Measuring blackleg’s impact on profitability

Blackleg may not be the most yield-robbing fungus that prairie canola producers face, but it is certainly a concern.

“Growers shouldn’t assume that blackleg doesn’t have a repercussion for their overall profitability,” said Clint Jurke of the Canola Council of Canada.

As the prevalence and severity of blackleg crept up over the past decade in on some prairie fields, members of the canola industry realized they didn’t have a model that could calculate the amount of yield loss due to the disease.

“No one had actually done that work, to correlate infection and severity of the pathogen to yield losses. So it (the blackleg yield loss model) was given a pretty high priority,” Jurke said.

Researchers from the University of Alberta and Alberta Agriculture teamed up to run trials from 2013-15, to figure out exactly how much yield one could expect at specific disease severity levels.

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“It’s kind of a joint research project that was part of Growing Forward 2 program that was funded through SaskCanola, the federal government and the canola council,” Jurke said.

“So that yield loss model, although the first version wasn’t entirely perfect, has helped us develop some estimates on how much blackleg was costing farmers as a disease. ”

U of A researcher Stephen Strelkov and Sheau-Fang Hwang of Alberta Agriculture led the research and grew several canola cultivars to determine the relationship between blackleg disease severity and yield in susceptible, moderately resistant and resistant canola varieties.

Strelkov said there is a fairly linear relationship between disease severity and yield decline.

“Basically what we found was that with each unit increase of the disease zero to five scale, there was an approximate corresponding decline in yield of 17 percent in the plant. So for basically every time the disease severity went up, say one to two, or two to three, we could expect more or less a 17 percent decrease in the yield,” Strelkov said.

The disease rating scale that the industry uses is a zero to five rating for every plant. Researchers cut the plant open and score the amount of disease from zero to five where zero has no sign of disease and with five the plant is completely dead.

Strelkov said the first phase of the study is finished, but there is still work to do with the second phase underway, which will help improve the yield loss model.

“Work is ongoing, the next stage is to refine the model and to look at a wider range of hosts to see how it holds up,” Strelkov said.

While Strelkov described the model thus far as a preliminary one, Jurke said the canola council has already found the tool valuable.

“When we last we ran this model, it was in 2015 and 2016, and the estimated yield loss was $330 million and $396 million, respectively.

“This may have been a high estimation, since the model has since been improved. To compare, sclerotinia stem rot roughly reduced yield in those years by four percent and 10 percent, which is far more costly,” Jurke said.

The problem with the first model was that it didn’t recognize there isn’t a significant yield difference between a zero and a one rating.

“The very first model assumed a linear relationship right from zero to five, and really there is a bit of plateau from zero to one, and it’s not until after you get severity rating above one that the yield begin to drop off. So they have come up with a better fit for that severity index,” Jurke said.

He said the severity data for 2018 is still in the interim phase and will be available later in the winter.

Jurke is unaware of any growers using the model to estimate losses due to blackleg on their farm, but he said there is nothing stopping them from doing so.

“The research paper is out there, it does have a scale that could be used,” Jurke said.

“Certainly if they do go out and cut stems open and estimate the amount of blackleg severity, they could use it in that regard.”

Rating blackleg

The severity of blackleg infection is evaluated on a minimum of 100 plants averaged over four replicates at crop maturity. Individual plants are uprooted, cut through the basal part of the stem and scored on the basis of the amount of disease using the following zero to five scale:

0: No diseased tissue visible in the cross-section.

1: Diseased tissue occupies up to 25 percent of the cross-section.

2: Diseased tissue occupies 26 to 50 percent of the cross-section.

3: Diseased tissue occupies 51 to 75 percent of the cross-section.

4: Diseased tissue occupies more than 75 percent of the cross-section with little or no constriction of affected tissues.

5: Diseased tissue occupies 100 percent of the cross-section with significant constriction of affected tissues; tissue dry and brittle; plant dead.

Source: canolacouncil.org

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