Blackleg battle requires longer canola rotations

BRANDON — Rotation is the easiest way to control blackleg, and if two types of rotation are employed, farmers can squeeze the disease.

One type of rotation is easy, said blackleg management specialist Justine Cornelsen of the Canola Council of Canada, while the other isn’t as obvious or simple.

But with an integrated approach, blackleg’s growing incidence in western Canadian fields can be suppressed.

“We really need to focus-in on the basics,” said Cornelsen in a presentation to the Manitoba Canola Growers Association.

“These new tools come into play when we’re not going the basics really well.”

The new tools include race-specific resistance, which farmers can employ to fight the particular strains in their fields, identified by lab tests, and then managed inside a resistance rotation system.

The basics include crop rotation, the age-old disease management technique that is still the single best defence against blackleg, a disease Cornelsen described as the “silent yield robber.”

Rotation of more than a one-year gap is essential because blackleg spore counts not only don’t fall after a year, they actually get worse.

“When we’re on tighter canola rotations, we don’t allow that stubble to break down naturally within the environment, so the disease is able to infect canola two years down the road when that new canola crop is there.”

Blackleg spreads by spores, and the population of spores actually peaks about 18 months after the disease has infected a crop, continuing to grow in the canola stubble long after a crop has been harvested. If a farmer is just following a two-year rotation, with a canola-wheat or canola-soybean rotation, the one-year gap doesn’t control the problem.

“That (18 months peak) pretty well puts us at the spring of seeding our new canola crop…. This is why we’re seeing a bit of a buildup,” said Cornelsen.

“We have to let the stubble in the field naturally break down.”

Blackleg is a widespread, chronic but not devastating disease today due to the introduction of resistant varieties of canola. However, its incidence in fields is rising, new races are appearing and developing, and its continued presence across most canola acres is a threat to Canada’s access to the crucial Chinese market, Cornelsen said.

Farmers can suppress blackleg most easily through a three -or four-year rotation, scouting for possible infections, and the use of resistant varieties. That combination of practices can do most of the blackleg-control job.

But if farmers end up with more serious problems, there are new and innovative tools and methods they can turn to.

“They’re things you use when you haven’t been doing the first things properly,” said Cornelsen.

Rotating resistant varieties helps avoid the disease itself becoming resistant to variety resistance, and can control specific races of blackleg.

Since the racial profile of each field is different, farmers needing better control can test their infected canola to find which types of blackleg are in their fields.

They can then grow canola varieties that have the best resistance to those types of blackleg.

Many seed companies label their canola seed to reveal the nature of its blackleg resistance and all seed companies should be able to tell a farmer what kinds of resistance their varieties hold, Cornelsen said.

Fungicides don’t appear to pay for themselves, since they are applied too late to stop the disease infecting the crop at the cotyledon stage.

Did you know?

  • Canola plants are susceptible to blackleg infection throughout the life cycle.
  • If basal infection begins early, stem cankers may appear from flowering onward. As the season progresses, cankers penetrate, deepen and may girdle stem bases, often completely severing the plant.

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