Researchers detected verticillium stripe, caused by the pathogen Verticillium longisporum, in canola in Manitoba in 2014. The disease is now across the Prairies and will cause yield loss.
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.
(Verticillium stripe, caused by V. longisporum, is the verticillium disease of canola in Western Canada. Verticillium wilt, caused by V. dahliae, is a common disease in various other crops around the world.)
Key distinguishing features:
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 is where the “stripe” name comes from. Leaves can show similar symptoms – healthy on one side, diseased on the other. You will not see stem or leaf striping 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 hyphae and conidia fill up the vascular system and give the stem cross section a greyish colour. This is easily confused with blackleg. With blackleg, stem tissue infection tends to be darker and cause distinct wedge shapes of black. Verticillium is lighter grey and more general throughout the cross section. Blackleg stem discolouration is also confined to the crown area at the base of the stem. Verticillium darkening can extend well up the stem.
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.
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 centimetres in size. If you see pink and specks confined to a lesion, it’s blackleg.
The disease is easiest to scout just prior to or just after harvest when symptoms are most obvious. The Identifying Verticillium Stripe video provides key scouting tips for effective scouting of this disease.
Differentiating between verticillium stripe, sclerotinia, blackleg, grey stem and fusarium wilt
|wdt_ID||Plant Disease||Sclerotinia||Blackleg||Blackleg||Verticillium Stripe||Grey Stem||Fusarium Wilt|
|1||Species||Sclerotinia sclerotiorum||Leptosphaeria maculans||Leptosphaeria biglobosa||Verticillium longisporum||Pseudocercosporella capsellae||Fusarium oxysporum|
|2||Stem Symptoms and distinguishing features||Bleached white appearance. Infected stems tend to shred and shatter very easily. White mouldy growth. Black sclerotia bodies will form inside the stem.||Stem lesions with pycnidia (black spots) forming inside the lesion. Base of stem (crown) becomes woody. Cross section cut reveals blackening.||Shallow stem lesions with pycnidia||Shredding of the stem tissue. Tiny black microsclerotia form beneath the peeling outer layer.||Large purple to grey-speckled stem. Pod lesions are also possible.||Discolouration of the stems; yellow or reddish-brown streaks on stems.|
|3||Pod||Withered pods if branch below is infected. Sometimes white mould, lesions.||No symptoms||No symptoms||No symptoms||Grey speckled pods.||No symptoms.|
|4||Crown (base of stem) exterior||Cankering|
|5||Crown cross-section||Clean, dried down||Solid black sections, often pie shaped. Fully black in extreme cases.||Typically does not reach stem in time||Greyish hue across entire cut. Gets darker as microsclerotia build up. Can extend many inches up the stem.|
|6||When to scout||Prior to swath timing||Prior to swath timing, 60% SCC.||Prior to swath timing, 60% SCC||Easier to ID post-harvest||Prior to swath timing, 60% SCC.||Prior to swath timing, 60% SCC.|
|7||Yield Loss||Potential yield loss in a field can be determined by: % Potential Yield Loss = % Infection x 0.5||For every unit of increase in disease severity, a 17% loss in plant seed yield can be expected.||Comes in too late in the season to cause a significant impact||Does occur but no system to measure at this time.||Not known.||Varieties are resistant, rare to find.|
Verticillium microsclerotia are soil-borne, so steps to keep soil in place could provide some reduction in spread. Two- or three-year breaks between canola crops are good disease management in general, but verticillium microsclerotia can remain viable for many years. Plant tolerance or resistance is likely to provide the best solution, and plant breeders are looking into this trait.
No fungicide or soil amendment is known to be effective on verticillium stripe.
Blackleg and sclerotinia stem rot, if those are the diseases present, are more manageable through genetic resistance, crop rotation and fungicides.
- Canola Research Hub verticillium stripe research summaries.
- It also features the Verticillium stripe: Researching a new canola disease threat blog.
- The Canola Encyclopedia’s Verticillium stripe chapter provides a succinct disease overview.
- This video summarizes verticillium stripe research projects.
- Also see the Verticillium Stripe in Canola – Lifecycle and Disease Severity video.
- The full Verticillium Stripe Workshop recording increases understanding of this disease.