A better understanding of major and quantitative resistance genes in blackleg was developed and it is suggested that labelling of individual genes by the canola industry will allow growers to rotate major genes, while still taking advantage of quantitative resistance.
Almost all canola varieties registered in Canada have good levels of resistance to blackleg disease. Unfortunately, Leptosphaeria maculans, the fungus that causes blackleg, can rapidly overcome resistance, resulting in serious yield losses. Repeated use of the same canola cultivars (which isn’t recommended) can speeds the fungus’ adaptation to resistance genes. However, simply changing varieties may be ineffective because producers have no way of knowing which resistance genes they are deploying.
The goal of this project was to determine if canola varieties could be grouped based on their susceptibility to different L. maculans populations. The initial approach was to emulate a system used in Australia, which involves testing the disease resistance of canola varieties by suspending crop residues over canola plants and then from scoring the resulting infections.
Methods and results
The research team collected crop residues from various sites in Alberta and Saskatchewan and exposed a selection of canola cultivars in this way. Initial results suggested that at least two cultivar groups were present, but these groups were difficult to validate because of differing amounts of spores produced by the crop residues. The team therefore decided to remove this problem by devising a new method where they could control the number of spores applied to the test plants.
To do this, the research team induced spore formation on the crop residues and pooled spores from many fungal fruiting bodies to represent the combinations of L. maculans strains present in field. The suspensions were used to simultaneously point inoculate cotyledons and spray and wound inoculate plants at the three-leaf stage. The team found that many varieties were highly susceptible to the pooled fungal populations, although some cultivars were more susceptible to specific populations than others were.
Some varieties responded differently to the same populations when inoculated as seedlings than when inoculated at the three-leaf stage, which suggests that major resistance genes, also known as seedling resistance, are not the only genes determining cultivar resistance to L. maculans. The results of these tests did not correspond with the performance of cultivars in canola fields. These results suggest that quantitative resistance is important, and that grouping cultivars using this method is not feasible.
As a result, a system based on planned deployment and withdrawal of resistance genes seems more practical. Therefore, the research team selected six representative isolates, that represent the most common pathogen gene combinations, using data provided by the Hossein Borhan laboratory at AAFC in Saskatoon. They tested a number of canola cultivars by inoculating them and scoring disease responses. They again observed that a large number of cultivars that were unable to withstand common L. maculans populations.
The WCC/RRC definition (Western Canada Canola/Rapeseed Recommending Committee 2009) of “moderately-resistant” or “resistant” is defined as mean disease severity (MDS) of ≤30% of Westar. Commercial fields tend to have two dominant blackleg pathotypes, but only about 6% of the cultivars tested could simultaneously resist two of the most common L. maculans strains. This implies that blackleg should be killing a large percentage of canola crops. This is obviously not true, probably because of quantitative resistance genes in existing varieties, which is good news for the canola industry, but also means that resistance grouping is not feasible.
Therefore, labelling of individual genes, as now being implemented by the canola industry, will allow growers to rotate major genes, while still taking advantage of quantitative resistance.