Top Crop Manager

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Oat crown rust races challenging breeders and growers

New varieties of oats help in battle with new races of crown rust.


November 19, 2007
By Top Crop Manager

Topics

65aIn Manitoba and eastern Saskatchewan, crown rust, also known as leaf rust,
cuts into oat yield and quality as disease incidence and severity increases.
While the presence of crown rust was quite low in 2006 in Manitoba due to unfavourable
conditions, some areas of eastern Saskatchewan did have crown rust infestations.
In contrast, 2005 infestations across the eastern prairies were very severe.

"We saw crown rust develop earlier than normal, early July compared to
late July in 2006. A couple of the new varieties were badly damaged. Morgan,
for example, is a very good variety in the absence of disease. Unfortunately,
this year, it suffered from crown rust in some areas, especially when planted
late," explains plant breeder, Brian Rossnagel of the University of Saskatchewan's
Crop Development Centre.

While the crown rust infestation was spotty on the eastern prairies in 2006,
the concern among plant pathologists and breeders is that with a return to more
normal weather, crown rust will be severe again. Another concern is that the
disease is highly adaptable and continues to develop new races that can overcome
new resistant varieties.

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Dr. James Chong, a plant pathologist with Agriculture and Agri-Food Canada
(AAFC) at Winnipeg, Manitoba, says that severity of crown rust in any year depends
on environmental conditions, abundance of the spores, stage of the crop and
the resistance of the varieties. While numerous genes have been identified and
have provided effective resistance to crown rust when first deployed, the rust
can overcome the new resistance genes very quickly, often within a few years.

"We have seen that happen over the years with cultivars first relying
on genes obtained from cultivated oat and then genes from a wild relative collected
from the Mediterranean regions. Pc38, Pc39, Pc48 and Pc68 were the genes that
were obtained from this wild oat relative and have been deployed in varieties
released at different times since the 1980s," explains Chong. "In
the 1980s, a series of cultivars were released with resistance to Pc38 and Pc39.
Since 1994, a series of new cultivars carrying the Pc68 gene in addition to
Pc38 and Pc39 were released. Since Pc38 and Pc39 had been defeated by the rust
in the early 1990s, these cultivars were basically protected by a single gene,
Pc68."

Prevalence can be high
In 2005, crown rust was very prevalent across Manitoba and into eastern Saskatchewan.
Isolates of the rust fungus collected from field surveys were established in
the greenhouse. Results based on tests on oats lines with single resistant genes
indicated that the level of resistance provided by the Pc68 gene has started
to decline since 2002, says Chong.

In the 2005 disease survey, more than 44 percent of isolates from wild oats
and 73 percent of isolates from tame (cultivated) oats were virulent (able to
attack cultivars with the Pc68 gene). Virulence of Pc38 and Pc39 remains high
at more than 82 percent since the early 1990s. This is because the Pc38 and
Pc39 genes are found in cultivars such as AC Assiniboia, AC Pinnacle, Ronald,
and Furlong that carry the Pc68 gene. The older varieties such as Dumont, Robert,
Riel and AC Preakness all had resistance based on these two genes.

In a never-ending battle to combat the new races, the AAFC breeding program
at Winnipeg released a variety named Leggett in 2004. Leggett contains the Pc94
gene along with the Pc68 gene. In addition, the new oats variety HiFi, released
from the breeding program at North Dakota State University, has Pc91, a different
effective resistance gene. For the time being, these two varieties offer the
best resistance against all races of crown rust on the eastern prairies. "However,
virulence to these two genes already occurs in the rust population, so it is
just a matter of time before we see these varieties become susceptible,"
says Chong.

For example, virulence to Pc94 was not detected in the rust population in the
2005 annual survey. However, an isolate capable of overcoming the Pc94 gene
was found in the eastern region. Similarly, virulence to Pc91 in the eastern
prairie region is still very rare. Isolates with virulence to this gene have
been observed in 2002, 2003 and 2005.

Rossnagel says growers in rust areas should try to get their hands on Leggett
or HiFi as soon as they can. HiFi may have good commercial availability in 2007,
but Leggett will be another year or two away from commercial production.

"Good alternate choices, while not as good as HiFi or Leggett for rust
resistance, are CDC Dancer and CDC Boyer. They have moderate resistance to the
rust, so they can provide some protection against yield and quality losses,"
says Rossnagel.

