Courtney Ross, Canola Council of Canada
Verticillium stripe is the hottest new disease in canola world, rising to yield-damaging levels in Manitoba and eastern Saskatchewan and found in all growing regions across the Prairies. Many farmers and agronomists are learning how to tell it apart from blackleg or sclerotinia stem rot. Farmers and agronomists can use the information in this article to prepare for verticillium stripe scouting at harvest time this year.
Symptom one: 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, which is known as “half-stem senescing.” This 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.
Symptom two: stem cross section discolouration. Verticillium stripe infects roots and enters the plant’s vascular system. Verticillium hyphae and conidia fill up the vascular system, the water conducting tissue, restricting the passage of water and nutrients throughout the plant. This gives the stem cross section a greyish colour, and is easily confused with blackleg. We have two tips to distinguish the pathogens. With blackleg, stem tissue infection tends to be darker and cause distinct black wedge shapes. Verticillium is lighter grey, more general throughout the cross section and can present in more of a starburst pattern. And two, blackleg stem discolouration is confined to the crown area at the base of the stem. Verticillium darkening can extend well up the stem.
Symptom three: black specks. As verticillium infection advances, microsclerotia will start to 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.
Symptom four: 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. Inside the stem, sclerotinia will form sclerotia bodies – the canola-seed-sized resting bodies. Verticillium stripe does not produce sclerotia bodies.
The ideal time to scout for verticillium stripe is at harvest when symptoms are most obvious. No fungicide or soil amendment is known to be effective on verticillium stripe, so accurate identification is all about future management.
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.
Blackleg and sclerotinia stem rot, if those are the diseases present, are more manageable through genetic resistance, crop rotation and fungicides.
For more on verticillium stripe identification and management, read the verticillium stripe chapter at canolaencyclopedia.ca and “Verticillium stripe – identification and management” at canolawatch.org/fundamentals.
–Courtney Ross is an agronomy specialist with the Canola Council of Canada. rossc@canolacouncil.org.
