Pest Profile: Verticillium Stripe
Photo Credit: Canola Council of Canada
OVERVIEW
Disease symptoms in canola include leaf chlorosis, early ripening, stunting and, as the disease progresses, shredding of the stem tissue. Tiny black dots, called microsclerotia, infect the tissue on the stem and can fall into the soil, surviving there for many years and potentially infecting other fields.
- Relatively new disease, first detection in Canada was in Manitoba in 2014.
- Soil born disease; spores can survive on stubble up to 10 years.
- Develops in hot & dry conditions. Soil: 15℃-19℃; air: 23℃+. When the plant is under stress- especially the xylem.
- Enters the plant via root: wounds or root tips.
- Time of flowering affects severity of infection.
- Monocyclic- 1 cycle/year.
- Affects the vascular system = grey coloration which restricts water and nutrient movement.
IDENTIFICATION
Symptoms of this disease can be noted on the leaves and pods, but are more noticeable on the stem on roots, varying according to environmental factors and plant stage. The disruption of water and nutrient uptake causes stunting, brittle stems and faint black vertical striping on the stems, which becomes more noticeable when peeling back the skin. Later in the season, microsclerotia will begin to develop, which are also more noticeable beneath the epidermis. The striping will become more obvious in more mature plants due to the dying tissues beneath the stem surface, and near the end ripening, the microsclerotia will begin to germinate and produce conidia spores, giving the stems a powdery appearance.
Verticillium Stripe is often mistaken for other diseases, such as sclerotinia stem rot and blackleg, but upon careful investigation, the differences are more obvious. Sclerotinia’s large sclerotia and hollowing inside the stem is different from the tiny microsclerotia of verticillium stripe, and while the discoloured stem and premature ripening could point to blackleg, the stem affected by verticillium stripe does not have blackening in the stem cross-section at ground level as it does with blackleg.
Disease Cycle
- Germination of fungal propagules in the soil.
- Infection first occurs at fast growing root tip.
- The most damaging infection occurs during flowering.
- The fungus can enter the vascular system directly from root tip or by wounds on a root.
- After the fungal hyphae enter the root they multiply and produce conidia (single celled spores) in the xylem. They are then able to continue multiplying and spread through the plant.
- Eventually the spread of conidia causes the xylem to plug, turn grey and collapse.
- As the plant begins to die the pathogen spreads from xylem to the surrounding vascular tissue.
- Once the pathogen in the vascular tissues, microsclerotia are formed.
- As the plant continues to die and dry down, the microsclerotia cause the stems epidermis to peel back = stem shredding appearance.
- Microsclerotia is then released back into the soil.
SYMPTOMOLOGY
- Appears late in season.
- Plant - stunting and premature ripening, with closer visual inspection striping on a portion of the stem occurs. As the plant dries down, shredding of the stem can be seen. The shredding typically has black flecks within it.
- Stem - cross section at base of stem appears grey and necrotic.
SCOUTING
Best results just prior to harvest.
- Stem striping – when canola is still green it will have 2 tones on stem (1/2 green, ½ premature ripening= “stripe”) this differs from sclerotinia as, it does not cause that striping.
- Cross Section – Grey in colour. Due to damaged vascular tissue, the symptoms will be seen throughout the whole plant.
- Stem peeling – stem outer layer will start to peel back. The newly exposed layer will have microsclerotia present. This will appear as black vertical striping.
- Black specks- microsclerotia form under the peeled stem outer layer which resembles a powdery appearance.
CONTROL TIPS
- At this time there are no treatment fungicides currently registered for control of verticillium stripe in Canada. Control measures for the closely V. dahlia (which can cause verticillium stripe, include:
- Scout just before or just after harvest when symptoms are most obvious
- Increasing rotation length of non-host crops
- Weed management
- Increasing soil futility
- Use of non-host trap crops
- Fumigation (vapam and chloropicrin)
- Reduce soil movement, sanitize equipment, extend crop rotation.
References and Additional Reading
“Verticillium Stripe” Canola Council of Canada. Online.
https://www.canolacouncil.org/canola-encyclopedia/diseases/verticillium-stripe/
“Verticillium Disease Etiology and Nursery” Canola Council of Canada. Online.
https://www.canolacouncil.org/research-hub/verticillium-disease-etiology-and-nursery/
“Verticillium Stripe” Manitoba Canola Growers. Online.
https://canolagrowers.com/resource/verticillium-wilt/
https://www.canolacouncil.org/wp-content/uploads/2021/03/Blackleg-Management-Guide-FINAL_Mar1.pdf
https://www.canolacouncil.org/canola-encyclopedia/diseases/blackleg/
https://www.canolacouncil.org/canola-encyclopedia/diseases/verticillium-stripe/
Blackleg Yield Losses and Interactions with Verticillium Stripe in Canola (Brassica napus) in Canada Yixiao Wang, Stephen E. Strelkov and Sheau-Fang Hwang *