Plant breeders are also continuing to develop new resistant varieties. New
sources of resistance have been identified in wild oats collected in Europe,
the Middle East and North Africa. Several advanced lines with resistance from
the University of Saskatchewan Crop Development Centre and Agriculture and Agri-Food
Canada were in the co-operative oat tests in 2006.

Home-grown crown rust
Typically, crown rust urediniospores carried by the southerly winds from the
US provide the primary source of inoculum for the prairie provinces. The particular
stage of the rust that attacks oats during the summer do not survive the cold
winter in Canada, but can overwinter on fall seeded oats and grasses in the
warmer southern states. However, Rossnagel is also concerned that the crown
rust fungus can cycle its sexual stage in the prairie region. He suspects that
this is part of the reason why crown rust showed up earlier in Saskatchewan
in 2006.

Indeed, Chong says the crown rust fungus can overwinter by cycling from oats
to its alternate host, buckthorn, in the prairie region. Buckthorn, a woody
shrub, is usually found in city parks, ravines, scattered wood lots and along
riverbanks. (Not to be confused with sea buckthorn, a common shelterbelt and
garden shrub.)

During the late growing season, crown rust turns into black telia, containing
teliospores, on oat-straw stubbles. This is the stage of the rust that remains
dormant during the cold winter months. In late spring, teliospores on straw
stubble germinate and produce basidiospores to infect nearby buckthorn plants.
Subsequently, the aeciospores are produced from this host, which infect only
the oat plants. From these infections, urediniospores are produced and this
is the stage of the crown rust fungus that can cycle every 10 days to build
up the epidemics in the field.

Chong says that while these local infestations on the buckthorn plants may
be involved in crown rust outbreaks, the primary source of inoculum most likely
comes from the US. "In addition to working on genetics of host resistance,
we are also studying wind pattern and trajectory to better predict outbreaks,"
says Chong.

Another area that the breeders are working on is a different genetic resistance
mechanism. The most commonly used genes are seedling-resistant genes, meaning
those that express their resistance at the seedling stage right up to the adult
plants. Chong says that adult-plant resistance may provide longer-term protection.
"In other cereal rust/host systems like wheat stem rust and wheat leaf
rust, this type of resistance has proven to be very reliable, lasting up to
40 or more years," he explains.

However, adult-plant resistance can be conditioned by single or multiple numbers
of genes, making breeding more difficult if the resistance is conditioned by
multiple genes. In addition, oat plants must be grown to the adult stage for
testing and selection, requiring greater financial and space resources.

What can you do?
Planting crops early is one of the key methods to escape severe infection. Early
planting may allow the crop to mature before the crown rust epidemics develop
to the dangerous levels.

Foliar fungicides are another tool and Rossnagel says growers of susceptible
varieties in potentially affected areas should pencil in an application when
developing crop budgets. "If you need a fungicide, then at least you have
that budgeted, but if it turns out that you don't need it, then you're ahead
of the game," he says.

Registered fungicides for crown rust on oats contain the active ingredients
propiconazole and/or trifloxystrobin. Products containing propiconazole include
Tilt 250E, Bumper 418EC and more recently, Pivot 418EC. Stratego 250EC (propiconazole
and trifloxystrobin) has just been reinstated for use on oats in Canada. Costs
were generally in the $7.50 per acre in 2006.

Fungicide application should take into account the time of arrival and amount
of inoculum (urediniospores) moving up from the south and weather conditions.
The USDA's Cereal Rust Bulletin is a tool that Canadian growers can use to help
keep track of crown rust's movement from the US. This bulletin starts in March
and provides information on the northward progress of cereal rusts from Texas
to the end of the growing season in the Northern Great Plains. Chong says the
information is useful for forecasting rust on the eastern Canadian prairies,
but cautions that it is just a forecast and growers need to assess all the other
environmental and agronomic factors associated with the disease before spraying.

The bulletin is found at: www.ars.usda.gov/main/ site_main.htm?modecode=36-40-05-00.

The ideal timing for application is to spray at flag leaf emergence to protect
the flag leaf. Since rust can develop very quickly, frequent scouting is required.
Once the flag leaf is covered with rust symptoms, it may be too late to apply
fungicide. -30-


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