By Top Crop Manager
Areas that have had it likely had it for years.
Growers have heard it all before. Then why is the concept of controlling soybean
cyst nematode (SCN) so difficult?
If the answer is out there, a host of researchers and breeders wants to know
because like or not, SCN is a fact of life for soybean growers in Ontario.
In addition, reports have the disease moving as far east as Brant County and
Peel Region, and north to Bruce County. Worse still is the fact that previous
varieties that showed resistance to SCN are now showing signs of losing that
For some, that might be ominous news, but to Albert Tenuta, the message he
has been repeating for years remains the same: it is manageable, provided a
grower determines the presence of cysts in the soil and then rotates resistant
varieties from one year to the next. "What you want to do, first and foremost,
is rotate your resistant varieties," says Tenuta, a plant pathologist with
the Ontario Ministry of Agriculture, Food and Rural Affairs at Ridgetown. "It
doesn't matter whether it's the same source of resistance or not, every variety
has a little degree of difference in resistance."
Tenuta reports that 2005 was a good year for SCN and a good awareness year
for growers. Many of them were keyed for the arrival of Asian soybean rust,
while the arrival of a mid-season infestation of soybean aphids heightened their
awareness. As a result, many growers were doing more scouting of their fields.
That had its benefits and its drawbacks. On the plus side, it meant a better
monitoring of insects, weeds and diseases. On the down side, it led to some
rather startling discoveries. "In new areas, Brant, Bruce and areas in
Huron-Perth, all have had cyst nematode in the past," explains Tenuta.
"When you get numbers as high as 12,000 eggs per 100 grams of soil, that's
not a one year wonder type of thing, they have been there for 10 or 15 years
and in that process, growers have lost yield every year and in some years, like
2005, more so than others."
Why SCN is so often missed or dismissed is its reputation as a 'silent yield
robber'. Tenuta says that in a good year, the symptoms can be masked, even though
the crop can experience up to 30 percent yield loss. "But a year like 2005,
a stressed year, hot and dry, the soybeans under stress: you don't need as many
cysts to cause an equivalent amount of injury," he adds. "In other
years that you wouldn't see something, like 2005, you did."
Tenuta is also fielding questions from growers as to why consistent use of
SCN resistant varieties is not cleaning up the problem. His reply is simple:
once SCN is in the field, it does not go away. Even going out of soybeans for
five to 10 years will not eliminate cyst from a field. If SCN populations were
high enough in the first place, they most likely will still be at economically
damaging levels even after this long period. "It's easy to deal with: you
rotate, use a resistant variety and you scout," says Tenuta.
Why resistance is occurring
agrees with Tenuta's assessment and applies the analogy that a grower's field
is much like a bank account. It must be monitored on a regular basis to know
what is happening. A researcher with Agriculture and Agri-Food Canada's Greenhouse
and Processing Crops Research Centre at Harrow, Ontario, Welacky understands
the process of developing resistance, and why the trend seems to be reversing
itself. In a very simplistic explanation, too many growers are relying on a
single SCN resistant variety as a management strategy. At present, most of the
genes expressed for SCN resistance originate from a parent, designated as PI
88.788. Other sources of resistance are being added such as Peking resistance
since new lines may have other gene sources such as PI 437654.
What is happening in Ontario, as well as the US, is a shift in the understanding
of how resistance is conferred. "The concept isn't that we know the resistance
is not changing, but that the SCN race or Hg biotype in the field is changing.
As a result, new sources of genes that are resistant to that race or Hg biotype
need to be introduced into the varieties," says Welacky.
The theory rests on two basic principles: either resistance is inherent in
a variety, or can be inserted or bred; or there is a degree of adaptability
of the SCN genes to resistance. The former puts resistance into the plant, the
latter places the ability to develop resistance in the nematodes. "It is
known that there were already mixed populations of SCN, where the ones that
were not resistant to one gene are starting to overcome the nematodes that were
resistant to that gene. Then the population increases and they've started to
Welacky concludes that growers need better resistance in the varieties grown
in Ontario, at least that are comparable to the parent PI 88.788. "All
the varieties that we've been looking at have a tendency to have infection from
SCN, but they may not be below the level of 10 percent resistance," he
says. "The message is that not all varieties have the same level of resistance."
Figure 1 shows only 22 percent of available varieties would be considered resistant,
whereas 35 percent would be considered tolerant. The remaining are moderately
resistant to susceptible. Tenuta and Welacky stress that knowing how many cysts
will be produced on the roots is critical to effective management of this disease.
A tolerant or moderately susceptible variety may yield well, but will not lower
SCN populations as effectively as a very resistant variety. As for which varieties
have better resistance, Welacky urges growers to ask their seed dealers directly.