“Quick Dive: Understanding Verticillium Stripe!” | (A crash course on verticillium stripe)

Verticillium stripe of canola is causing yield loss in Manitoba and probably other areas of the Prairies. And it’s getting worse.

The disease, caused by the Verticillium longisporum pathogen, was first reported in Manitoba in 2014. In 2015, Canadian Food Inspection Agency surveyors found the pathogen in six provinces, including all three Prairie provinces.

The 2021 Manitoba disease survey marked the first year that surveyors had the protocols to accurately identify verticillium stripe. (More on accurate identification later.) In that year, 30 per cent of Manitoba fields had the disease. This is “prevalence.” And, on average, 15 per cent of plants in those fields had obvious symptoms. This is “incidence.”

A year later, Manitoba prevalence rose to 38 per cent and incidence to 23 per cent. These numbers come from the Western Forum on Pest Management oilseed diseases report, based on 116 Manitoba fields surveyed. A 2022 survey, targeted at eastern Saskatchewan areas deemed more likely to have the disease, found symptoms in many fields. Alberta numbers in 2022 were very low.

Verticillium stripe levels were lower in Manitoba in 2023, with prevalence of 29 per cent and incidence of 11 per cent. The forum’s 2023 report notes that the Saskatchewan Ministry of Agriculture ran a verticillium-specific survey targeting 100 fields.

“Preliminary observations seem to suggest that verticillium stripe is no longer a rare disease in Saskatchewan,” the report reads. Alberta surveyors suspected verticillium stripe in three per cent of fields surveyed.

David Kaminski, Manitoba Agriculture field crop pathologist, in talking to reporter Richard Kamchen for a Canola Digest article in 2023, says he’s not sure why Manitoba levels are higher than the rest of the Prairies.

“Clubroot originated in Alberta and made its way east, and some people say blackleg originated in Saskatchewan and made its way east and west, and in that kind of analogy, we could say that verticillium has been more prevalent in Manitoba and seems to be moving westward,” Kaminski says. “But we don’t have full documentation on that.”

The bottom line is that the disease is likely here to stay. Agronomists need to know how to recognize it and make science-driven recommendations for management.

What is verticillium stripe?

The fungal species Verticillium longisporum causes verticillium stripe, a plant disease that leads to stem striping, blockage of the xylem and stunted seed fill in canola.

The Canola Encyclopedia chapter on verticillium stripe cites a study by Inderbitzin et al, printed in PLoS ONE in 2011, showing that V. longisporum is a diploid hybrid of three species, including V. dahliae.

V. dahliae is present in Canada as a pathogen of potato, tomato, sunflower, strawberry, cabbage and maple. Based upon genetic analysis, the researchers showed that V. longisporum originated independently at three different times, involving four different lineages and three different parental species.

Each V. longisporum lineage was genetically homogenous and contained one set of alleles that were identical across lineages. This has a potential effect on screening for resistance genes in Brassica napus and other related species and implications for disease management.

A more recent Canadian study, Verticillium disease etiology and nursery, with principal investigator Dilantha Fernando from the University of Manitoba, looked into the genetic diversity and behaviour of V. longisporum in Western Canada.

Fernando, writing in his final report, noted that “molecular DNA studies of V. longisporum indicated that the pathogen is a diploid hybrid of V. dahliae and two unknown species forming three lineages A1/D1, A1/D2 and A1/D3. V. longisporum lineage A1/D1 is considered the most virulent group across Brassicaceae crops, especially canola, followed by A1/D3 and A1/D2.”

Fernando and his team tested infected canola samples from different locations in Manitoba, Ontario and Saskatchewan. All 14 samples from Manitoba were confirmed as A1/D1, the most virulent group. All three samples from Ontario were confirmed as V. dahliae. And out of six samples collected from Saskatchewan, one each was confirmed as A1/D1, A1/D2 and A1/D3, and the other three were V. dahliae.

Disease cycle

The soil-borne fungus infects roots and travels up the water-transporting xylem in the stem. It will eventually plug the xylem, cutting off the flow of nutrients.

The Canola Encyclopedia provides a detailed description of the verticillium stripe disease cycle and is the source for content in this section of the article. Verticillium stripe is a monocyclic disease, meaning it only goes through one cycle each year. While it is hard to pinpoint a start point in a cycle, the encyclopedia starts with rapid germination of fungal propagules from microsclerotia in the soil. Host plant root exudates trigger this germination, and these propagules infect the canola roots at their most susceptible location — the fast-growing root tip.

