Study proves concept of grouping varieties by blackleg resistance class

Key Result: This study found that matching genetic resistance to dominant pathotype groups in a field is possible, but more work is needed to make this practical for long-term cultivar rotations and overall blackleg management.

Project title, Principal investigators: “Development of canola cultivar blackleg resistance groups: Feasibility evaluation,” Ralph Lange, Alberta Innovates – Technology Futures

Funding: Alberta Canola, WGRF, ACIDF

Blackleg disease, caused by the fungal pathogen Leptosphaeria maculans was responsible for near-total crop losses experienced in Canadian canola fields in Canada prior to adoption of resistant cultivars in the mid-1990s. Unfortunately, L. maculans can recombine avirulence (Avr) genes to rapidly overcome cultivar resistance, resulting in frequent, serious yield losses.

The Canola Council of Canada and other agencies recommend against re-use of canola cultivars in individual fields, particularly in fields where inoculum load is high, for example under rotational intervals of less than one canola crop in four years. Changing cultivars should slow pathogenic adaptation because different combinations of resistance genes (R-genes) would be presented to pathogen populations that are adapted to previously planted cultivars. But this would only work if the cultivars have different R-genes.

The goal with this study was to determine if canola cultivars could be grouped on the basis of their reaction to L. maculans. Producers could then use this information to select cultivars that will perform against blackleg in their fields.

Early experiments confirmed that cultivars could be placed into two loose groups, but grouping was subject to a number of confounding factors, such as inoculum, pathogenic variation and quantitative resistance.

So, Lange and his team began testing a system based on planned deployment and withdrawal of R-genes based on the primary blackleg pathotypes present in a field.

To test this, they challenged a set of Canadian canola cultivars with isolates representing the common pathotypes found across the Prairies. They selected a set of six L. maculans isolates from a subsample of 98 cultures collected from 33 locations in Manitoba, Saskatchewan and Alberta that represented the most common Avr gene combinations. With its L. maculans test population chosen and defined, the team tested a number of canola cultivars by point-inoculating each chosen isolate onto the cotyledons of these cultivars, and evaluating blackleg disease according to standard methods.

An example of their results is presented in the table, which shows the mean disease severity of cotyledon point inoculations, converted to per cent disease severity. The table shows that some cultivars are resistant to each of the six pathotypes tested. Commercial fields tend to have two dominant pathotypes (Larkan and Borhan, unpublished data) which means that cultivars must be simultaneously resistant to two pathotypes in most cases. Pathotypes D and E are most prevalent in Alberta. Therefore, farmers in Alberta may see better results picking cultivars with an R or MR rating for both D and E, but only 15 per cent of cultivars tested had this combination. Pathotypes B and C appear to be most prevalent in Manitoba, but only 12 per cent of the cultivars tested would be simultaneously resistant in this scenario. So, choosing cultivars resistant to two or more pathotypes may be a challenge.

This information will help canola growers make better blackleg resistance choices. But for this resistance grouping system to work in the long term, we need additional major-resistance genes, better knowledge of the efficacy and genetics of quantitative resistance, as well as better information on L. maculans population structures over all geospatial scales, and the changes in these structures over time. Emphasis on crop rotation and other blackleg control methods should be increased to reduce reliance on genetic resistance.

Disease severity rating¹ after inoculation with L. maculans isolate