Multiple genes improve clubroot resistance
Researchers investigated the efficacy and durability of canola lines carrying single and multiple clubroot resistance (CR) genes against ‘new’ and ‘old’ pathogen pathotypes. Stacked CR genes of different modes of action improved resistance and enhanced durability.
Cultivar resistance is a cornerstone of clubroot management. Most canola hybrids have resistance to the ‘old’ pathotypes 2, 3, 5, 6 and 8, but research in 2014-2016 found a total of 17 ‘new’ pathotypes using the Canadian Clubroot Differential (CCD) set Strelkov et al. 2018.
Researchers at Agriculture and Agri-Food Canada in Saskatoon, Saskatchewan initiated a unique three-year study in 2016 to assess the efficacy and durability of canola lines carrying single and multiple CR genes. For the study, 20 canola-quality B. napus inbred and hybrid lines carrying single, double and triple CR genes were produced in collaboration with Nutrien Ag Solutions. All lines in the study were resistant to the old pathotypes 2, 3, 5, 6 and 8. The study looked at the efficacy of selected lines against the newly identified pathotype 5X. Westar and 45H29 (resistance to old pathotypes), both susceptible to the 5X, were included as controls.
Durability of the selected lines were also assessed against the predominate pathotype 3H (old pathotype 3) under heavy and lighter disease pressure, mimicking clubroot infestation levels in Alberta versus in Saskatchewan and Manitoba. Stacking of CR genes was also included to investigate the efficacy and durability against this common clubroot pathotype.
The study found that CR genes on chromosome A8 (CRB) are effective on the old pathotype 3H, but only partially resistant to two of the new pathotype 5X populations, and susceptible to a third population of pathotype 5X.
When the CRB gene was combined with one of the CR genes on chromosome A3 (Rcr1 or CRM), moderate resistance was achieved against all 5X populations, as well as high resistance to the older pathotypes. The researchers say this indicates that the range of resistance can be increased by stacking two CR genes with different modes of action.
Representative clubroot severity (gall size) for canola lines carrying a single resistance gene on chromosome A03 or A08, or both genes (bottom photo) exposed to a field population of new pathotype 5X in five general cycles under controlled-environment conditions.
The study also showed that in response to 5X infection, many genes involved in pathogen immunity pathways were more strongly activated in lines carrying these two CR genes, relative to those controlled by either of the single CR genes alone. Despite an intermediate level of resistance, the resistance appears quite durable after five generational cycles of exposure to the same 5X population, with disease severity index (DSI) generally <30%. This highlights the value of using the multi-genic approach for CR efficacy and durability.
Against pathotype 3H, a single CR gene was found to lose the resistance gradually, especially when exposed to high initial inoculum levels (107 spores/g soil). The resistance erosion is noticeably slower under lower inoculum pressure (104 spores/g). The researchers highlight the importance of extended crop rotation (a >2-year break from canola) to reduce the load of resting spores in heavily infested fields to assist in the performance and durability of clubroot resistance.
The overall results highlight the value of stacked CR genes of different modes of action for resistance performance and durability. More work is warranted to look at resistance against additional new pathotypes (3A and 3D, for example) to confirm the validity of this multi-genic strategy for enhanced resistance efficacy and durability.
Research funding was provided by WGRF and SaskCanola to allow the hiring of postdocs and students on the project.
Enhancing the Durability of Clubroot Resistance with Multiple Genes. Gary Peng, T. Song, N. Tonu, R. Wen, Agriculture and Agri-Food Canada, Saskatoon SK, Nutrien Ag Solutions. Final Project Report. CGDP ADF 2015.64 Peng. 2019