Early scouting urged for blackleg

However, it can sometimes be difficult for growers to tell the difference between blackleg and root rot in June and July


Canola growers and agronomists often look for blackleg in August or September because it’s a good time to spot signs of the fungal disease.

However, Justine Cornelson, an agronomist with the Canola Council of Canada, said growers should also scout for blackleg earlier in the season.

“If you’re only scouting for blackleg prior to harvest, you don’t know of all the plants you potentially lost earlier on in the year. They’ve dried out and are gone,” she said earlier this spring at a CanoLab session in Dauphin, Man.

“That’s why I encourage growers to scout all throughout the year.”

Blackleg is a top of mind disease for Canada’s canola industry.

For much of 2016 China threatened to shut the door to canola imports from Canada over concerns that traces of blackleg could contaminate the Chinese rapeseed crop. China was concerned about dockage, which is the amount of foreign material shipped with canola seed.

In September the two countries reached a three-year agreement on the blackleg issue, but Chinese officials are still concerned about blackleg levels in Canada.

“One way to mitigate risk to China is to start focusing on lowering levels of blackleg in the field,” Curtis Rempel, vice-president crop production and innovation with the canola council, said last fall.

“While this is good for stable and open trade (with) China, I would submit … it’s also good for us. It increases our yield, it increases our profitability.”

As part of that effort, it’s important to recognize the symptoms of blackleg early in the growing season so that growers get a sense of the problem on their farm.

However, distinguishing blackleg from other diseases isn’t easy in June and July.

“There’s a lot of confusion with other diseases: root rots, foot rots,” Cornelson said.

At the meeting in Dauphin, Cornelson showed samples of infected plants to growers and ag industry reps. She held up two plants: one with blackleg and the other infected with rhizoctonia solani, one of the soil-borne pathogens that causes root rot.

She explained that blackleg starts on the leaves of canola and works its way down the plant.

Once in the root, it can cause a pinching at the stem.

“With blackleg, you’re going to see that constriction, which is very similar to root rot,” Cornelson said.

“But if you were to cut that root and that stem right at the base of the plant, you’re going to see that blackened tissue.”

Root rot, in contrast, starts in the soil and moves up the plant. The disease makes the root soft and mushy, while blackleg dries out the root from the inside out.

“The blackleg plant, it’s dried out. It’s woody,” Cornelson said. “That’s one of the major ways I like to diagnose the difference between the two, is that woodiness in the plant.”

There are other early symptoms for blackleg and root rot.

  • Lesions can be grayish with a white centre.
  • Black pycnidia, or black specks, are found on the lesion.
  • With early infection on cotyledons, lesions will end up spreading and drying the cotyledon completely down.
  • Lesions vary in colour but typically range from gray to brown.
  • Lesions can have irregular shapes.
  • Decay and rot of roots moves up the stem and further down the root.
  • Girdling is found at the base of the stem.
  • Another key for diagnosis is that blackleg flourishes in warm, moist conditions, such as 16 to 20 C.
  • Root rot typically prefers cool or cold conditions.

Growers who can’t tell if the disease is blackleg or root rot can send plant samples to a provincial lab.

Cornelson said knowing the type of disease is important, but it’s probably more critical for upcoming crops.

“Most diseases have limited management options, in-crop,” she said. “(It’s) something to note and better manage before putting in future canola crops.”

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