch up soybeans and don't use a susceptible soybean variety."

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"Choosing resistant varieties gives us a chance for a good yield anyway,"
says Leo Guilbeault, a farmer and Pioneer sales representative who grows about
800 acres of soybeans each year near Belle River, Ontario. On his operation
which he and his brother-in-law Paul Cazabon run, they knew that SCN was present
in some fields, but other fields were clear of the problem. Their plan was to
use good crop rotation practices and carefully select SCN-resistant soybean
varieties in order to reduce the damage caused by the pest.

Tenuta calls soybean cyst nematode a silent yield robber. While it can drop
yields in susceptible varieties by up to 80 percent, it often goes unnoticed.
Yield losses are often attributed to other common problems that occur in the
field. Eventually producers realize that the losses can no longer be attributed
to aphids or the weather and start digging up plants looking for cysts.

In 2005, Guilbeault discovered SCN in fields that had previously been unaffected.
"We could see it all summer," he reports. The plants were shorter;
some were half as tall as other unaffected plants. Because the plants were smaller,
they also produced fewer pods. The soil tests revealed infestations of 5000
to 7000 eggs per gram of soil. By industry standards, this number of SCN eggs
per 100 gram soil sample would be classified as high risk and associated with
a greater than 50 percent potential yield loss (see Table 1).

Guilbeault has seen SCN in his fields for more than 20 years. Once there, the
population gradually expands and has an increasing impact on yield. It can spread
naturally in soil transported by wind, water or even birds or be transported
from field to field on equipment.

"By the time you start seeing SCN symptoms you're probably already losing
between a quarter and a third of the yield," Tenuta says. "The key
to soybean cyst nematode management is early detection. The earlier a producer
can determine or find that he has nematode in his field the less likely that
the disease or the nematode populations will build up to levels that will cause
him much of a problem. Once you find out you have it, we have very effective
management tools. It's just a matter of initiating those management tools to
get them started."

Producers can check for SCN by scouting in-season or by soil testing anytime.
Scouting is done by digging up soybean plants and looking at the roots. It is
quick, it is cheap and in most cases a grower just needs to do it a couple of
times a year to determine whether SCN eggs are present. However, in-season scouting
can only be done once a field is in soybeans, so Tenuta recommends that producers
also do a soil test. A soil test will not only reveal if SCN eggs are present,
but also quantifies the infestation level. The level can help determine what
varieties should be chosen.

Over the past three years, Pioneer sales representatives have conducted soil
sampling with selected growers in southwestern Ontario. In 2004, 56 percent
of the samples tested by Pioneer Hi-Bred had some level of SCN present, based
on 227 samples. Of these samples, one third would be classified as high risk,
with a potential yield loss of more than 20 percent. Results from 2005 showed
similar results. Out of 223 samples, 69 percent had cysts and 45 percent fell
into the high risk category. Final results from 2006 are expected to be similar.

Guilbeault continues to switch a greater proportion of his fields to SCN-resistant
varieties. The new varieties cost about the same as non-resistant varieties
and they yield just as well, so there is no reason not to switch, he adds.

Terry Smith, a farm operator near Dresden, Ontario, conducted an on-farm trial
when his Pioneer sales representative suggested he try growing cyst-resistant
varieties, along with the non-resistant variety he had chosen. "That is
how we noticed we had a lot of cyst pressure in the field," says Smith.
"There was a very noticeable difference in the field. The plants had a
real height difference and difference in the number of pods per plant. At harvest
time, the cyst resistant variety yielded 50bu/ac, while the non-resistant variety
yielded 39bu/ac. It opened our eyes." Without having the side-by-side plots
for comparison, Smith says he probably would have blamed the poor growth on
weather conditions.

SCN resistant varieties have come a long way since the early 1990s. "In
most cases, under most environments, resistant varieties outperform susceptible
soybean varieties," Tenuta says. ÒIf you use them on fields with cyst nematodes,
they are far superior."

Pioneer is working on developing more new varieties that are resistant to SCN
damage. One of the research tools currently being used to develop this technology
is marker-assisted selection (MAS). "MAS is the use of genetic markers
on the chromosome to identify the presence of a gene that carries a given trait.
We take a DNA sample and run genetic tests looking for evidence of the traits
we are searching for," says John Soper, director of soybean research for
Pioneer.

Table 1. Number of eggs per 100 grams of soil.
Number of eggs Risk level Potential yield loss
One to 1000 Low Zero to 20 percent
1001 to 2000 High 20 to 50 percent
2001 to 10,000 High 50 to 75 percent
Greater than 10,000 High 50 to 100 percent

This is good news for growers. It allows plant breeders to identify the best
possible lines, which also means that yield does not have to be sacrificed to
obtain plants that are specialized in one area. "Ten years ago, SCN-resistant
varieties were at the bottom of our research trials for yield performance. Now
they are right at the top: competitive for yield with varieties lacking SCN
resistance. We've been able to increase the rate of yield progress in our SCN-resistant
products to almost one bushel per year, compared to historic levels of 0.3 to
0.4 bushels per year on average," says Soper.

"Customers don't have to choose between yield and a defensive package.
They can have both," says Tim Welbanks, product information manager with
Pioneer. The company has been looking at MAS since the early 1990s. "Our
first efforts were focussed on identifying soybean cyst nematode resistance.
SCN historically has been difficult to screen for with field-screening techniques.
You face different levels of pressure across fields, resulting in different
degrees of damage and inaccurate rating of varieties," says Soper.

MAS also gives Pioneer a tool that is cost effective. It is less costly to
diagnose a disease trait and the technology is also more accurate. "With
MAS, we screen hundreds of thousands of experimental lines very early in the
process, eliminating those without the traits you want. We only funnel the most
promising experimental varieties into our yield test program," says Soper.
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