Top Crop Manager

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Anti-aphid soybeans hold much promise

But availability still a few years away.


November 14, 2007
By Top Crop Manager

Topics

16aAs hard as it may be to accept, Ontario growers are still two to three years
away from having the latest 'tool in the toolbox' to fight soybean aphids. In
November 2004, researchers at the University of Illinois announced the discovery
of a genetic trait that would make soybeans resistant to aphids. The news came
as extension personnel and researchers were trying to dispel rumours that soybean
aphids are a problem one out of every two years. In addition, they were steeling
themselves for a possible encounter with Asian soybean rust.

The research team at the University of Illinois narrowed its study of hundreds
of cultivars down to just two, Dowling and Jackson, which were used in the southern
US, but have been commercially unavailable for a number of years. "We don't
know the actual chemical that's responsible for the antibiosis as the aphids
get sick eating from these particular types that have resistance," says
Dr. Glen Hartman, the lead researcher and plant pathologist with the USDA's
National Soybean Research Laboratory at the University of Illinois. "Reproduction
goes down, they don't produce offspring and if you let them feed on the resistant
types and not give them any choice, the aphids will die."

The development comes from natural selection for the gene, without the use
of current biotechnology methods.

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Although some might see that as an impediment to having it ready for commercial
availability, Hartman notes that many of the seed companies have been working
with the resistant germplasm and varieties should be available in two or three
years. "It shouldn't take that long because the Illinois group has provided
adapted breeding lines that already have the gene in them," says Hartman,
noting that with a few backcrosses and subsequent yield trials, the time to
market should be fairly short.

What the team at the university is working to develop now is similar resistance
with other gene sources. "We have to determine if we're dealing with the
same gene and potentially new genes from these other sources," explains
Hartman, citing the process of locating these genes, then mapping them for the
purpose of examining their viability. "Which would be very important because
if we ever encounter strains of the aphids that overcome the initial resistance,
we'll have other ones in place."

Developing the trait for Ontario
Since Dowling and Jackson are cultivars from the southern US, the trick for
breeders in northern climate regions is to backcross this gene with cultivars
found in Ontario. Private companies are involved in that process, as are breeders
at the University of Guelph. On both sides of the border, there is uncertainty
as to why the antibiosis occurs.

"Whether the damage is because aphids feed less on it or that the soybean
plants are better able to cope with the damage caused by aphids, I'm not exactly
sure," says Dr. Istvan Rajcan, a soybean breeder in the plant agriculture
department at the University of Guelph. "But the study is comprehensive
and meticulously done, and the good news is that Dr. Gary Ablett and I have
been making crosses and probably will have to make several backcrosses because
the donor lines were from Maturity Group 6, 7 or 8. The backcrossing takes more
time but is necessary because the progeny would never be able to reach maturity
and produce seed in Ontario. So we crossed it with our material, then backcrossed
it, and we're now at the stage of testing the progeny of the first backcrosses
by exposing them to high aphid populations in the greenhouse, where we are going
to stress test two populations for resistance."

Dr. Art Schaafsma at Ridgetown College campus is involved in the evaluation
of aphid resistance in the Guelph soybean material, as well. From there, they
will send the resistant varieties to Costa Rica to speed up the breeding process.
And while growers are anxious to have a variety with this new trait, Rajcan
reminds everyone that these things take time. "You have to be cautious,
especially about the promises you make," he says. "It is a genetic
technology with a good potential but it takes time to put it in the right background,
have it tested and confirm resistance while retaining good yield."

Private sector feeling pressure
Soybean breeder, Don McClure has experience with grower angst over the supposedly
lengthy wait for these lines. At a Syngenta field demonstration day near London,
Ontario, in September 2005, McClure answered questions as to when the varieties
would be available. And 2008 was not soon enough for some growers. "That
is fast, that's using every season we can in winter nurseries and pushing the
project as hard as we can, that's as quick as we could get something out,"
says McClure. "Even then, we won't have very large quantities of seed."

Like Rajcan and Hartman, McClure notes the mode of resistance is yet unknown,
but that the key is the interruption in reproduction. "With the way these
things multiply, it's exponential growth, so if you can control the first generation
or two, or at least reduce their impact, or their ability to reproduce, then
you can really have a huge impact on the total final population," he explains.
"At least we'll have some options for growers beyond spraying large acreages
with insecticides in the middle of summer."

Dealing with the undesired
One certainty in the discussion on pests is that growers will have to adjust
their management practices to deal with aphids on an annual basis. Soybean aphids
will become the same as soybean cyst nematode and, likely in 2006, Asian soybean
rust. Growers will have to deal with these pests every year, not sporadically.

"People forget that eastern Ontario had soybean aphids in 2004, and again
in 2005, so it really just depends on the conditions that play out in your area
each year," says Tracey Baute, field crop entomologist with the Ontario
Ministry of Agriculture, Food and Rural Affairs in Ridgetown. "If there
are enough ladybugs and other natural enemies in your area and conditions aren't
right for aphids, you might not suffer from soybean aphids very badly, but if
everything works out, you could have consecutive years of aphid problems."

She adds that some growers are having trouble with redefined thresholds of
250 aphids per plant on 90 percent of plants in a field. "We're still trying
to make sure the threshold is working, and a lot of people had to get used to
the concept that it wasn't 250 per plant that is the damaging level. It is that
we set that number to give them time to get in there and spray before reaching
that injury level," says Baute. "Some waited much too long and had
thousands on the plant. It was definitely a learning experience."

But, the good news is that all of these pests are manageable. What is even
better news is that growers are scouting more as they prepare for aphids, cysts
and rust. "People were finally looking, and you'd get calls on miscellaneous
things that they usually don't pay attention to on their plants because they
were finally looking," says Baute. "And they were recognizing even
the beneficials that are there, so a lot of them really did do a good job of
keeping ahead of the situation."

Problem on the horizon
If there is a problem
dealing with these various pests, and others, it will come in the availability
of sprayers. In 2005, some custom sprayers cautioned the agribusiness sector
and extension personnel that Ontario does not have sufficient equipment to deal
with a severe aphid infestation and a possible rust problem in 2006.

"They recommend that growers might have to do things themselves in some
cases," says Baute. The logistics of the issue, be it with rust or aphids,
complicate things, from the nozzles to the acres being sprayed to water volumes
and pressures. "We really have to hone in on that in 2006 and say, 'This
is what works and this is what doesn't work, and you guys really have to follow
this'."

Definitions: antibiosis versus antixenosis
Antibiosis is the ability of a variety or hybrid to reduce the survival,
growth or reproduction of insects that feed on it. It is often caused
by the production of a toxin in the plant. The most common example of
antibiosis in field crops is Bt corn.

Antixenosis is the ability of a variety to repel insects, causing a reduction
in the position of eggs or feeding. There is a general repellent quality
to the plant and can be chemical or cellular, and may be detected before
an insect lands on the plant or after they taste the plant tissues.

Cellular qualities that can affect insect populations include leaf hairiness
and stem hardiness.

 

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