Top Crop Manager

Features Agronomy Diseases
Waiting game on soybean rust continues

But indications are growers need information, action.

November 12, 2007  By Top Crop Manager

6aIn spite of numerous uncertainties, the unanimous sentiment among researchers
and extension personnel is that soybean rust is now a disease with which Ontario
growers must cope, the same as with soybean aphids and fusarium in wheat. Its
presence in nine states in the US (Alabama, Arkansas, Florida, Georgia, Louisiana,
Mississippi, Missouri, South Carolina and most recently Tennessee) means it
will likely become a fact of life in Ontario.

The seriousness of the disease cannot be underestimated: yield losses of up
to 90 percent have been recorded in South America. According to Albert Tenuta,
field crop plant pathologist with the Ontario Ministry of Agriculture and Food,
soybean rust's arrival in 2005 is an 80 to 90 percent probability. "We
have to be aware of it and respect it, but we don't want to be scared of it,
and we don't want to underestimate it or be complacent," says Tenuta, adding
that timing will determine the disease's damage potential.

Progression of the disease is fast
May 1: Little long-range symptoms. May 5: Obvious discolouration of canopy.
May 16: Defoliation advanced. May 27: Complete defoliation.

'Weather' it hits at vegetative or reproductive
As with so many factors, weather will play the ultimate role, especially since
most weather patterns that strike the US midwest and southern Ontario are the
product of two phenomena; warm air masses from the southern Great Plains and
moisture that is drawn from the Gulf of Mexico as those masses migrate north.

If soybean rust arrives during the early vegetative stage when plants are young,
there will be more problems than if it arrives at the later reproductive stage
of development. At that later point, a disease has less impact on an established
plant, and weather conditions tend to be drier during mid-August and September.
"If it comes in at growth stages R4 or R5 and beyond, and we have a typical
late August and September, it won't matter if the spores are there, they're
not going to develop under unfavourable conditions," says Tenuta.

The importance of scouting and tracking the disease cannot be underestimated,
says Dr. Marty Draper, a plant pathologist with South Dakota State University.
"We have to be sure we detect this disease earlier than we do with aphids,"
he says. "By the time rust has done so much damage that we can find the
disease easily, we're not going to get it under control."

6bNews is not all bad, but…
One bit of good news is that soybean rust does not overwinter well north of
Kentucky. That, plus the fact that cold temperatures descended over much of
the southern US late in 2004, may mean 2005 is a build-up year, says Dr. David
Wright, director of production technologies with the Iowa Soybean Association.
But he also takes issue with those who suggest it will not be a problem for
one or two years yet. "There are some people who say, 'Well, we're not
going to worry about it, because it's going to be more of a problem in the south,
we'll just kind of watch it'," says Wright, who also works with the Iowa
Soybean Promotion Board. "It could show up on our doorstep, and be recognized
in the northern regions before it's recognized anywhere in the south, so we
must be vigilant."

To Dr. Loren Giesler, a plant pathologist and professor with the University
of Nebraska, it is the sense of urgency that has changed during 2004. "It
just emphasizes the need to scout, the need to be aware of conditions, and this
one really, more than aphids, brings in the idea of needing to know what's happening
in regions south of you, as this disease approaches."

Concerns remain over fungicides and application
For the time being, the four fungicides growers need to remember are Headline
(pyraclostrobin), Quadris (azoxystrobin), Tilt (propiconazole) and Folicur (tebuconazole).
Registered for emergency use for 2005, they have been broken into two groupings,
the strobilurons (Headline and Quadris) and the triazoles (Tilt and Folicur).
According to Tenuta, the strobilurons are the protectants while the triazoles
act as curatives, and can help limit colonization of the tissues at later stages.
"If you have a one, three or five percent infection that's started, you
can cure that or defend against that, with the timely application of a triazole,"
he says. If the disease progresses to the 10, 15 or 20 percent infection level,
it is likely too late to spray. "At 20 percent, you're not going to get
the economic return on the fungicide that you'd want under normal conditions."

