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Awareness is essential for soybean rust

Disease is now in North America.


November 12, 2007
By Ralph Pearce


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32aNow that soybean rust has been found in Louisiana, Florida and possibly other
US Gulf Coast states, grower concern has increased considerably. Besides soybean
cyst nematode and aphids, growers must concern themselves with developing management
strategies surrounding soybean rust.

It was just a matter of time before the disease found its way into North America.
Although it did not surprise plant pathologists and researchers, they were hoping
it could have been later rather than sooner.

It is a delicate balance to raise awareness without raising alarm, but the
flag must now be raised, says Albert Tenuta, extension plant pathologist for
field crops with the Ontario Ministry of Agriculture and Food in Ridgetown,
Ontario. "We've been anticipating soybean rust and preparations have been
ongoing towards the inevitable introduction into North America," says Tenuta.
"These include tools to assist producers and agribusiness in the scouting
and identification of soybean rust as well as its management."

A series of workshops is scheduled for the winter of 2004/05 across Ontario
to increase grower and industry awareness and management techniques for soybean
rust.

The threat to Ontario and Canadian soybean producers has increased substantially
with the recent confirmation of the disease in the southern US. The damage it
has inflicted in other parts of the world is astounding, with yield losses ranging
from 10 to 90 percent. Recent estimates by the US Department of Agriculture
now suggest economic losses of nearly $3.0 billion, with most of the damage
occurring in the Mississippi delta and southern US regions.

Soybean rust can be divided into two fungal species. Phakospora pachyrhizi
is the Asian strain that has been identified in North America and is the more
aggressive of the two, which was reported in Japan as far back as 1903. Phakospora
meibomiae
has been detected in the western hemisphere, as close as Puerto
Rico but is not considered as great of a threat.

Dr. Loren Giesler, an extension plant pathologist with the University of Nebraska,
agrees with Tenuta that current prediction models accurately predicted the southern
US as being the initial entry point of soybean rust. "That's based on a
lot of movement that we've seen throughout the world in the past several years,"
says Giesler, referring to reports that have rust moving as an airborne spore
from Japan and Australia to South Africa and to South America. "Now that
the disease has arrived, it's been predicted that it would overwinter in the
southern part of the US and then work its way up, similar to wheat leaf rust
or corn rust." The disease organism has now been identified in nine US
states as far north as South Carolina, Tennessee and Missouri.

Past and present of the disease
South America is a good example of how quickly this disease can spread. From
the initial infection in Paraguay in 2001, it continued to spread into the main
soybean producing areas of Brazil and Argentina. Then, in August 2004, the disease
was found across the equator in Cali, Columbia. "And once it jumped the
equator, we were just one major storm front away from having it in North America,"
says Tenuta.

For Giesler, the concern is most pressing in the southern US, in part because
kudzu, one of the primary host plants for soybean rust in South America, is
also abundant in Florida, along the eastern seaboard and into the Mississippi
delta. But controlling kudzu would not mean control of soybean rust. "Soybean
rust is unique over other rust diseases in that it has a lot of different hosts,"
says Giesler, adding that most rusts have only a primary and alternate host.
"But the aggressive strain can go to 32 different plant species."

Adding to the profile of just how damaging rust can be was the report from
the fall of 2003, when Brazilian researchers found the disease in soybeans that
were just 16 days old, the earliest it has been seen. Much of that 'early start'
was attributed to kudzu's abundance as a host.

Identification an issue
As with many other diseases that occur in the fields of Ontario and throughout
the US midwest, soybean rust looks like several other problem diseases, including
bacterial pustules, brown spot, downy mildew or bacterial blight. In soybean
rust, tan to dark brown or reddish-brown lesions, 2mm to 3mm in diametre appear,
usually on the underside of the leaves, although they also can be found on the
petioles, pods, stems and the upper leaf surface. The reddish-brown pustules
are raised, much like other rusts, with tan pustules having more spores. Mottling
on the upper leaves can also appear. "It can be confused with bacterial
pustules, but there's a low incidence in Ontario," says Tenuta, adding
the bacterial pustule condition is usually only a problem during summer months
with higher humidity. "It can also be confused with brown spot, which is
one of our earlier leaf diseases."

The higher humidity may be more important to growers within the Great Lakes
basin. Giesler theorizes the disease may have less of an effect on growers further
west, given the drier climate. And Tenuta adds that humidity will play a role
in the viability of spores. Some studies show that 10 to 30 percent of spores
can still germinate after 28 days, although Tenuta suggests the 'true risk'
for Ontario growers may be closer to 14 days.

Management choices in the interim
In the short-term, use of fungicides is the best protection, with particular
attention to the number of applications and the timing. In the long-term, researchers
and breeders are already at work on developing resistance and tolerance. There
are no commercially available varieties resistant to soybean rust in North America.

Although presently there are no fungicides registered for soybean rust, the
provincial extension personnel in Ontario, Manitoba, Quebec and the Maritimes
were proactive in submitting emergency use applications to the Pest Management
Regulatory Agency for four fungicides, pyraclostrobin (Headline), azoxystrobin
(Quadris), propiconazole (Tilt) and tebuconazole (Folicur).

Both Tenuta and Giesler are members of the North American Soybean Rust Technical
Committee, and as members, have access to the USDA plots in South America and
Africa, and are looking at various management applications, including different
combinations of fungicides.

"It is important to stress that soybean rust has not been detected in
Ontario or Canada and we anticipate the disease's impact to be less here than
the southern US," says Tenuta. "Our geographic distance from the pathogens'
overwintering locations in the southern US and our environmental conditions
are less favourable which lowers our risk to the disease."

If there is a silver lining to be found, adds Tenuta, it is that confirmation
at this time allows for the education and continued proactive development of
management plans to combat the disease prior to next year's growing season.

 


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