Disease surveying in the fall is one of those tasks many of us drag our feet on. Nothing can be done that late in the season to protect the crop from damage, so why survey? The value of surveying is to assess your current management practices and hybrid selections to help make better-informed decisions that will maximize yield potential in future years.
For a disease like blackleg, the canola crop may not show visible signs of damage. Only after disease surveying may the true level of blackleg damage be determined, along with the bushels lost to the disease.
Step1: Detecting Blackleg
To detect blackleg and calculate yield lost from the disease, fall disease surveying is a must. Justine Cornelsen, Agronomic & Regulatory Services Manager with BrettYoung, mentions that the closer to harvest timing, the better for yield loss assessment. “Canola disease surveying is set for around 60% seed colour change because surveyors want to assess plants before they are swathed or too far damaged to differentiate. Now with many acres being left to straight cut, blackleg assessment could be left till later to better determine yield lost.” Blackleg signs are seen throughout the growing season, but prior to harvest the yield damage can be calculated. To do this, pull plants, cut through the root tissue then determine the percentage of decayed or blackened tissue. Cornelsen explains that not all discolouration within the root tissue is caused by blackleg. “It takes time to train your eye for blackleg infection, and it’s important to cut through the root tissue at the base of the plant to get a true assessment of infection.”
Step 2: Measuring Losses
To properly assess yield loss, pull and cut plants that best represent the entire field. Plants from several locations across the field should be used. Blackleg disease severity is rated on a 0-5 scale, with a rating of “0” being clean with no blackened tissue and a rating of “5” having blackened tissue across the entire root cross-section cut or a dead plant. While surveying a field for blackleg, two measurements are going to be taken. The first is disease incidence, the number of plants showing the disease, and the second is disease severity, the average percent of root cross-section infection.
Blackleg yield loss models have been developed through work at the University of Alberta that correspond with the disease severity scale. A recent project found that seed yield losses occurred after the plant was over a disease severity of “1”. A rough rule of thumb is that for every unit increase in disease severity, 20% seed yield can be expected. “Even at low levels of disease severity, major yield losses can be experienced and that’s why I call blackleg the silent yield robber. The crop may be standing with no noticeable damage, it's when you pull and cut plants, you can see the infection”, Cornelsen says. The blackleg infection starts on young leaves and travels through the plant to the base of the stem and root. From there it starts to infect the tissue and limits the plant from taking up moisture and nutrients, this causes the plant to lodge and dry down, minimizing yield.
Use the new blackleg yield loss calculator from the Canola Council of Canada.
The only way to know your crop’s disease severity and infection levels is through surveying. Information gathered through surveying can lead to changes in management practices and hybrids selected in future years. Blackleg is prevalent across western Canada, but it can be minimized through the adoption of an integrated pest management approach and not relying on just one tactic to manage the disease. There is no standard threshold to when practices need to be adopted to minimize disease pressure, it comes down to personal preference and how much yield loss you can handle from blackleg.
Resources: Canola Council Blackleg