The plant is most susceptible to V. longisporum infection when canola starts flowering, according to Zhou et. al. in a report published in Plant Pathology journal in 2006. Zhou found that the fungal hyphae entered the root vascular system through an opening, often a wound. After the hyphae multiply in the root, hyphae and single-cell spores called conidia are produced locally in the xylem and moved through the vascular system of the plant to multiply.

This prevents the regular flow and functionality of water and nutrients to the plant tissues and eventually causes the xylem to plug, turn black, collapse and shrivel, as described in Eastburn and Paul’s verticillium chapter in the 2007 Compendium of Brassica Diseases.

As the plant begins to mature and senesce, the pathogen moves from the xylem into the surrounding non-vascular tissues, where multi-cellular microsclerotia are formed in the dying tissue, as Zhou describes. Likewise, conidia production in the stem base and roots causes the tissues to turn dark grey, then black and can cause the lateral roots to eventually break down, making it easy to pull from the ground.

While the plant begins to show progressively more intense symptoms of the disease, the microsclerotia in the stems cause the stem epidermis (thin outer layer) to peel back. At this point the stem may take on a shredded appearance and the microsclerotia will be released to the soil, where they will rest until stimulated to germinate again. The microsclerotia remain viable in the soil for several years, especially if infected residue is incorporated into the soil after harvest.

Environmental influence

Verticillium stripe tends to be more damaging in hot, dry conditions. These stressful conditions, the Canola Encyclopedia notes, tend to reduce root and xylem function, while excess moisture makes the disease less of an issue. Plants with damaged roots also make it easier for the disease to enter the vascular system, note Heale and Karapapa in the Canadian Journal of Plant Pathology, 1999.

Verticillium stripe can use weedy host species such as wild mustard and others outside the Brassicaceae family as host species. The Canola Encyclopedia again cites Eastburn and Paul, who report that V. longisporum can affect several annual and perennial plant species in both temperate and subtropical zones.

The pathogen is also present in oilseed rape in Sweden, Germany, Poland, Ukraine, Russia, France, Czechia, Belgium, the Netherlands, the U.K. and Japan, and can infect many crops including broccoli, cabbage, cauliflower, horseradish, radish and wild mustard.

Symptoms

Infection can occur in a few scattered plants, in small areas or across the whole field. Symptoms are not typically noticed until plants are near maturity.

The Canola Encyclopedia provides a detailed description of symptoms:

• Verticillium stripe’s restriction of water and nutrient uptake leads to stunting and premature senescence.
• Faint black striping on the stems can appear darker and more obvious when rubbed.
• By peeling back the epidermis and outer cortex of the stem, observers can find blackening on the inside of the stem and microsclerotia later in the growing season.
• Striping can be more obvious closer to harvest.
• At the end of ripening, microsclerotia can germinate and produce conidia spores, giving the stems a powdery look.

I also wrote a 2022 Country Guide article with a few more details. Early infection can show up as grey or tan diseased stripes up one half of the stem — hence the “stripe” name. As verticillium infection progresses, the epidermis will peel away from weakened stems to reveal tiny specks called microsclerotia underneath. Eventually, verticillium infection blocks the transfer of water and nutrients, weakening the stem and killing the plant.

Canola Council of Canada staff note in the Country Guide article that, “because we’re letting canola stand longer, either for straight combining or later swathing, we may be noticing the disease more than we used to.”

Country Guide also included observations from Manitoba farmer Brad Crammond. In 2020, he had stems breaking off and toppling, making it look from a distance like a case of severe lodging.

“We had some big winds in August that year and a lot of talk on Twitter was about crops going down and making harvest difficult,” he says. “In retrospect, the cause for many of these cases may have been verticillium stripe.”

Symptoms may be hard to notice in less severe cases, especially since mild cases will look like regular senescence. The encyclopedia adds that hail or physical damage can also cause lesions similar to verticillium stripe, but these lesions will not have microsclerotia beneath the epidermis.

Because symptoms continue to develop right up to harvest, the late season is a good time to scout for verticillium stripe. That is when scouters are most likely to see the bleached and brittle stems, the peeling epidermis and the microsclerotia underneath that peeled skin. At that time, stem cross sections will also show a starburst-like pattern as conidia build up.

