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Canola foes the same — but different

Blackleg and verticillium stripe can be confusingly similar

Blackleg and verticillium stripe are sometimes even found in the same plant. 

Is it verticillium stripe or is it blackleg?

That’s a question canola growers may find themselves asking more in the future, as verticillium stripe (VS) gains a stronger foothold on North American farms, a recent webinar hosted by the Prairie provinces’ canola associations heard.

It was first discovered in Manitoba in 2014 and is caused by the fungal species verticillium longisporum. The pathogen has a longer history in Europe, dating back to the 1960s in Sweden and subsequently spreading through the rest of north-central Europe.

Justine Cornelsen, an agronomy specialist with the Canola Council of Canada said the first step in managing the problem is going to be getting out and scouting the field and identifying it.

“You’ve got to get into your fields and know what you’re working with,” she said.

Left to right: canola infected by verticillium stripe, canola infected by blackleg, and uninfected canola. photo: Canola Council of Canada

The VS infections “snuck up on” the canola sector, she said, due to undiagnosis or misdiagnosis as it’s been confused with blackleg. Some of the symptoms can be similar and the two pathogens can even at times be found in the same plant at the same time. It also has similar symptoms to fusarium wilt (another fungal infection). And because the symptoms typically occur later in the season, it may get missed entirely.

“We’re not going out in the field and looking for that old residue and that’s where verticillium is very obvious,” she said. So, what should you look for?

“What we are seeing as an early symptom was half-stem senescing: half of the stem still being green, healthy and mature and the other half starting to prematurely ripen.”

Another cue is that a cross-section of the stem has a greyish hue to it. Later in the season, the microsclerotia begin to appear.

“That is the real key diagnostic feature when we are looking for verticillium,” she said.

For blackleg, the best time to scout is just prior to swathing or around that 60 per cent seed colour change.

“When the plant starts to die off, it becomes very confusing to know what you’re working with,” said Cornelsen.

“Ideally, you pull up plants that well represent the field,” she said. For best results, you need to snip through the root tissue right at the base of the stem.

Equally important in managing these diseases is crop rotation. Research has shown that it is possible that verticillium lasts in the soil for 10 to 15 years. So, planting canola every year is just going to increase the level of infection year after year. Crop rotation is important for both verticillium and blackleg.

Because there is more known about blackleg, the best management practices are better defined than for verticillium stripe. So beyond scouting and crop rotation, there are some standard practices that are well established, such as choosing the hybrid with the right resistance.

Canola plant infected with blackleg. photo: Canola Council of Canada

Foliar fungicide applications have been available for a long time but aren’t commonly used because the treatment is preventive and has to be done very early (two- to four-leaf stage). It also provides no benefit if your resistant hybrid is functioning.

“If it’s not acting like a resistant hybrid, if a major gene is being overcome, that’s where an application pays off,” Cornelsen said.

All canola seed is treated with a fungicide that acts to prevent transference of blackleg within hybrid seed bags or from the field, but until now, seed treatments haven’t been available that provide in-field protection. “This will be the first year where canola growers can access or buy a seed treatment to protect the plants when they’re growing,” said Cornelsen.

Because VS is so new to Canada, beyond scouting and crop rotation, management practices are not well defined.

“We’ve taken a lot of work and research out of Europe but even there, they are still struggling to manage this particular disease,” said Cornelsen. However, it can help to control the brassica weeds and canola volunteers.

“If you can get rid of the hosts, you’re not going to allow that inoculant to build.”

One of the challenges for managing the disease is because it is a soil-borne pathogen that can survive in the soil for 10 to 15 years. However, steps can be taken to help mitigate the spread of the disease. These steps include equipment sanitation and developing an on-farm biosecurity plan.

Sheau-Fang Hwang of the University of Alberta also spoke to the webinar, detailing current research into VS, both in the greenhouse and field.

Because it’s a new disease in Canada, little is known about yield loss, but European estimates range between 10 to 50 per cent yield loss.

To help strengthen the knowledge base of the disease in Canada, a ‘Verticillium Stripe Management’ project was established through the Canola Agronomic Research Program (CARP) in 2019. The final results of the four-year study won’t be available until 2023, but Hwang discussed some of the early results in her presentation.

In 2020, Hwang’s team was involved in field testing to evaluate VS severity and impacts under different inoculum levels.

The results of those field tests indicate that VS:

  • Reduces seed germination rate;
  • Reduces plant height; and that
  • Disease severity increased as inoculums level increased.

“Yield losses were observed in all the treatments,” she said.

Hwang was also involved in three greenhouse studies. The first compared the effects of verticillium longsporium with that of verticillium dahliae (another species of verticillium fungus). These experiments determined that VL was significantly more virulent. In the second greenhouse study higher mortality rates were observed when plants were inoculated with higher concentrations of the disease. And the third greenhouse study looked at the timing of when the pathogen was introduced. That study discovered disease symptoms increased, plant height decreased and plant mortality increased when plants were inoculated at three weeks, compared to plants inoculated at one week.

Dilantha Fernando of the University of Manitoba also made a presentation on his work on blackleg.

First detected in northeastern Saskatchewan in 1975, it’s a widespread canola disease throughout the Prairies, and could be a blueprint for how to combat the looming VS threat.

In 2011, Fernando was involved in the first gene study of blackleg to determine what resistant genes (r-genes) were available in canola.

“We were able to successfully show that there are several r-genes that are available, but the predominant one was the Rlm3 gene,” said Fernando.

Further research determined that this particular gene had become quite susceptible to blackleg infection.

The work that Fernando and his team did back in 2011 resulted in the establishment of r-gene labelling for canola seeds which now plays an important role in the management of the disease.

Hybrids are now labelled as R (resistant), MR (mostly resistant), MS (mostly susceptible) and S (susceptible).

The blackleg research Fernando is currently doing is drilling down deeper into the r-gene ratings through a systematic r-gene rotation study.

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