Features Agronomy Plant Breeding
Breeding fusarium wilt resistance into field pea

Researchers at the Agriculture and Agri-Food Canada (AAFC) Cereal Research
Centre in Morden, Manitoba, are working with genetic sources of fusarium wilt
resistance in field peas. Fusarium wilt is a soilborne fungal disease that can
cause yield losses and, in some cases, premature plant death in pea crops. It
is one of the top diseases affecting pea crops in parts of western Canada. Because
fusarium wilt is a soilborne disease that can survive up to 10 years in the
soil, the consensus of researchers is that the best way to deal with this disease
is through the development of resistance.

"There are basically four races of fusarium wilt that are a concern to
field pea production, including races 1, 2, 5 and 6," explains Dr. Bob
Conner, pulse crop pathologist. "In the first part of the study, which
we completed in 2003, we looked at more than 100 commonly grown pea varieties
to see which of them might be resistant to which race, and if any varieties
were resistant to more than one race."

Researchers on the project included Dr. A.G. Xue, Dr. Tom Warkentin (CDC) and
Dr. D.J. Bing. "The research results showed that a few of the cultivars
were resistant to at least three of the four races." Some of the cultivars
were susceptible to all of the races.

Researchers also tried to determine how common the four races of fusarium wilt
were across the prairies. "We worked with people in Alberta and Saskatchewan
who surveyed fields and collected plant samples for us over three seasons,"
says Conner. "We isolated the different races of fusarium wilt, and then
tested those isolates on a set of different lines that carry different genes
for resistance to fusarium wilt."

Through the results, researchers could identify which fusarium wilt race was
present. "It appears that all four races of fusarium wilt are fairly common
in Canada." Researchers are taking this information to their pea breeding
programs and are aiming for varieties that will be resistant to all four races.

Another component of the study searched for molecular markers for fusarium
wilt resistance. "We worked with Dr. Ron Knox, a molecular biologist at
AAFC in Swift Current, to identify molecular markers closely linked to the genes
for resistance, which would assist breeders in identifying lines that might
be resistant to fusarium wilt," explains Conner. "By using molecular
methods, we can screen a lot of material fairly quickly, and the more promising
lines can then be screened against the pathogen to confirm resistance."
The screening process without molecular methods is lengthy and laborious, as
tests have to be done in laboratory growth chambers using a limited number of
cultivars each time.

There are two types of symptoms for fusarium wilt. "Early wilt symptoms
appear when races 1, 5 and 6 are present," explains Conner. With early
wilt, plants usually die soon after they emerge from the soil. "The second
type of symptoms are referred to as near wilt symptoms and appear with Race
2."

Near wilt symptoms appear much later in the season when the plants are starting
to form pods. The pathogen causes plants to turn yellow and wilt. "Any
information we can gather on disease resistance or susceptibility, we bring
to the attention of the plant breeders so they can take that into consideration
in variety development.

"Based on the research work we've done so far, we know there are varieties
out there that already have resistance to some of the fusarium races,"
says Conner. "In the annual screening process for new co-op test entries,
we use our disease nursery at Morden to evaluate cultivars for various diseases,
including fusarium wilt."

At the nursery site, peas have been grown for more than 15 years, so levels
of the pathogen have built up, making analysis of resistance fairly easy. Researchers
are using the disease nursery to screen much of the advanced materials from
the pea breeding program for resistance. They are already using this information
to improve fusarium wilt resistance in current breeding programs, and will continue
to work towards improving the development of new varieties.

Other disease resistance also incorporated into plant
breeding
At the Crop Development Centre at the University of Saskatchewan, advanced pea
lines are screened for fusarium wilt resistance. "Although the priority
of our program in Saskatchewan is on powdery mildew and mycosphaerella blight,
as are the programs at AAFC Lacombe and Morden, we do also screen our materials
for fusarium wilt resistance," explains Warkentin. "We've been working
with researchers in the Pacific northwest, and every year send a series of advanced
lines for evaluation at Washington State University. They have a nursery specifically
for fusarium wilt, and give us a rating of how our varieties are doing in those
trials. It gives us some information on our most advanced materials."

Warkentin notes that there are also some indirect breeding efforts for resistance
to diseases like mycosphaerella and fusarium wilt. "Our breeding materials
are exposed to moderate levels of disease inoculum every year because of the
close proximity of our plots to previous trials. So we're indirectly selecting
for resistance by selecting higher yielding lines in the presence of moderate
levels of those two pathogens."

In terms of powdery mildew resistance, pea breeders have been doing a good
job and almost all of the new varieties being released have resistance. "Powdery
mildew resistance is a single gene trait, so varieties that have the resistance
gene will be basically free of disease," explains Warkentin. Essentially
all new varieties are powdery mildew resistant, and growers now have several
choices when they go to select varieties.

Mycosphaerella blight (ascochyta) is quite different, as all varieties are
at least moderately susceptible. "This is one of the most important pea
diseases, but a difficult one to make progress on developing resistance,"
says Warkentin. "Mycosphaerella is present essentially each year, especially
under wetter conditions, and can cause significant yield losses. We do have
ratings in the variety tables for mycosphaerella blight, with the ratings being
either fair or poor in terms of resistance."

Growers can check cultivar descriptions or talk to seed companies to get more
information about fusarium wilt and other disease resistance of varieties. Information
should be available for most Canadian varieties. "However, we are seeing
some varieties that have been developed in Europe and are now being sold in
Canada, which may or may not have been selected for resistance to that disease,"
says Conner.

Growers will be able to find varieties resistant to powdery mildew and fusarium
wilt. They can continue to look forward to more resistant pea varieties for
various diseases in the future as researchers and pea breeders continue their
efforts to improve resistance in new varieties.

The Bottom Line
It appears we are starting to gain on improving disease resistance in
all plant types. With the push on for gene mapping and genomics, we have
been able to speed up the process of screening for desirable traits in
many plant species. Fusarium wilt, up until a few years ago, was not a
major problem in our area. A couple of years ago, it devastated non-tolerant
canola crops.

Relatively new diseases such as fusarium wilt identify why research investment
needs to be expanded in all our important commodities so our breeding
institutions both public and private can keep ahead of the issues, as
well as be prepared to react to new epidemics that will arise suddenly.

Nature is forever evolving and will continue to throw new problems at
us. Soybean rust is another good example of why having the resources available
for detection, control and then breeding resistance into a species will
be important to continue to sustain a species. Warren
Kaeding, Churchbridge, Saskatchewan.