The bigger challenge, perhaps, is to distinguish verticillium stripe from other common diseases, especially blackleg and sclerotinia stem rot.

Other diseases

This section describes how to distinguish verticillium stripe from other common canola diseases. Below are the four major symptoms of verticillium stripe, each with similarities to symptoms of blackleg or sclerotinia stem rot.

Stem striping. When the crop is full height but still green, canola plants infected with verticillium stripe will often have a two-toned stem — half healthy and green and half discoloured and drying down. This half-stem senescing is where the “stripe” name comes from.

You will not see half-stem senescing with blackleg or sclerotinia stem rot. Sclerotinia will cause stem discolouration, but it will not stripe half the stem. The only other disease to cause similar symptoms is fusarium wilt, but current canola cultivars all have resistance to that pathogen.

Stem cross-section discolouration. Verticillium stripe infects roots and enters the plant’s vascular system. Verticillium hyphae and conidia fill up the vascular system, restricting the passage of water and nutrients throughout the plant. This gives the stem cross-section a greyish colour that is easily confused with blackleg.

There are two tips to distinguish the pathogens:

• With blackleg, stem tissue infection tends to be darker and causes distinct black wedge shapes. Verticillium is a lighter grey, more general throughout the cross-section and can present in more of a starburst pattern.
• Blackleg stem discolouration is confined to the crown area at the base of the stem. Verticillium darkening can extend well up the stem.

“Verticillium will present some general discolouration, generally in a starburst-like pattern. Blackleg will present itself in a distinct solid black wedge covering a percentage of the stem,” says Canola Council of Canada agronomy specialist Courtney Ross.

Stem peeling and weakening. Peeling stem skin is a symptom of verticillium stripe. Under that peeled outer layer will be the microsclerotia, often taking the shape of faint black vertical striping. Severely diseased stems may break off and can be confused with lodging. Sclerotinia stem rot will also cause weakened, brittle stems, but sclerotinia will not have the stripy, speckly microsclerotia. Sclerotinia stem rot will cause the entire stem tissue to shred, not just the outer layer.

“The shredding of the stem symptom is comparable to sclerotinia. However, the hollowing of the stem and larger sclerotia bodies from sclerotinia are different from the smaller microsclerotia present from verticillium stripe,” Ross says.

Black specks. As verticillium infection advances, microsclerotia will form on the underside of peeling stem skin. These can be found all the way up the stem. Verticillium specks may seem similar to blackleg pycnidia, but they’re much smaller — more like powdery pepper. In some cases, blackleg pycnidia will have a purple-pinkish ooze of pycnidiospores around them.

Blackleg pycnidia are also confined to a lesion no more than a couple of centimetres in size. If you see pink and specks confined to a lesion, it’s blackleg.

Yield loss

If infected plants ripen prematurely, they can show significant yield reduction.

Verticillium longisporum could cause significant yield damage in canola, but only when disease severity and infection are high, states the Canola Encyclopedia, citing Heale and Karapapa. As disease onset is late in the growing season, verticillium stripe is less damaging than other diseases such as blackleg or sclerotinia stem rot.

However, Dunker et al, in a 2008 article in the Journal of Phytopathology, reported on European research showing significant rapeseed yield loss when infection establishes early. They report yield loss in this situation at 10 to 50 per cent. A big factor in yield loss is smaller seed size.

In 2016, Jasper Depotter, a plant pathology researcher at the University of Cologne, published results from field trials in the United Kingdom. His study showed yield loss as high as 34 per cent, with differences among cultivars.

“In a bad year on a susceptible cultivar, the estimates of Dunker seem realistic,” Depotter was quoted as saying in the Country Guide article.

There is no rating scale for the severity of verticillium stripe, and thus no way to quantify yield loss. Ross notes that research is underway to uncover yield implications in Western Canada.

In Country Guide, Crammond says he noticed, on Aug. 10, 2020, some sudden and premature die-off in a seemingly healthy field of canola.

“We’ve had issues with blackleg in the past and I could tell this was something different,” said the farmer from Austin, Man.

So, he called his Canola Council of Canada agronomy specialist to take a look. Samples sent to Manitoba’s PSI Lab confirmed the diagnosis: advanced verticillium stripe.

Unlike other diseases, it can be more severe in dry conditions, which could explain why the disease reached new heights in 2021.

