Features Agronomy Diseases
Changes in races of oat stem rust intensify

While 2002 was the last time oat stem rust caused some losses in Manitoba and
eastern Saskatchewan, researchers and plant breeders are rushing to catch up
to a race of oat stem rust that previously only existed at low levels. The race,
called NA67, is on the rise, and in 2003, close to 50 percent of the oat stem
rust races found were NA67. Unfortunately, none of the current registered varieties
have resistance to NA67.

"I hesitate to call it a new race of oat stem rust, because there is evidence
that it has been in Manitoba in the past, but for some reason it never increased
or adapted," says Tom Fetch, a cereal plant pathologist at Agriculture
and Agri-Food Canada's (AAFC) Cereal Research Centre in Winnipeg.

Plant pathologists annually survey Manitoba fields for the presence of oat
stem rust. They not only survey oat fields, but also look at wild oats in other
fields as well. Wild oats are a non-selective host and provide a good indication
of the races of stem rusts found in the field. From these surveys, they are
able to determine the infestation and severity levels of oat stem rust, and
also identify the various races that were prevalent.

NA67 was first noticed to be increasing in 1998. In 1997, NA67 was found at
very low levels, but during the spring of 1998, the levels had jumped to around
20 percent of oat stem rust races. "That is a dramatic rise for a race
that previously was not very prevalent. 1998 was certainly the threshold year
for NA67," says Fetch.

Since 1998, the proportion of NA67 stem rust has steadily increased. In 2002,
the proportion of NA67 found on non-selective wild oats was 45 percent in Manitoba.
In 2003, the proportion was 55 percent in Manitoba and 43 percent in Saskatchewan.
While the overall stem rust infestation was low in 2004, Fetch expects that
the proportion of NA67 race will likely again be in the 50 percent range.

Stem rust generally does not overwinter on the prairies. Rather, the spores
blow up from the US from late June to mid-July. The severity of the prairie
infestation depends on the spore production in the US, wind direction during
spore release, and weather conditions on the prairies once the spores arrive.
Typically, Manitoba and eastern Saskatchewan have the highest risk of rust infection.
Daytime temperatures near 28 degrees C and nighttime temperatures near 18 degrees
C with heavy dew formation favour rust development.

Plant breeders are working to develop NA67 resistance in oat breeding programs.
Fetch says that several previously discovered genes, which provide resistance,
are currently being used. He is also looking for newer sources of resistance.
Fetch has had success in identifying oat lines with NA67 resistance, but getting
the resistance into domestic oats is difficult.

"The resistance that was found were in wild relatives of oats that were
mostly diploid plants. Cultivated oats are hexaploid, which is problematic when
trying to incorporate genes from a diploid," he explains.

So, metaphorically speaking, until Fetch can figure out how to breed a mule
to a horse, oat growers will have to rely on several management practices to
cope with NA67 oat stem rust.

What can growers do?
Fortunately, oat growers have not had to contend with NA67 during the past two
years since the overall rust severity was low. However, since prairie stem rust
infestations occur based on the whim of Mother Nature, Fetch suggests a management
plan to help minimize the impact of all stem rust races, NA67 included.

"I still recommend choosing a variety with some stem rust resistance,
even though it won't be resistant to NA67. At least the variety will have resistance
to about 50 percent of the races that can affect oat crops."

A second key strategy is to plant oats early. Early planting will hopefully
mean the oat crop is further along in maturity when the rust spores show up.
If the plant is already headed out when the disease develops, the yield impact
will be much lower.

Of course, early seeding is a relative term, but Fetch says definitely seed
before June, and in the first week or two of May, if possible. "If you
plant in early June, the crop may only be out of the ground for one month before
the spores arrive. That can have a devastating impact on crop development. But
if you plant in early May, you might miss most of the rust infection."

While no fungicides are registered for control of oat stem rust, propiconazole
(Tilt) is registered in oats for control of crown rust and septoria leaf blotch.
Fetch says that some growers want to control these leaf diseases in an attempt
to keep the crop as healthy and free of disease as possible. Grower comments
have indicated that some control of stem rust is also achieved.

"Fungicide applications can be effective for controlling disease, but
you have to consider the economics and whether the application will reward you
with increased yields," cautions Fetch.

Most growers using fungicides appear to be applying them from the boot to panicle
emergence stage. Fetch advises that the grower needs to follow the label with
regard to maximum rate and growth stage of fungicide application.

With any luck, plant breeders will be able to incorporate NA67 resistance into
oat varieties before the next big oat stem rust infestation hits. In the meantime,
following Fetch's recommendations will help to minimize losses should that spring
wind from the south blow ill. -30-