Vigilance needed with blackleg on the rise

Adopt best management practices and rotate resistance gene groups

Blackleg in canola hasn’t got out of hand across Western Canada yet, but it is on the rise, says a Canola Council of Canada (CCC) specialist who advises farmers to scout fields and consider rotating into different disease-resistant varieties to reduce the risk of disease buildup.

There is still time this spring before seeding to check last year’s canola crop residue to see if there is any sign of black, pepper-like disease spores on stubble. Those spores aren’t reason for panic, but are a warning the disease is present in your fields.

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Some central Alberta canola fields in 2019 stand as an extreme example of what can happen if conditions align and blackleg suddenly explodes through a crop, says Justine Cornelsen, CCC agronomy specialist.

A tight wheat-canola rotation for several years, growing the same older canola variety combined with ideal spring environmental conditions — good moisture and 15 C temperatures — triggered a disease outbreak that wiped out these fields of canola.

“There was 100 per cent infection and complete loss,” says Cornelsen. “The disease lives on crop residue and with the tight rotation and ideal environmental conditions it just took off. Blackleg severity is measured on a scale of zero to five, with zero being a clean field and five being maximum severity, so this pocket of fields had a four-plus rating. The disease spreads through the plant, causes the stem to constrict cutting off nutrient supply and the crop lodges or just falls over. It was a writeoff.”

Those fields near Red Deer, Alta., were a worst-case scenario. Cornelsen says most western Canadian farms are not in that ballpark of disease severity, but she wants farmers to be aware the risk is out there.

With virtually all canola varieties registered in the past 20 years carrying a rating of moderately resistant (MR) to resistant (R) against blackleg, awareness or concern over the disease has been relatively low. For the most part, blackleg just hasn’t been a concern.

Blackleg levels increasing

“Particularly over the past 10 years or so we are beginning to see an increase in blackleg levels,” says Cornelsen. The CCC says average disease levels should be under 10 per cent and ideally under five per cent. “In the last few years, the average across the Prairies has increased to about 12 per cent, so producers need to be checking their fields and at least be aware the disease may be out there.”

The gradual increase in blackleg severity is blamed on changes in production practices over the past decade. Canola acres have increased so the disease has a large host crop. Canola rotations have tightened up — in some cases back-to-back crops or with only a one-year break in rotation. And farmers have been growing varieties with the same genetic resistance for several years in a row.

“With this combination of factors, the pathogen is adapting itself to the resistant genes,” says Cornelsen. Some varieties that were rated R, for example, when they were first registered, are now found in some areas to have a MS (moderately susceptible) to S (susceptible) rating as the disease changes.

Many genes are responsible for blackleg resistance in canola. Much like the approach taken with rotation of crop protection products, rotating varieties or resistance gene groups creates the opportunity to bring a mix of resistance genes to the field over time, which can reduce selection pressure and improve durability.

Cornelsen says if the disease is present in canola fields and farmers aren’t impressed with crop performance and yield, they may need to have infected root samples analyzed to determine what race of blackleg is present in their crop and then select a canola variety with a different resistance gene to that race of blackleg.

The four most common groups of genetic resistance in Canada are described as Group A, Group C, Group G and Group E1. “Group C resistance is the most common genetic resistance found in canola varieties,” says Cornelsen. “It has been the backbone of genetic resistance for the past 20 years. If the disease is present in your fields and it doesn’t appear to be responding to that resistance, you may need to change to a variety with a different group of genetic resistance.”

Some but not all seed suppliers will show the group of genetic resistance on the seed batch label. She advises producers to consult their seed supplier or an agronomist for advice on variety and resistance selection.

Along with seeding canola varieties with genetic resistance, Cornelsen says farmers can also use a fungicide to control blackleg. However, to be effective it must be applied early at the two- to six-leaf stage and before the pathogen reaches the leaves of the plant. She also notes that companies are working to introduce a seed treatment that is effective in controlling blackleg, which is expected to be available for the 2021 growing season.

Reducing risk

Producers interested in reducing the risk of blackleg disease development in their canola crop need to adopt three key “best management” practices, says a Canola Council of Canada specialist.

While the disease hasn’t been a huge concern for growers since the introduction of disease-resistant varieties some 20 years ago, it appears to be making somewhat of a comeback, says Justine Cornelsen, an agronomy specialist.

Cornelsen’s recommended risk management practices include:

1. Follow a proper crop rotation that allows for at least two and ideally three or more years between canola crops. The blackleg disease overwinters on canola residue and will break down over a couple of years.

2. Scout fields for signs of the disease. While blackleg hasn’t been an issue for several years, it is becoming more prevalent across Western Canada. The best time to check for signs of the disease is after fall harvest. Inspect crop residue for signs of black pepper-like spores on stubble, and if you cut open the root crown you will find a black mass of infected root material.

3. Although virtually all canola varieties developed over the past 20 years carry an MR to R (moderate to resistant) rating against the disease, it is important to rotate the genetic resistance being used in one variety with a variety that carries a different type of genetic resistance. Using different forms of genetic resistance reduces the risk of the disease pathogen developing resistance to the control measures. One approach used in Australia, for example, has producers growing canola varieties with a particular genetic resistance to blackleg for two or three seasons, then retiring that resistance package for 10 years before reintroducing it again. It helps to increase the longevity or durability of genetic resistance.

About the author

Field Editor

Lee Hart

Lee Hart is editor of Cattleman’s Corner based in Calgary.

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