Management

Scouting. Accurate identification will be easier with experience. Verticillium stripe is easiest to scout just before or just after harvest when symptoms are most obvious. Accurate identification is an important step in disease management.

Blackleg and sclerotinia stem rot, if those are present, are more manageable through genetic resistance, crop rotation and fungicides. Verticillium stripe has few proven management steps.

Test plant tissue. If scouters find symptoms that look like verticillium stripe, they could use lab tests for confirmation. Labs in Canada that provide this service include:

Contact labs for their sampling protocols.

Reduce soil movement. Verticillium microsclerotia are soil-borne, so steps to keep soil in place could reduce spread somewhat. The outlines a few biosecurity measures. As V. longisporum is a soil-borne pathogen, biosecurity practices can help mitigate the spread of this disease, on- and off-farm. These include equipment and tool sanitation, controlling off-farm traffic, monitoring seed, feed and fertilizer sources, and developing an on-farm biosecurity plan.

Extend the break between canola crops. Two- or three-year breaks between canola crops are good disease management in general. However, verticillium microsclerotia can persist 10 to 15 years in the soil, as Agriculture and Agri-Food Canada research scientist Hossein Borhan notes in a Canola Research Hub report.

Despite that persistence, Crammond’s on-farm experience, as described in Country Guide, shows that rotation could work. One of his canola fields in 2020 was on a half section with a mixed cropping history. Eighty acres in the middle had quinoa, soybeans and wheat over the previous five years and no canola.

The rest had been in a wheat-canola rotation for “quite some time,” he says. While the rest of the field turned brown prematurely due to verticillium stripe, the 80 acres with a longer break between canola crops stayed green and healthy. It was all the same cultivar.

The Canola Encyclopedia notes that in northern Europe, where this disease has been an important issue for more than 30 years, researchers recommend that growers leave three years between canola crops. This allows pathogen populations in the soil to naturally decline.

But, due to the long-lived microsclerotia, rotation alone is not enough to manage this problem effectively.

Ask about genetic resistance. Seed companies, if asked, might be able to shed some light on differences. Crammond’s on-farm experience shows differences between cultivars do exist. In 2021, he ran out of seed in one field and finished the final eight acres with a different cultivar. While most of the field was at risk of shelling out due to high levels of verticillium stripe, the eight acres of a different variety had no issues, he says.

Dilantha Fernando, a professor in the department of plant science at the University of Manitoba, tested germplasm from international sources and dozens of lines supplied by Canadian seed companies. He says some have superior levels of resistance, but lines supplied to him were not identified so he doesn’t know if any were commercial cultivars.

Hossein Borhan leads a new study that will provide an understanding of genetic resistance to verticillium stripe. He and other researchers on the project identified two locations within B. napus DNA that seem to convey resistance to V. longisporum. Borhan also screened 50 B. napus lines within the AAFC nested association mapping program to compare their verticillium stripe resistance. Some lines are very resistant and some are very susceptible, he says.

Papers from Eynck et. al., in the Journal of Plant Diseases and Protection, 2009, as well as Heale and Karapapa, show that other Brassica species, namely B. carinata and B. oleraceae, have low susceptibility to verticillium and show almost no resulting yield loss. This adds further evidence to the potential for breeding solutions to manage this disease.

No treatments. No fungicide or soil amendment is known to be effective on verticillium stripe. As part of Borhan’s study, researchers found a microorganism that could potentially protect against V. longisporum. If proven to work and if it can be effectively mass-produced and applied to the soil or roots, it could provide growers with a biological control product.

Manage blackleg. Alberta research shows an interaction between blackleg and verticillium stripe pathogens. So, while growers have limited tools to manage verticillium stripe at this time, steps to manage blackleg could ultimately reduce yield loss from verticillium stripe.

University of Alberta researchers Yixiao Wang, Stephen Strelkov and Sheau-Fang Hwang, in an article published in the journal Plants in 2023, report on their assessment of L. maculans/V. longisporum interactions under field and greenhouse conditions.

Their conclusion, as written in the article: “When L. maculans was co-inoculated with V. longisporum, blackleg severity and yield losses increased. In some cases, verticillium stripe caused greater yield losses than blackleg. The results suggest that the interaction between L. maculans/V. longisporum may cause more severe losses in canola, highlighting the need for proactive disease management strategies.”

While many treatments could potentially work, all we know for certain is that there seems to be a connection to blackleg. Verticillium stripe has few proven management steps.

