Don’t be silly, discover the verticillium

On August 10, 2020, Brad Crammond noticed sudden, premature mortality in a seemingly healthy canola field. “We’ve had issues with blackleg in the past,” says the Austin, Man., farmer, “and I could tell it was something different.”

So he called Canola Council of Canada agronomist Angela Brackenreed to take a look. “The symptoms were advanced verticillium stripes,” says Brackenreed. “It was pretty obvious.” Samples sent to the PSI laboratory in Manitoba confirmed the diagnosis.

The disease, caused by the pathogen Verticillium longisporum, is relatively new to canola in Canada. It was first detected in Manitoba in 2014. In 2015, Canadian Food Inspection Agency (CFIA) investigators found the pathogen in six provinces, including all three Prairie provinces. In 2020, the incidence rate in some fields was high enough to cause yield loss. For the 2021 Manitoba Disease Survey, the first year that surveyors had an effective system for including verticillium wilt, 30% of fields had the disease. In these fields, 15% of the plants, on average, showed symptoms.

Verticillium streak, unlike other diseases, can be more severe in dry conditions, which could explain why the disease has reached new heights in 2021. Crammond, experienced with the symptoms, also saw verticillium streak last season.

“Now that we know what we’re looking for, we find it all over the Prairies,” says Brackenreed.

Early infection can show up as diseased gray or tan stripes down the half of the stem – hence the name “stripe”. Similarly, infected leaves may be dying on one side and healthy on the other.

As the verticillium infection progresses, the epidermis peels away from the weakened stems to reveal tiny spots called microsclerotia underneath. These spots may look similar to blackleg pycnidia, but verticillium spots are much smaller. Verticillium stripe, like blackleg, also causes darkening of the cross section of the stem. While blackleg causes distinct black wedge shapes, verticillium discoloration is diffuse throughout the cross section and continually darkens as microsclerotia accumulate. Verticillium infection can extend up the stem, while blackleg is concentrated in the crown around ground level. Eventually, verticillium infection blocks water and nutrient transfer, weakening the stem and killing the plant.

“Because we’re letting the canola sit longer, either for straight harvesting or later swathing, we may be noticing the disease more than before,” says Brackenreed. “Verticillium infection can also continue to progress after canola plants are swathed, but people don’t usually watch the swaths that closely.

In 2020, Crammond had stems that broke and toppled over, making her look like a case of severe lodging from afar. “We had high winds in August of that year and a lot of the Twitter chatter was about the crop dropping and making it difficult to harvest,” he says. “In retrospect, the cause of many of these cases may have been a verticillium band.”

Differences between cultivars

Crammond knows the disease has reduced yields on his farm, but Canada does not yet have a system for estimating the amount of losses.

In Europe, which also has a verticillium wilt problem in oilseed rape (Brassica napus), a 2008 study by Dunker et al. Yield loss estimated between 10 and 50%. In 2016 Jasper Depotter, a plant pathology researcher at the University of Cologne, published the results of field trials in the UK. Depotter’s study showed yield loss as high as 34%, with differences between cultivars. Based on this, he says, “in a bad year on a susceptible cultivar, Dunker’s estimates seem realistic.”

Genetic differences also appear in Canadian research. Dilantha Fernando, a professor in the Department of Plant Science at the University of Manitoba, tested germplasm from international sources and dozens of lines provided by Canadian seed companies. He says some have significantly higher levels of resistance, but the lines provided to him have not been identified, so he does not know if any were commercial cultivars.

In Saskatoon, Agriculture and Agri-Food Canada researcher Hossein Borhan is examining 50 Brassica napus lines as part of AAFC’s Nested Association Mapping (NAM) program to compare their resistance to verticillium scratching. Some lines are very resistant, others are very sensitive, he says.

Crammond’s on-farm experience shows differences between cultivars. In 2021, he ran out of seed in one field and finished the last eight acres with a different cultivar. While most of the field was in danger of being destroyed due to high levels of verticillium wilt, the eight acres of a different variety had “no problem,” he says.

Rotation helps

No fungicide solution is available at this time. Genetic resistance will likely provide the highest level of protection, with effective crop rotation support.

While researchers don’t know how many years between canola crops are needed to provide effective risk reduction, Crammond’s experience shows that rotation can work. One of his canola fields in 2020 was on a half section with a mixed crop history. Eighty acres in the middle had quinoa, soybeans and wheat in the previous five years – no canola. The others had been in a wheat-canola rotation for “quite a while,” he says. While the rest of the field turned brown prematurely due to verticillium wilt, the 80 acres with a longer break between canola crops remained green and healthy. It was the same cultivar.

“The increase in cases of verticillium stripe makes crop rotation even more important, in my opinion,” says Crammond.

“Rotation has so many other benefits, so it’s worth it,” says Brackenreed. Some evidence suggests that incorporating stubble into tillage may also increase spore longevity, she adds, so minimal tillage may provide additional help.

Brackenreed recommends canola growers across the Prairies monitor for verticillium wilt in 2022. “Learn how to identify the disease so you know what you’re dealing with,” she says. “And because there seems to be a genetic component, be sure to compare results from different cultivars. Although we do not currently know which cultivars have increased resistance, the genetic diversity resulting from the use of different cultivars could cushion the risk.

-Jay Whetter is Manager of Communications at the Canola Council of Canada.

Yixiao Wang, a doctoral student in applied plant pathology at the University of Alberta, compared cross-sections of stems infected with verticillium wilt to those of blackleg. With the verticillium, the darkening may extend up the stem (above) while the darkening of the blackleg stem is concentrated around ground level. By splitting the infected stems lengthwise, Wang found that she could tell the two diseases apart based on the upward extent of stem darkening.

Photo:
Yixiao Wang

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