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This article outlines what we know (and mostly don’t know) about potential verticillium stripe management. In most situations, we don’t have Prairies-based research to confirm the practice or make specific recommendations.
The first known case of verticillium stripe on the Canadian Prairies was in Manitoba in 2014. The soil-borne disease, caused by the pathogen Verticillium longisporum, has been a long-term problem for rapeseed crops in Europe. The disease blocks nutrient flow within the plant, leading to early senescence and yield loss. Incidence and severity seem to be accelerating quickly in Manitoba and across the Prairies.
Research will start to answer vital questions about verticillium stripe management in Canada. We have a lot to learn about its biology, yield impact and management practices.
Genetic resistance
Current situation for Western Canada: We need an assessment system to test resistance levels. With this, breeders can accurately compare verticillium tolerance in current and future canola cultivars.
Cultivar resistance is a cornerstone of canola disease management. No commercial cultivars claim resistance, but some do claim higher tolerance. To screen for resistance, we need a standardized disease severity scale utilizing standardized resistant and susceptible check varieties. University of Alberta researchers Sheau-Fang Hwang and Stephen Strelkov, in a study concluded in 2024, proposed a 0-6 assessment scale to evaluate disease severity in canola seedlings, and a 0-4 scale for mature plants. This could distinguish between different types of quantitative resistance that may be active at different times.
No known major-gene or qualitative resistance exists for verticillium stripe. (Qualitative resistance comes from a major gene or genes and provides a clear benefit when it matches the pathogen race present. Quantitative resistance can come from many genes and is often considered a baseline of protection from major losses.)
Disease assessment at different growth stages also provides a measure of disease progression. Canola breeders could use the scale to evaluate the disease in canola breeding lines. We need more research to understand how disease severity scores correlate to actual yield loss in the field.
Current commercial cultivars likely have differences in verticillium stripe tolerance because pre-commercial canola germplasm does show large differences. Harmeet Singh Chawla, a verticillium stripe researcher at the University of Manitoba, has screened breeding lines. He confirms increased levels of resistance in some.
Current cultivars
Growers could ask seed companies about their most tolerant cultivars. Some seed companies may have an idea which of their cultivars show increased tolerance. Growers could split a field with two cultivars to compare disease infection on the plants, timing of infection and yield. However, multiple sites and years will be required to evaluate the resistance. Varying environmental conditions from year to year can have an enormous impact on the expression of resistance in the field.
In Europe, Verticillium longisporum, the most pathogenic verticillium for brassica crops like canola, has been known for decades. Quantitative resistance has basically nullified the yield loss from V. longisporum in napus rapeseed on that continent. “In Germany, the issue has somehow become calmer as no further spread of the disease is noticeable and damage appears to be manageable,” says Andreas von Tiedemann, plant pathologist and professor from Georg-August University Göttingen in Germany. Even though genetic resistance is “only partial and quantitative” it has steadily increased over the past two decades, he says.
In Canada, Dilantha Fernando at the University of Manitoba is investigating the genetics of quantitative resistance to V. longisporum.
Pathotype diversity
Within a pathogen species, scientists often find different “pathotypes” with greater levels of virulence or different interactions with the host. As the resistance discussion matures, we will learn how or if genetic diversity among verticillium pathotypes on the Prairies affects the performance of resistance traits.
As we know, diversity of blackleg and clubroot pathotypes in a field does affect the performance of blackleg and clubroot resistance traits. We know that V. longisporum group A1/D1, the only virulent group, dominates in Canada. Part of Chawla’s current project is to identify the more aggressive isolates within that group. The project will then create markers for those isolates, which test labs can use to qualify the virulence level of races found in a field. Canola breeders could also use that information to select for cultivars with resistance to these most virulent isolates.
Days to maturity
Another trait that may make a difference in verticillium stripe severity is days to maturity. Agronomists from Manitoba have some anecdotal evidence that cultivars with longer maturity show lower disease severity. This could relate to the disease survey timing – it may just miss the heaviest infection on late crops. However, if true, there could be many reasons. One is that plants that mature later are vegetative longer, and in that longer vegetative state may put more energy into fortifying cell walls within the xylem. Verticillium stripe infects through the xylem walls.
Nursery
Ahmed Abdelmagid, research scientist with Agriculture and Agri-Food Canada in Morden, is establishing a verticillium stripe field nursery to screen canola lines for resistance levels. The nursery is a protected area with high levels of V. longisporum in the soil (added or natural). Because the pathogen is present at high levels, nurseries are a good place to test cultivars for resistance. The University of Manitoba also has verticillium stripe nurseries, as do many commercial seed developers. Field nurseries have been instrumental in screening canola lines for resistance to diseases, such as in the cases of blackleg and clubroot.
