Features Agronomy Diseases
First sclerotinia tolerant canola variety in the works

It is an exaggeration to say that sclerotinia is on the ropes, but at the least this yield-hammering disease is starting to stagger in the ring. Developments in canola breeding could change the way growers farm as early as the summer of 2009.

Two years after commercial availability, Clearfield lentils are meeting expectations of growers and processors.

Fungicides offer one defence but it is public and private canola hybrid development that is landing the most blows against sclerotinia. It has been a tough slog, mostly because sclerotinia is such a “powerful disease attacking many broadleaf crops,” as Dr. Dilantha Fernando, plant pathologist at the University of Manitoba says.

“Forecasting models aren’t as well-developed for sclerotinia in canola as they are for something like late blight in potatoes,” says Ralph Lange, plant pathologist with the Alberta Research Council. “It’s the most difficult disease to manage. For instance, I know from experience that it’s possible to grow a good crop with a hybrid susceptible to blackleg if you’re willing to do certain management practices. Not so with sclerotinia. If there’s a risk, you have to spray.”

Lange says that because the sclerotinia models are not as reliable, some producers are left searching the skies, not to judge the weather but to look for airplanes. If they see airplanes spraying other growers’ canola crops, they spray too.

“It will take multiple genes to develop resistance to sclerotinia,” says Fernando. “With rust in wheat and blackleg in canola, you can achieve resistance through good breeding mainly because one gene in the plant corresponds to a gene in the pathogen. Once you find the resistance gene in the plant, it’s transferable through breeding. You can’t get resistance to sclerotinia with one gene.”

In the background, behind the frontlines where farmers battle each summer with sclerotinia, public and private researchers have been busy searching for a solution. Dr. Lone Buchwaldt, a plant pathologist with Agriculture and Agri-Food Canada in Saskatoon, is screening germplasm, looking for lines that are resistant to sclerotinia. Up until now, she and her colleagues have worked with one source of resistance in particular, from China. She talks about places on the chromosome that confers resistance to the canola plant. “We have to combine two or more of these loci to get good resistance in the field,” she says. “When combined, they confer a biochemical defence that stops the fungus on the main stem.”

It is time-consuming work. Her team screens lines and their progeny and extracts DNA and have found a correlation between the DNA and resistance to sclerotinia.

Part of the problem, ironically, is that sclerotinia does not show up in every field, every year. “We can’t screen in a field on a reliable basis,“ she says. “It’s so sporadic where it occurs. We can’t guarantee the weather or plan a good infection. However, we have developed screening methods that work well in the growth room and under semi-field conditions where we use petals infected with ascospores. We have recently identified new lines with promising levels of sclerotinia resistance.”

One company that has had a breakthrough is Pioneer Hi-Bred. Representatives are saying their new hybrid, Pioneer brand 45S51, offers an improved level of field tolerance to sclerotinia when it is left unsprayed. When sprayed, they are saying it has a significantly higher level of disease tolerance than other, similar hybrids.

Field tolerance of this new hybrid is a combination of the reduced transfer of disease into stems resulting in fewer plants infected, coupled with reduced severity of stem symptoms resulting in lower yield losses on diseased infected plants.

The company has added sclerotinia to the scale it uses to measure disease. A rating of “1” indicates a high level of diseased plants and “9” is highly tolerant with trace levels of diseased plants. Complete resistance to sclerotinia does not exist but 45S51 is one step closer for canola producers.

Using the hybrid is like buying an insurance policy in areas with low risk of sclerotinia development. In other words, a grower might not have to pull the trigger on a fungicide application. If the sclerotinia checklist suggests the need to spray, 45S51 will give growers some extra time in making that decision. “Sometimes farmers can’t get a plane to spray in time or get into a wet field,“ says Igor Falak, research scientist with Pioneer Hi-Bred. “In essence, it’s built-in insurance.“

“It will reduce sclerotinia impact and it may reduce unnecessary spraying,“ says Falak. “Very often the best prediction might say you need to put on a fungicide and then it turns out you didn’t need to spray.“

The 45S51 hybrid will mean less disease in the crop resulting in less inoculum in the field down the road. In times of heavy disease pressure, the hybrid outperforms similar hybrids when both are sprayed with a fungicide. “It’s very hard to pack tolerance with high yield, they don’t go hand in hand,“ says Falak. “It takes time to improve the package.“

The hybrid could also help spread out a grower’s risk level and improve time management. The 45S51 hybrid gives growers some extra breathing room. “In the Red River Valley where growers have to spray almost every year, this hybrid may give them more choice about whether or not to spray,“ says Falak. “If it’s a year with low pressure then the impact is even further reduced in 45S51. If it’s dry, you may get away without spraying but you’ll have to watch carefully, we’re not advocating not spraying.“
Falak says the sclerotinia checklist will still drive spraying decisions but the economic threshold has changed with the new 45S51 hybrid.
This is just the first hybrid with sclerotinia tolerance for canola growers. And with public and private breeders on the case, there will be more to come in the fight against sclerotinia. n