Multiple genes improve clubroot resistance
October 4, 2021 By Bruce Barker, P.Ag CanadianAgronomist.ca
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 to 2016 by Stephen Strelkov at the University of Alberta found a total of 17 “new” pathotypes using the Canadian Clubroot Differential (CCD) set.
To further understand how clubroot-resistant (CR) genes interact with these new pathotypes, researchers at Agriculture and Agri-Food Canada (AAFC) in Saskatoon 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, led by research scientist Gary Peng, 20 canola-quality Brassica napus inbred and hybrid lines carrying single, double and triple clubroot-resistant 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 pathotype, 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 clubroot-resistant genes was also included to investigate the efficacy and durability against this common clubroot pathotype.
The study found that clubroot-resistant 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 clubroot-resistant genes on chromosome A3 (Rcr1 or CRM), moderate resistance was achieved against all 5X populations, as well as high resistance to the older pathotypes. Peng says this indicates that the range of resistance can be increased by stacking two clubroot-resistant genes with different modes of action.
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 clubroot-resistant genes, relative to those controlled by either of the single clubroot-resistant 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 less than 30 per cent. This highlights the value of using the multi-genic approach for clubroot-resistance efficacy and durability.
Against pathotype 3H, a single clubroot-resistant gene was found to lose the resistance gradually, especially when exposed to high initial inoculum levels (107 spores/g soil). The resistance erosion was noticeably slower under lower inoculum pressure (104 spores/g).
Because resistance erosion was slower under lower inoculum pressure, the researchers emphasized the importance of extended crop rotation – at least a two-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.
Research by Strelkov found that having a greater than two-year break from canola resulted in a 95 per cent decrease in clubroot resting spore concentrations. In the third year after the harvest of clubroot-resistant canola, resting spore concentrations were similar to those of host-free control plots. By implementing longer rotations and having an awareness of clubroot severity or inoculum levels in their fields, producers may be able to prolong the effectiveness of clubroot-resistant varieties and contribute to more sustainable management of clubroot of canola.
The overall results of these research projects highlight the value of stacked clubroot-resistant 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.
According to the Canola Council of Canada, it is understood that no clubroot-resistant variety, including new ones with multiple resistance genes, are resistant to all of the clubroot pathotypes detected in Western Canada. As a result, growers and agronomists should talk to their seed suppliers to gain an understanding of how to implement a strategy for rotating and stacking clubroot-resistant genes on their farm. This strategy will become more useful as more clubroot-resistant genes are bred, and stacked, into new canola varieties.