Crop rotation
Current situation for Western Canada: We do not know the viable lifespan of V. longisporum microsclerotia on the Prairies. The common break of two or three years between napus crops in Europe does reduce infection levels. We don’t know if a longer break is more suitable for Western Canada.
Verticillium stripe may require longer rotation than blackleg and clubroot, but research on the Prairies has not been done to confirm this. Research shows a two-year break between canola crops is the best practice for blackleg and clubroot. Farmer experience in Manitoba suggests that a three-year break may not be enough for verticillium stripe.
Von Tiedemann says crop rotation of two or three years between Brassica napus crops is “recommended and practiced” in Europe. This helps to manage verticillium stripe in Europe.
European research shows that Verticillium dahliae microsclerotia can persist in the soil for 10 to 15 years in the absence of a host. In their “The Verticillium threat to Canada’s major oilseed crop: canola,” published in the Canadian Journal of Plant Pathology in 1999, Heale and Karapapa commented that the larger microsclerotia of V. longisporum may survive in soil for a shorter time than V. dahliae. “A four- to five-year rotation with non-Brassica crops may be considered,” they concluded.
The required break between host crops is based on how long Verticillium longisporum microsclerotia remain viable in the soil. Microsclerotia form through the aggregation of fungal hyphae, which then accumulate melanin, making them tough and resistant to environmental stress and microbial degradation. Microsclerotia serve as the primary inoculum for infection when favourable conditions allow them to germinate and produce infective hyphae. These hyphae penetrate plant roots, leading to vascular colonization and subsequent disease symptoms.
Spore movement
Current situation for Western Canada: We know how verticillium microsclerotia spreads, but we don’t know how far microsclerotia on the soil surface will move with wind and water erosion. They could cross over a number of fields.
Verticillium microsclerotia spread through wind, water, machinery, seed and people (wearing muddy boots, for example). When infected plants die, microsclerotia on their tissue fall on the soil surface and eventually move deeper into the soil with tillage or rain. They can also move laterally to other locations with water runoff or wind. Note that point of entry into host plants is through the roots, so microsclerotia that infect canola must eventually move into the soil.
Fouad Daayf, researcher from the University of Manitoba, says because verticillium microsclerotia are found on residue on the soil surface, this could make them more susceptible to movement.
Weed control
Current situation for Western Canada: We know the common weeds that host V. longisporum, and control of these weeds will reduce the green bridge that maintains diseases levels even when the host crop is not present.Swedish research (Johansson et al) suggests oats may also be a host, but we don’t know if that is true at the field level on the Prairies.
Common weeds that host V. longisporum are basically the same weeds that host clubroot: shepherd’s purse (Capsella bursa-pastoris), field pennycress (a.k.a. stinkweed, Thlaspi arvense), any mustards and, of course, volunteer canola. Fall germination of canola volunteers will not likely provide enough time for V. longisporum to cycle to sexual maturity.
A 2015 report by Depotter et al noted that inadequate weed management may jeopardize the effects of crop rotation.
A Swedish greenhouse study (Johansson et al, Plant host range of V. longisporum and microsclerotia density in Swedish soils, 2006) compared various crops and weeds for microsclerotia formation. Brassica napus and charlock, a mustard weed, had very high infection rates. Oat and scentless chamomile were somewhat susceptible to infection and could produce microsclerotia, though at much lower levels than canola.
Soil factors: Balanced nutrition and pH
Current situation for Western Canada: We know that balanced nutrition makes for healthier plants. We don’t know if this actually reduces the verticillium risk.
Nutrition
In general, a plant not limited in any required nutrient will be healthier, and healthier plants tend to be more resilient to disease and other threats. Canola crops with balanced nutrition cannot avoid infection, but may experience lower severity. A struggling plant may have infection move in sooner, shutting down the plant sooner and leading to more severe symptoms and yield loss potential.
Von Tiedemann does not know of any oilseed rape studies relating nutrition to verticillium risk. “There were studies with sulphur indicating improved resistance against phoma [blackleg] and botrytis, but studies on V. longisporum are lacking. However, I would not expect significant effects in the range of fertilizer levels usually applied.”
We do have reports from other crops, mostly to do with the more common V. dahliae pathogen. The following two paragraphs from a published report from Spain describes effects from nitrogen (N) and potassium (K):
…The effect of N on V. dahliae infection has been previously reported in many herbaceous hosts, but with contradictory results in the literature. For instance, although the general understanding is that N excess increases disease incidence, direct effects of NO3− and NH4+ have been reported by reducing the number of propagules of V. dahliae in the soil and consequently mitigating the disease.