One issue growers should understand is the strobilurons have developed resistance
in other parts of the world because of their single mode of action. "But
we need to be aware of that, and our recommendation will be to try to alternate
the chemistries and keep things off balance."

Understanding keys to application
When spraying to control weeds, timing and using the right product are key factors.
But with fungicides, it is coverage that counts. "With soybean rust, just
because we get the fungicide there doesn't mean the disease is controlled,"
says Helmut Spieser, an engineer dealing with pesticide applications for the
Ontario Ministry of Agriculture and Food.

Since coverage is so important, Spieser recommends growers check the labels
but says 15 to 20 gallons per acre is a good starting point. In terms of droplet
size, medium to coarse sprays are best, he says, and are achievable with flat
fan, cone nozzles, TwinJets, Turbo TeeJets and drift reducing nozzles.

As for pressure, Spieser cites a range of 40psi to 100psi, with particular
emphasis on proper nozzle to target distance. "In most cases, when we have
a boom with nozzles on 20 inch centres, we need 20 inches from the nozzle tip
to the target," says Spieser, adding that it is similar to spraying a cereal,
however the boom may be a little higher.

Sprayer and nozzle manufacturers are gearing up with products and advice for
proper application of fungicides. Operators will need to set up their equipment
to ensure coverage of the lower leaves and stems of the plant, even at advanced
growth stages.

If the soybean crop is in full canopy when the disease strikes, Spieser says
a straight boom will do the job. Only if row middles can be seen would there
be an advantage to multi-nozzle configuration, despite its added costs.

Early communications also vital
Growers scouting their fields and believing they have found soybean rust are
asked to contact the OMAF Crop Pest Hotline, by calling 1-877-424-1300. Growers
also can check the following web sites:

  • Ontario Ministry of Agriculture and Food at:;
  • Agriculture and Agri-Food Canada at: and
  • Ontario Soybean Growers at:

Further down the road to improved germplasm
As North American growers become more adept at managing soybean rust, they may
get some help from researchers working to incorporate resistance into soybean
germplasm, hopefully within five to 10 years. "Ultimately, the best management
tool would be to have a combination of good effective resistance, because there
are different races," he says. "Even though there are resistant lines,
there are four different resistance genes out there, but those genes are not
durable against all the different races of soybean rust that have been tested."

Tenuta has seen cases where a private company can successfully test a resistant
variety in Thailand. Yet the same germplasm in a Brazilian variety, grown under
different environmental conditions, disease pressures and different races of
soybean rust, might fail. At present, there may be as many as 20 different races
of soybean rust, but Tenuta concedes that number is not conclusive.

"From year to year, we're going to see variability," he concludes.
"In the southern US, you can be assured you're going to see rust every
year at some level, and probably significant levels, especially in subsequent
years as it builds up more, and gets more of an overwintering foothold in the

Questions about marketing
If there is a concern with soybean rust and product acceptability, it is in
the withdrawal times and potential residuals of the fungicides that have been
granted emergency use registrations. There are certain tolerance levels and
timing windows that cannot be ignored without affecting sales. "On all
of these fungicides, there are 'days to harvest' restrictions, and you'd better
pay attention to that label and those days to harvest under that acceptability
issue," says Albert Tenuta, field crop plant pathologist with the Ontario
Ministry of Agriculture and Food in Ridgetown, Ontario. He agrees with US assessments
that other regions of the world have adapted to soybean rust's presence without
adverse impacts on markets.

In answering the question of whether soybeans will be marketable once rust
sets in to Ontario fields, the quick answer is 'yes'. Soybean rust has existed
for decades in other regions of the world and since it is not a seedborne disease,
it will have limited impact on soybean sales. If there is an impact, it is the
withdrawal times and adherence to specific label guidelines. "That's essentially
the same for any crop protection product," says Matt McLean, research and
bioproduct manager with the Ontario Soybean Growers in Guelph, Ontario. "Even
if you look at soybean aphids, there are insecticide products with time to harvest
requirements on them, as well."


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