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Controlling oat stem rust

PLANT BREEDING

Controlling oat stem rust

Bruce Barker

The races of stem rust may be changing, but researchers are continually working on new resistance.


November 19, 2007
By Bruce Barker

Topics

By all accounts, the incidence of oat stem rust (Puccinia graminis)
on the eastern prairies was light in 2006. Dry weather and fairly wide-spread
fungicide applications for crown rust control combined to keep the disease at
bay. However, plant pathologists and plant breeders are keeping their eyes on
several new races of rust that are emerging here and in the US.

"We saw extremely light stem rust across the prairies in 2006. In the
midwest US, they had an extremely dry year, which reduced the amount of stem
rust blowing into Canada," explains cereal disease research scientist,
Tom Fetch at Agriculture and Agri-Food Canada (AAFC) in Winnipeg.

As part of the Canadian Plant Disease Survey, pathologists survey fields to
determine the prevalence of stem rusts, and analyze the samples to determine
the races of stem rust that are causing the disease. According to Fetch, who
summarized the oat stem rust results, stem rust in cultivated and wild oats
was absent or at trace levels in western Canada in 2005 (the 2006 survey results
are still being tabulated). The reason levels were low in 2005 was that low
stem rust inoculum pressure from the US, combined with extremely high oats crown
rust pressure, led to severe crown rust infection and left little leaf and stem
tissue available for stem rust infection.

Fetch says that the predominant race in Manitoba since 2001 has been NA67,
but it declined to 25 percent of samples found on wild oats and 55 percent on
cultivated oats in 2005. He says that all oats cultivars recommended for production
in Canada are susceptible to stem rust races NA67.

In 2005, the predominant race of stem rust in Manitoba and Saskatchewan on
wild oats was NA27 (42 percent), along with NA29 (17 percent) and NA30 (10 percent).
These races are not virulent on most commercial Canadian cultivars grown in
Manitoba or Saskatchewan as most varieties have resistance to these races.

For 2005 (data for 2006 has not yet been summarized), Fetch says that NA67
was still observed in Manitoba, but that prevalence of NA27 and NA29 appeared
to be up. "This is good news because we have good resistance to NA27 and
NA29. I'm not sure what the reason is that those two races are up in prevalence.
Perhaps farmers are growing varieties that are susceptible to races NA27 and
NA29, allowing them to develop and increase in prevalence compared to NA67."

While Canadian varieties do not yet have NA67 resistance, oats lines with effective
stem rust resistance (Pg16, Pg-a genes) to NA67 are in advanced agronomic trials
in breeding programs at the Cereal Research Centre of AAFC at Winnipeg.

"In our breeding programs, we use cultivars with the Pg2, Pg9 and Pg13
genes as the basis of stem rust resistance. We're now taking steps to incorporate
Pg10, Pg11, Pg16 and Pg-a genes into oats cultivars," explains Jennifer
Mitchell Fetch, an oats breeder with AAFC in Winnipeg. "Those combinations
can give us resistance against a wide range of races."

Of recent note is the appearance of two new races in the US. Tom Fetch says
that one of the races is probably a mutation of NA67 that is showing a little
more virulence. One isolate of this new race showed up in Manitoba last year,
and it was found in Texas in 2004 and 2005. Texas is a key overwintering ground
for stem rust, where the disease multiplies in the spring and blows northward
to infect eastern prairie crops.

Tom Fetch says this new race also appears to have virulence to the Pg-a resistance
gene as well. "This is bad news as the new race would take several resistant
genes out of the breeding programs, and we have few enough genes to work with
in the first place," he explains.

The other new race was just discovered in Louisiana, but not enough is known
about it to cause immediate concern here.

Crop management helps
While plant breeders toil to bring new cultivars to farmers that can resist
the evolving stem rust races, growers can also use management to help minimize
the disease. Mitchell Fetch says that planting in the first weeks of May can
allow the crop to advance to later stages by the time rust spores blow up from
the south. In addition, using resistant varieties, even if they do not have
complete resistance to all the races, can help.

"Use resistant varieties, even if their resistance genes provide resistance
to the 'older' stem rust races and only offer partial protection," she
says. Varieties like AC Assiniboia, Ronald, Pinnacle and Furlong, while not
resistant, can offer some protection against stem rust.

Fetch agrees and says that, for example, if the NA67 population accounts for
50 percent of the race frequency, at least growers are protected from the other
half of the stem rust population when growing a variety with resistant genes
to the other races.

Varieties rated very poor in the seed guides, such as Triple Crown or AC Morgan,
have no stem rust resistance and Mitchell Fetch says growers need to be prepared
to spray with a registered fungicide to protect the crop. -30-