Concerning the effect of K on plant diseases, it has also been reported that K alone, or in combination with N or other nutrients, influences the severity of diseases caused by many soil-borne pathogens, including verticillium wilt diseases. Indeed, applications of K in K-deficient soils increase host resistance against diseases such as verticillium wilt of cotton; nevertheless, the mechanisms implicated are still poorly understood.…
pH
Because the disease started in Manitoba and is, currently, more serious in Manitoba, some speculated that the pathogen may prefer higher soil pH. New lab-based petri dish research from the University of Alberta suggests that V. longisporum can grow over a wide range of pH values from acidic to alkaline, but further testing is underway.
Topography
A Manitoba farmer talking to Canola Watch observed very low verticillium stripe on hill tops while the rest of the field had a high incidence rate. We don’t know for sure why that could be. One possible reason is simply that more microsclerotia end up in the wet low spots.
Weather factors
Current situation for Western Canada: Field observations suggest that verticillium is worse in hot, dry conditions.
Verticillium tends to be worse in hot, dry conditions. This could be related to plants drying down faster and being generally weaker. Verticillium stripe tends to be more severe in already stressed plants.
Von Tiedemann led a two-year study in Germany in 2003 and 2004 to measure disease incidence and severity at different levels of added verticillium stripe inoculum. He observed higher incidence and severity in 2003, a year with higher average soil and air temperatures than 2004.
Sheau-Fang Hwang from the University of Alberta and Fouad Daayf from the University of Manitoba have a new study to investigate conditions favouring verticillium stripe development and yield losses in canola.
Complete disease management
Current situation for Western Canada: Verticillium stripe often co-exists with other canola diseases in the field, and there seems to be a connection between verticillium severity and the severity of other diseases. Research continues and could help confirm how or why this happens.
Based on field observations, plants combating other issues – such as blackleg or sclerotinia stem rot – can show more severe infection of verticillium stripe. Reasons could be that a stressed plant doesn’t have the same “fight” in it or may have shut down sooner than an otherwise healthy plant. In short, plants suffering from another disease are likely to have higher verticillium stripe severity.
Hwang and Strelkov, in their study that finished in 2024, ran field and greenhouse experiments showing that canola infected with both blackleg and verticillium stripe showed increased blackleg severity. Verticillium stripe severity also tended to increase.
Hossein Borhan, research scientist with Agriculture and Agri-Food Canada in Saskatoon, continues this work with a new study looking into the synergistic interaction between V. longisporum and L. maculans, the blackleg pathogen, and the effect on canola yield.
Fungicide
Current situation for Western Canada: No fungicides are registered for verticillium stripe. We’re not sure any of them actually work, either.
Von Tiedemann in Germany is working with companies on fungicide-based management but they “have not reached conclusive results.”
Chadrick Carley works with Syngenta in Western Canada, and Syngenta markets popular canola seed treatments. He says they don’t have anything showing efficacy on verticillium stripe at this time.
A key factor is that verticillium is a soil-borne disease that tends to infect later in the season. Fungicide seed treatments wear off prior to the pathogen moving in the soil. And because the pathogen enters canola through the roots, that also presents a targeting challenge for foliar applications.
Harvest practice
Current situation for Western Canada: On-farm trials in Manitoba in 2025 will look into the relationship between harvest practice and verticillium stripe severity.
Canola crops standing in wait of straight combining have that much more time for the verticillium stripe to progress. Does this reduce yield and increase inoculum returned to the soil? Manitoba Canola Growers will do on-farm trials in 2025 to compare verticillium stripe severity in swathed and straight-combined canola.
While growers wait for those results, one management question is whether to swath canola fields that are lodging due to verticillium stripe? A Manitoba farmer with experience with verticillium stripe and lodged crop would still straight combine. “The yield gain from waiting negates the headaches from having to straight combine lodged crop,” he says.
Scout
Current situation for Western Canada: Distinguishing verticillium stripe from blackleg and sclerotinia stem rot is important for growers and agronomists. Confirming the presence of verticillium stripe is the first step in management.
Confirming the presence and severity of verticillium stripe in a field is good first step before taking on any other management steps.
Hwang and Strelkov, in their study that concluded in 2024, found that verticillium stripe infecting canola in the early stages inhibited canola establishment and growth. Later infection led to deterioration of the stem and vascular tissues. Infection at both stages affected yield, and yield loss increased as the infections became more severe. Yield loss occurred without obvious symptoms under low moisture.
Based on field observations, yield loss tends to be much greater in fields with striping in early August versus late August. Plants in early-infected fields are cut off from nutrients and water during critical pod fill and seed development time, and field observations show they tend to have more small seeds and lower-than-expected yield. A field that completes that window and fills pods before infection may see considerably more yield potential.
Labs can test canola stem pieces for the presence of verticillium stripe, adding confirmation to field observations.