Scouting for blackleg is an essential step in understanding disease pressure, assessing yield loss, and informing future management decisions. Fall surveying provides the clearest picture of infection levels because plants are mature and symptoms are easier to evaluate.
Step 1: Detecting Blackleg
Fall is the ideal time to begin blackleg detection. According to Justine Cornelsen, Agronomic & Regulatory Services Manager with BrettYoung, surveying “around 60% seed colour change” helps ensure plants are evaluated before they are swathed or damaged. She also noted that with more acres being straight cut, assessments can sometimes occur even later to better understand yield loss.
Blackleg symptoms can appear throughout the season, but yield impact is best assessed close to harvest. To do this, pull plants, cut through the root tissue at the base, and determine the percentage of blackened or decayed tissue. Cornelsen explained that it “takes time to train your eye for blackleg infection,” and that cutting into the crown is important to get a true evaluation since not all discolouration is caused by blackleg.
Step 2: Measuring Losses
A proper assessment includes sampling plants from several representative areas across the field. The blackleg disease severity scale ranges from 0 to 5, where:
- 0 = clean plant, no blackened tissue
- 5 = full blackening across the root cross-section or a dead plant
Two measurements are recorded during surveying:
- Disease incidence: the number of plants showing symptoms
- Disease severity: the average percentage of infected crown tissue
Research from the University of Alberta has developed yield loss models that correspond with the severity scale. A recent project found that yield losses begin once severity exceeds 1. A general rule of thumb is that each unit increase in severity can result in about 20% yield loss.
Cornelsen emphasized that blackleg can be deceptive. Even when fields look healthy, “major yield losses can be experienced” because the disease limits the plant’s ability to take up moisture and nutrients, eventually causing lodging, premature dry-down, and reduced yield.
Growers can also use the Canola Council of Canada’s blackleg yield loss calculator to help understand the relationship between severity and expected yield reduction.
Next Steps After Surveying
Surveying provides the information needed to adjust management practices. Infection levels can guide hybrid selection, crop rotation decisions, and other integrated pest management (IPM) strategies. While blackleg is widespread across Western Canada, Cornelsen noted that minimizing its impact depends on using multiple tactics, rather than relying on a single approach.
There is no defined threshold at which management changes must occur — it ultimately depends on a grower’s comfort with potential yield losses. Regular surveying ensures those decisions are informed.
Canola Council Blackleg Resource
This resource provides guidance to help growers understand blackleg severity and the impact it can have on yield. It includes tools such as the blackleg yield loss calculator, which allows growers to estimate yield reductions based on disease severity ratings. These tools support decision-making around hybrid selection and overall blackleg management strategies.
FAQ
How can blackleg be detected in canola fields?
By pulling plants before or near harvest, cutting through the crown tissue, and assessing the percentage of blackened or decayed tissue.
Why is it important to scout for blackleg before harvest?
Surveying before harvest provides the most accurate evaluation of yield loss and infection levels because plants have matured and symptoms are easier to differentiate.
How is blackleg severity rated?
On a 0–5 scale, where 0 indicates no infection and 5 indicates a fully blackened crown or dead plant.
What effect does blackleg have on yield?
Once severity surpasses 1, yield loss begins. On average, each additional severity level can result in about 20% seed yield loss.
What should growers do after surveying for blackleg?
Use the results to guide hybrid selection, management practices, and integrated pest management strategies in future seasons.
