Features Agronomy Diseases
Waiting game on rust continues

For much of 2004, growers in southern Ontario heard, at first, a little, and
then a lot about soybean rust. It was covered at seminars, in articles from
the US, and analyzed and reviewed from seemingly every angle.

In spite of the uncertainties, the unanimous sentiment among researchers and
extension personnel is that soybean rust is now a disease with which Ontario
growers must deal, 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 American fields. Most of those involved
in the drive to increase awareness agree with the need to understand and act
accordingly when the disease finally arrives. According to Albert Tenuta, field
crop plant pathologist with the Ontario Ministry of Agriculture and Food, soybean
rust's arrival in 2005 is still a possibility, depending on conditions in the
southern US during June and early July.

"We have to be aware of it, we have to 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, reminding growers that other rusts, in crops such as wheat and
corn, are common, albeit unwelcome visitors each year to Ontario. But it may
be soybean rust's damage potential that is causing anxiety.

Vegetative versus reproductive
Based on the South American experience, infection during the early vegetative
phases is low. As the plant goes into the reproductive phase (flowering, pod
set and pod filling) disease development increases. "We don't know whether
the South American observations will be similar in Ontario," says Tenuta.
So far, early observations in Florida and Georgia show that there is limited
infection on vegetative soybeans. "We will see if this changes as the southern
US crop flowers and what effect weather conditions at that time will have. Disease
has less impact on an established plant in hot, dry weather conditions during
mid-August and September.

Figure 1. Effects on rust fungal development.
"Timing is also important: if it comes in to Ontario 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.

To some in the US, one of the key issues will be the variability of the disease
as it strikes parts of the midwest and into Canada. Although Ontario growers
may see a cut-off date for the need to spray fungicides at R5 or R6, as Tenuta
states, Dr. Martin Draper, a plant pathologist with South Dakota State University,
points out that an R5 soybean in Ohio is different from a similar stage plant
in Ontario. As such, scouting and spraying applications will have to be adjusted
as a reflection of that variability. "If you look at an R4 here in South
Dakota, it's a safer decision to perhaps stop scouting than it is in Ohio or
Ontario," says Draper.

As for whether an infection at the vegetative stage is worse than at the reproductive
stage, Draper says the variability and uncertainty of soybean rust in 2005 will
make definitive statements about its infection more challenging. Infection on
later vegetative stages could lead to a greater source of local inoculum for
the reproductive stages when potential yield losses could be greater. As such,
2005 will be a tremendous opportunity to learn more about rust as the disease
progresses, particularly with respect to spraying recommendations.

At the same time, Draper recommends growers scout as early as possible, drawing
a parallel for the urgency with that of tracking aphids. "The problem is,
we have to be sure that we detect this disease earlier in the outbreak than
we do with aphids," says Draper, noting that with insecticides and aphids,
the curative is better than with soybean rust. "With rust, we don't have
that kind of activity from our product and generally, by the time we can find
the disease easily, the damage is done and we're not going to get it under control."

News is not all bad, but…
If there is some goods news about rust, it is that it does not overwinter well
north of Kentucky. "All the rusts, whether it's wheat, corn or soybean
rust, don't overwinter in Ontario, so we're always dependent on movement up
here," says Tenuta. "For 2005, the cold winter led to fewer overwintering
southern habitats and this has resulted in a lower amount of soybean rust. Therefore,
we can expect 2005 to be a build up year."

Dr. David Wright, director of production technologies with the Iowa Soybean
Association, agrees with Tenuta's assessment, especially in light of the cold
temperatures that descended over much of the southern US late in 2004. 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. "That's taking a bit of a risk and not looking at the
fact that 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."

Soybean rust is not new, urgency is
To Dr. Loren Giesler, a plant pathologist and professor with the University
of Nebraska, it is the sense of immediacy that has changed since he made a presentation
with Tenuta at the 2004 Southwest Agricultural Conference in Ridgetown. "It's
that urgency that's causing growers to pay attention," he says, adding
that he has been telling growers in Nebraska to make plans to spray. "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."

Giesler, like others, points to the importance of understanding the origins
of weather systems affecting the US midwest and southern Ontario. "I've
been telling Nebraska producers that we really don't know for sure how this
is going to play out, based on the weather records that we have," explains
Giesler. He acknowledges that rust may be more of a problem for Ontario given
the higher humidity levels in early to mid-summer.

For Dr. Terry Anderson, there is little reason to question the impact of soybean
rust, adding it is 'almost certain' the disease will appear in Ontario in 2005.
Reports from Louisiana have indicated viable infections. "Everybody is
just speculating right now, but I'm hoping it won't be a major problem here
in Ontario," says Anderson, a plant pathologist with Agriculture and Agri-Food
Canada's Greenhouse and Processing Crops Research Centre near Harrow, Ontario.
"The glitch is that we have the Great Lakes around us and that increases
our humidity and our dew periods in August." It is during the month of
July and the early days of August that humidity and dew periods will be highest.

Like Giesler, Anderson is hopeful of a positive effect on scouting as soybean
rust moves closer. He is working with Tenuta to establish 'sentinel plots' in
Ontario as part of a North American chain, and although they will be monitored
on a daily basis, Anderson refers to them as mere 'snapshots' of what is actually
happening in the fields. As such, growers must find the time to scout their
own fields to help with informal monitoring for Ontario and points in the US.