For now, look for the disease. If identified and if it’s causing yield loss, consider, at a minimum, the time-honoured disease management strategy — longer breaks between canola crops.

Verticillium stripe is harming canola crops in Manitoba and likely in other Prairie regions, and the problem is worsening.

The disease is caused by a pathogen known as Verticillium longisporum, which was first detected in Manitoba in 2014. By 2015, surveyors from the Canadian Food Inspection Agency found it in six provinces, including all three Prairie provinces.

In 2021, the Manitoba disease survey revealed that 30% of surveyed fields had verticillium stripe, with an average of 15% of the plants in those fields showing symptoms. The following year, these numbers climbed to 38% prevalence and 23% incidence. The data came from the Western Forum on Pest Management.

In 2023, the levels dropped slightly to 29% prevalence and 11% incidence in Manitoba. However, an ongoing survey in Saskatchewan indicates the disease is no longer rare there. In Alberta, the incidence remains low at around 3%.

David Kaminski, a field crop pathologist, noted in an interview that while Manitoba has higher levels of the disease, it’s unclear why it hasn’t spread as much to other parts of the Prairies.

The key takeaway is that this disease is likely here to stay. Agronomists need to be informed about how to identify it and to provide science-based management advice.

What is verticillium stripe?

Verticillium stripe is a plant disease caused by the fungus Verticillium longisporum. It leads to symptoms like striping on the stems, blockage of water transport, and poor seed development in canola crops.

According to the Canola Encyclopedia, V. longisporum is a hybrid species related to another pathogen called V. dahliae, which affects various crops such as potatoes and tomatoes.

Recent studies have shown that V. longisporum has three lineages, with one being particularly harmful to canola. These studies confirmed the presence of these strains in plants from Manitoba, Ontario, and Saskatchewan.

Disease Cycle

The fungus starts by infecting the roots of the plant, then travels up the stem, blocking water flow. It goes through a single cycle each year. The germination of the fungus in the soil is triggered by substances released from the roots of the canola plants.

Plants are most vulnerable to infection when they flower. The fungus enters through wounds in the roots and can eventually cause the plant’s tissues to turn black and shrivel. As the plant matures, the pathogen spreads and produces tiny spores.

Environmental Influence

Hot and dry conditions can worsen the effects of verticillium stripe, while wet conditions are less favorable for the disease. Additionally, weeds can serve as hosts for the fungus.

Symptoms

Symptoms usually appear close to harvest. Key signs of verticillium stripe include:

• Stunted growth and early aging of plants.
• Black striping on stems, which becomes more visible with rubbing.
• Peeling back the stem may reveal dark discoloration inside.
• Closer to harvest, the disease can produce a powdery appearance on stems.

Symptoms can be confused with other plant diseases, such as blackleg and sclerotinia stem rot. Proper scouting before and after harvest is essential for accurate identification.

Other Diseases

It’s crucial to distinguish verticillium stripe from diseases like blackleg and sclerotinia stem rot. Symptoms include:

Stem striping. Infected plants often have a green part and a discolored part on the stem, but blackleg or sclerotinia will not show this half-senescence.

Stem cross-section discoloration. The cross-section of a verticillium-infected stem is typically grey compared to the darker wedge shapes seen in blackleg.

Stem peeling. Peeling of the outer stem layer reveals tiny black specks, which can be confused with blackleg but are different in appearance.

Yield Loss

If plants mature too early, the yield can significantly drop. Verticillium’s impact on yield is generally lower than other diseases but can still be severe, especially when infection hits early in the season.

Studies in Europe have shown significant yield losses during early infections, and Canadian farmers are beginning to notice similar issues.

Management

Scouting. It is essential to check for symptoms, especially close to harvest, for accurate diagnosis.

Testing plant tissue. If verticillium symptoms are found, lab tests can confirm the presence of the disease.

Reduce soil movement. This helps prevent the spread of the pathogen.

Extend breaks between canola crops. Rotating crops can help manage disease, with a recommended break of three years.

Ask about genetic resistance. Some canola varieties may have better resistance to verticillium stripe.

No known treatments. Currently, no fungicides or soil amendments are effective against the disease, but research is ongoing.

Manage blackleg. Since blackleg and verticillium stripe can interact, managing one may help with the other.

In conclusion, scouting for the disease and understanding its management strategies is crucial for minimizing loss in canola production.