There’s been a lot of effort put into breeding fusarium-resistant varieties, Dr. Anita Brûlé-Babel told CropSphere delegates in Saskatoon in January.
But developing cereal varieties resistant to fusarium head blight (FHB) is more like a marathon than a sprint.
“There’s been a lot of breeding challenges. And I know a lot of farmers have been very frustrated — especially in Manitoba — frustrated about the fact that it took us a long time to get anything better than what we have,” said Brûlé-Babel.
Brûlé-Babel is a wheat breeder and researcher with the University of Manitoba, based out of Carman. She’s been working on FHB since 1999.
Plant breeders are making progress, especially in the Canada Western Red Spring (CWRS) wheat class. In Seed Manitoba 2014, more than 25 per cent of CWRS lines were rated moderately resistant.
The Canada Prairie Spring Red and General Purpose classes have some improved varieties coming through as well, Brûlé-Babel said. But Hard Whites and durums have a long way to go, she added.
Dr. Curtis Pozniak, a University of Saskatchewan researcher, is introducing resistant genes from other species into durum to boost resistance.
“He’s made some good progress there, but we’re still quite a ways from actually having a variety,” she said.
Plant breeders have identified several types of FHB resistance, but they’re focusing on two, Brûlé-Babel told delegates. Type one is resistance to initial infection. Often it’s linked to differences in flowering type — for example, whether anthers push out.
Type two is resistance to disease spread within the spike itself, Brûlé-Babel said. It’s one of the strongest types of resistance researchers have at their disposal, she added.
Researchers have also pinpointed resistant genes. Brûlé-Babel said one gene, FHB 1, is particularly effective.
“Because it’s got type two resistance, it also seems to have some good DON resistance in it as well. It is our strongest source of resistance. And it is quite widely used,” she said.
A second gene, FHB 2, also offers resistance. Plants with FHB 2 alone may have high disease levels. But plants with both genes have better FHB resistance, Brûlé-Babel said.
While researchers can map markers linked to resistant genes in the lab, they still have to do field work to see how plants react to FHB infection. They prepare an inoculum in the lab to infect the plants. When the plants flower, researchers spray them with the inoculum. Three days later, researchers inoculate plants again. And researchers mist-irrigate the plants to create the humidity FHB thrives on.
“We’re creating a perfect storm for this disease,” said Brûlé-Babel. Without these measures, it would be more difficult to get FHB infections every year, she said.
Researchers then look at the plants’ responses to the infection. They measure the percentage of infected spikes, and the percentage of infected spikelets on the spike. These numbers are used to rate the disease resistance.
Disease ratings reflect a range of resistance rather than a single set percentage. For example, a variety could have 30 per cent infected spikes, with 30 per cent of the spikelets affected. Another variety could have 40 per cent infected spikes, with 50 per cent of the spikelets affected. Both varieties would be rated moderately resistant.
Plant breeders face many hurdles in the race to FHB resistance.
There isn’t a single gene that guarantees resistance. Fusarium resistance comes from “many, many genes,” each with a small effect, that create resistance when combined, Brûlé-Babel explained.
The very nature of fusarium presents a challenge. The pathogen is a necotroph, meaning it kills the plant and feeds off dead tissue. “It’s not like a rust, where a rust needs a living plant to survive on. This pathogen will continue to grow quite happily on the crop residues and it doesn’t need the plant to be alive to do that.”
Abundant inoculum in fields causes problems for farmers, of course. For researchers, it means “plant defence responses are really complex,” says Brûlé-Babel.
Another stumbling block is the lack of understanding of the way plants and fusarium interact. Brûlé-Babel says they’ve done many studies looking at how plants respond to infection. “And a whole slew of things happen all at once. And yet we still are trying to figure out what are the key things we need to work on.”
There are also several different fusarium types. Luckily for plant breeders, the same resistance genes seem to work across the board, Brûlé-Babel said. This means plant breeders don’t need a specific program for each fusarium type.
Linkage drag is another obstacle. When plant breeders introduce new genes to wheat, undesirable genes tag along, Brûlé-Babel explained. She said it takes a lot of time to break those linkages, although researchers have managed to cut many of them.
Measuring disease and setting resistance levels are also challenges, she added.
Unlocking AC Emerson’s resistance
AC Emerson is a relatively new winter wheat, and the first of any class rated resistant to FHB.
Asked by a delegate whether researchers have been able to identify which genes are resistant, Brûlé-Babel said they haven’t yet. She suspects it’s a type one resistance, based on what she’s seen in other winter wheat. A graduate student will try to map the resistant genes this summer, she said.
Brûlé-Babel said that when they first registered AC Emerson, she was worried the resistance might not hold up.
“But it seems to be holding up quite well,” she said.