How are canola products bred for blackleg resistance?
There are currently two types of blackleg resistance in canola: minor gene resistance and major gene resistance. Minor gene resistance is a combination of many genes working together to help keep the plant healthy. Canola hybrids are tested for blackleg resistance and given a field resistance rating (resistant (R), moderately resistant (MR), moderately susceptible (MS), or susceptible (S)) prior to coming to market. This is a rating of infection severity compared to the highly susceptible Westar canola cultivar. Minor gene resistance is accounted for in this field rating. Major gene resistance is matching genes in a canola hybrid to the disease-causing gene in the pathogen. The genes are denoted as Avr genes in the pathogen and R genes in the plant. When the Avr genes in the pathogen match the R genes in the plant then no infection occurs. Major gene resistance is identified by some seed companies who use the standardized labelling system [MC1] for Resistance Groups (A-Rlm1 or LepR3, B-Rlm2, C-Rlm3, D-LepR1, E1-Rlm4, E2-Rlm7, F-Rlm9, G-RlmS, H-LepR2 or X-unknown).1
I have been selecting canola hybrids with an R rating for blackleg for many years. It has been a successful way of avoiding blackleg infection until this year. Why is my resistant canola infected with high levels of blackleg?
Though a hybrid has an R rating, the blackleg pathogen can shift and overcome the plant’s resistance especially if the same hybrid has been grown multiple years in a row. In terms of major gene resistance, the pathogen and the hybrid are no longer matched. This is when you may see unexpectedly high levels of blackleg infection in a canola field. To help slow this process it is best to rotate resistance genetics. Select a different hybrid. Use the Canola Council’s standardized rating system to select a hybrid with different major gene resistance than your current hybrid. Continue using hybrids rated “R” or “MR” for resistance in combination with major gene resistance. An integrated pest management program is also essential to maintaining blackleg resistance. A 3-year crop rotation where volunteer canola and related species are controlled in the non-canola years greatly reduces the pathogen population.2
What else can I do to reduce blackleg pressure on my farm?
- Crop rotation continues to be an important tool. Using a minimum of a three-year rotation can help reduce blackleg pressure in the field.
- Control volunteer canola and wild mustard in rotational crops to prevent inoculum build-up on these host plants.
- Collect canola stem pieces infected with blackleg from fields where you intend to plant canola in the spring. These stem pieces can be tested for blackleg race identification. Use the lab results to select a hybrid with matching major resistance genes. For more information on sampling methods and to find a testing lab near you visit the website www.blackleg.ca[MC2] from the Canola Council of Canada.
- Scout at swath timing. Take samples of blackleg-infected plants and have them tested for blackleg race identification. This is the best time to take stem samples.
1 Canola Council of Canada. 2018. Understanding blackleg resistance.
2 Canola Council of Canada. Blackleg. Canola Encyclopedia. https://www.canolacouncil.org/canola-encyclopedia/diseases/blackleg/.
Performance may vary from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.
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