Researchers look to seed treatment for blackleg resistance

LACOMBE, Alta. — The resurgence of blackleg in the prairie canola crop has the industry looking for more tools to beat back the disease. 


Crop researchers have been calling for longer rotations to reduce the amount of blackleg inoculum on prairie fields, but their calls have largely gone unanswered.


Instead, many growers rely heavily on resistance to the pathogen within the canola germplasm, a strategy that is beginning to wear thin. 


Gary Peng, a research scientist at Agriculture Canada, said during his recent presentation at Murray Hartman’s Science-O-Rama in Lacombe that the major resistance genes available to growers are no longer effective against the blackleg pathogen.


Peng said a study that coded the major resistance genes used by 206 canola varieties found that most of the varieties relied on two major resistance genes. 


Rlm1 was used in 10 percent of canola cultivars, while Rlm 2 was used in almost 70 percent of the varieties. 


“Those are the ones (hybrids) from the big players with 80 percent of the market share, those are common cultivars,” Peng said.


“They all carried Rlm 1 and Rlm 3. That’s a very common major gene background that we are seeing right now. If you heard them say they have multi-gene resistance, that’s Rlm1 plus Rlm 3.”


Even though Rlm1 and Rlm 3 are the only major resistance genes commonly found in Canadian canola cultivars, they are currently the least effective on Canadian fields out of the 16 or 17 major blackleg resistant genes identified so far.


“For each of those genes to work, there has to be a high frequency of responding avirulence gene in the pathogen population,” Peng said.


“So what happens is that when you have a corresponding Av-gene in the pathogen that can be recognized by the resistant gene, it’s almost like a lock and key system — it triggers a cascade of defense response in the plant.”


A cultivar that doesn’t have a resistance gene won’t be able to recognize this type of avirulence gene. As a result, the systems of reaction will not be triggered and the pathogen will go from the leaf into the stem. 


An Agriculture Canada project has been monitoring the blackleg pathogen in the three prairie provinces for almost 10 years for the frequency of specific Av-genes, and it found that Rlm 4, Rlm 6 and Rlm 7 are common while Rlm 2 is likely less common.


“What that means is if you have a resistant variety that carries any one of those resistant genes, corresponding to these Av-genes, it will work with a high percentage,” Peng said.


“At the same time, if a variety carries Rlm 3 or Rlm, 1 it will be ineffective. That’s understandable because in the past we have used Rlm 1 and Rlm 3 for almost 25 years.” 


He said pathogen race monitoring is critical because it helps seed companies know what resistant genes are available, and over time it helps track the pathogen population to foresee where the different Av-genes will go.


The major gene, or quantitative resistance, commonly used in commercially available canola varieties is now largely ineffective, but canola breeders have made great strides in building up the qualitative resistance to blackleg in their cultivars.


“Qualitative resistance is kind of a backbone of the current varieties for now,” Peng said.


“It is a valuable resource to us. The drawback of only relying on qualitative resistance is when we have things like hail damage, that’s where that resistance will start falling down.”


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Damage to canola plants from root maggots and flea beetles can also allow blackleg to bypass qualitative resistance and infect the stem, which happens often on the Prairies with current varieties. 


Major gene resistance allows even wounded plants to resist a blackleg infection and the associated yield loss that it causes. 


The problem with major gene resistance is that it can quickly break down if it is relied on too heavily. 


“A grad student’s work identified at least 90 different races which will have different combination of the Av-genes in them,” Peng said.


“Even with the known R genes, there is a race out there already virulent to it. So no matter what R genes we are going to put out, it’s not an if, it’s a when, it will be broken. It will depend how we use the single R genes.”


The Western Canada Canola/Rapeseed Recommending Committee recently approved a new labelling system for major gene canola resistances as a way to extend the life of the resistant genes that are in the seed development pipeline.


The new system will identify specific resistant genes in the varieties and place them into one of 10 groups. 


“These R genes we know are being worked on and coming down the pipeline, and others that will come,” Peng said.


The existing labelling system for canola, including the R and MR annotations, will also remain to help distinguish the qualitative resistance in the canola cultivars.


The labelling system is voluntary and it may take years for all seed companies to buy in, but growers will see the new label on packaging from some canola companies soon, and it will be included in the 2018 seed guides.


“There are quite a (few) reservations from a few seed companies. Their concerns are that an over-emphasis of this would not be a good long-term strategy in terms of the durability of the resistance. And also their programs, their current efforts, is on the qualitative resistance, and would this dilute that effort?” Peng said.


“That might just be an excuse. For me, I don’t see a major downside at this point, from a producer’s perspective.”


Growers with blackleg problems can now better understand which major gene resistance is breaking down in their field, and they can rotate to a different resistance package. They previously had to switch to a different R rated variety, which may have used the same resistant genes. 


There is no urgency for growers who do not have a major blackleg problem to switch, although it is often a good idea to rotate resistances. 


Knowing exactly what Av-gene is in a specific field’s pathogen population will greatly help growers make variety decisions.


“It will be the best information if you can test the pathogen on site on the farm, but it’s not practical at this time because of the testing procedure,” Peng said.


“It’s labour and time consuming.”


New molecular tools may eventually be developed that can be used at provincial and private labs, but for now growers will have to rely on using regional information on which blackleg race is present. 


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In areas where blackleg pressure is heavy, such as in southern Manitoba, some growers have been using fungicides to help reduce yield losses. 


Peng said a study with data from 17 site years across all three prairie provinces sprayed Headline, Quadris, Tilt and Quilt at the two to four leaf stage.


Headline was sprayed at bolting on a separate plot, and a treatment with a dual application with Headline was sprayed at the two to four leaf stage and a second application of Tilt was sprayed at bolting. 


The study found that early applications can reduce the disease incidence and severity and also increase the yield. 


It also found that Tilt didn’t work very well, the late application of Headline did not work and two applications worked no better than just one application. 


None of the treatments helped under low disease pressure when the data was broken down into plots with low and high disease levels.


“None of the treatments mattered. It did not affect the disease level, it did not affect the yield.”


Only the plots where there was high disease pressure benefited from treatments, and there was no yield response from the treatments on resistant varieties. 


“If a fungicide is used, it needs to be applied early, at the two to four leaf stage,” Peng said.


“The key issue is when looking at the date is the cost effectiveness, and a lot of fields from different studies do not show significant yield benefits from many different trials. And it’s understandable because in most cases on the Prairies the disease levels are still a bit on the light side.”


A problem with early fungicide treatments is that the majority of the spray misses the plants and ends up on the ground.


“So we’re thinking a seed treatment would be a better choice,” Peng said.


Australia has been using a seed treatment for blackleg in canola, and Peng said Agriculture Canada is working with companies on new active ingredients and products coming down the pipeline to see if they will work in Canada.


He is involved in a study that is in its early stages in which two cultivars — one resistant and one nonresistant to blackleg — are infected with the pathogen. He said the standard seed treatments that are available didn’t provide any protection, but there are two new compounds that are promising. 


“These compounds in particular had a remarkable effect, and we are doing a lot more with this and hope to work with the company to push this into a product for canola seed treatments in the near future,” Peng said.


“It’s very, very promising.”


He said the new seed treatments look like they will be far more effective than foliar applied fungicides. 


“The rate is being fine tuned,” he said.


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“The good news is that it (one of the tested seed treatments) is a registered product in Canada already, but for soybeans for different diseases. So I figure the registration process will not be as difficult as a new active ingredient coming into the market.”