The blackleg threat for canola

One of the most common diseases in canola is blackleg. As a fungal disease, it needs moisture to spread but infections still occur in the dry years we’ve been seeing lately in the Prairies.

Justine Cornelsen, who was an agronomy specialist with the Canola Council of Canada before joining BrettYoung as agronomic and regulatory services manager in January, says if it’s a warm spring, all it takes is a few scattered showers to get the blackleg pathogen, Leptosphaeria maculans, started on spreading spores and infecting plants.

“The temperature is going to impact how quickly they start producing lesions and when those lesions will be able to start creating that secondary infection, creating further infections in surrounding plants,” she says. “If we’ve got that humidity there (as well), it’s going to be very favourable for the pathogen to be producing spores and it doesn’t take a lot of rainfall.”

Blackleg in its most severe form causes stem cankering, which can lead to premature ripening, pod drop and plant lodging in canola crops, Cornelsen notes. She says when temperatures are unusually hot in July and August, as was the case in many parts of the Prairies last year, this causes additional plant stress that can make blackleg symptoms even more apparent in infected canola.

“You will notice the disease in the fields that are starting to dry out because they’re already moisture restricted. Those (infected) plants are going to be a lot more obvious, and you’re going to see that premature ripening,” says Cornelsen.

“We are seeing warmer temperatures, which are very conducive for the cankers to start to form and where we start to see our yield loss, so we have been having conditions for blackleg to do well.”

Cornelsen says when lots of rain falls during the summer months, this can cause blackleg damage to accelerate as well. “The high moisture is just going to allow that pathogen to spread even further. The plants might look healthier, but the infection is likely more severe because that pathogen is going through that plant a lot quicker.”

Canola residue is one of the main sources of blackleg infection. It usually takes a year or two for this residue to break down, but Cornelsen believes drought conditions in recent years have likely slowed this process down in many fields — which could conceivably increase the disease risk in some areas.

“There hasn’t been any work to really validate that, but just looking around, you can very easily find old canola residue from three or four years back that’s still fully intact over fields because we haven’t had a lot of moisture to break it down,” she says.

Provincial surveys

The provincial crop disease surveys conducted by the Alberta, Saskatchewan and Manitoba governments every year are a way to keep tabs on the blackleg situation. The numbers show just how broadly the disease has taken hold in farmers’ fields in Western Canada.

The surveys measure blackleg prevalence (the number of fields where blackleg was detected) and incidence (the number of plants exhibiting blackleg symptoms within the fields).

Cornelsen says the official tally from 2021 hasn’t been published yet but preliminary figures show the percentage of surveyed fields where blackleg was present was 89 per cent in Alberta, 84 per cent in Manitoba and 66 per cent in Saskatchewan. The preliminary incidence rate numbers in 2021 were 18 per cent in Alberta, 20 per cent in Manitoba and seven per cent in Saskatchewan.

One of the first signs of blackleg infection is the appearance of small beige lesions on canola leaves. The dots on the lesion you see on this leaf are pycnidia produced by Leptosphaeria maculans.  Photo: Justine Cornelsen

Cornelsen says looking at the averages for all three provinces there has likely been a slight increase, but she believes the numbers are more or less in line with the average for the Prairies over the past 10 years or so — a prevalence rate in the 70 to 80 per cent range, generally, and an incidence rate somewhere around 10 to 15 per cent.

This indicates that while the disease is present in a lot of fields, there are relatively few plants that exhibit blackleg symptoms, Cornelsen says, adding the number of plants with severe yield-robbing symptoms is lower still.

“We know the disease is here, but we know it’s at really low levels and we want to keep it that way,” she says. “It’s not taking a lot of our yield. It’s only on those extreme fields where we are seeing some losses occurring.”

That’s all good news for farmers but, even so, Cornelsen would like to see the canola industry back to where it was in the early 2000s, when the incidence rate was under five per cent for the Prairie provinces.

New canola hybrids with resistance genes have helped contain the spread of blackleg on Prairie fields and control yield losses due to the disease. In recent years, though, it has become clear relying on disease-resistant hybrids alone likely isn’t enough to keep your farm safe from blackleg. Fortunately, there are other strategies you can use to manage the disease and maintain your canola yields.

Major and minor gene resistance

Cornelsen says because of shifts in the L. maculans population, new races are emerging that can overcome the major resistance genes farmers have relied on for many years.

“When you use the same resistance gene over and over and over again, the pathogen adapts to overcome it, so it’s gone from an avirulent population to a virulent population,” she says.

Cornelsen maintains even with this development, resistant hybrids on the market generally work well, thanks in large part to products that have genes with minor or “quantitative” resistance as part of their blackleg protection package.

Cornelsen explains the way major or “qualitative” genes work is by stopping L. maculans in its tracks and preventing it from infecting the canola plant. It initiates a defence response instantly, but only against certain races of the pathogen. Minor genes, on the other hand, aren’t race specific and work by slowing down the pathogen once a plant is infected and by reducing blackleg symptoms.

“Ideally, you want a hybrid that has an effective major gene that matches the dominant races, but also a really strong quantitative background so you have a full protection package,” says Cornelsen. “It helps to really suppress the disease pressure in the field.”

Agriculture and Agri-Food Canada scientists and others are working toward gaining a better understanding of minor genes and to come up with a way to measure their effectiveness, which should provide a clearer picture of a hybrid’s quantitative potential, notes Cornelsen. But it’s not an easy thing to do.

“The issue here is there are so many different genes … working at different times of plant development. To try to find a system to quantify this type of resistance is really challenging,” says Cornelsen.

“The other thing with quantitative resistance, you’re going to have those genes react and work very differently under different environmental conditions, so this is where the complexity comes into play with trying to put a number to it,” she adds.

“That’s something the industry is working on, but we’re not at the point yet of having something where we can quantify that quantitative resistance.”

Once that happens, there could be two different rating systems for blackleg resistance in canola, one for major genes and another for minor genes indicating where they rank on a resistance scale, says Cornelsen. With this kind of information, canola producers would have a better idea of which products might work best for their farms.

A greater understanding of the genetics of blackleg resistance could lead to farmers deploying canola hybrids more effectively, she adds, not only to protect their crops but also to increase the longevity of new disease-resistant hybrids coming to the market.

“We don’t want to burn through the resistant sources we have, so when we know what we’re working with, you can better steward it for the future,” she says.

Crop scouting

Boots on the ground is the best way farmers can see for themselves if the blackleg-resistant hybrids they’re using are doing the job, or whether it’s time to try something else.

“When you’re making these types of decisions, you have to be surveying. You have to be going out into all of those canola fields and knowing your disease levels,” Cornelsen says.

“If they are increasing over time, that’s an indicator the resistant sources you’ve been deploying are being overcome or are no longer working to their full potential,” she adds. If you’re surveying each year and the levels are extremely low and they’re staying that way, that’s a good indicator your resistance sources are working.

“Unfortunately, often surveys don’t happen, and people are making uninformed decisions on that hybrid selection.”

Cornelsen says if farmers are seeing an increase in disease pressure due to a breakdown in resistance, they might consider using products with different resistance genes to switch things up.

This could require some guesswork since not all companies offering blackleg-resistant hybrids label their genetics, she says. One option is to send canola stubble samples out for testing to identify what L. maculans races are present in the field and try to match resistant genes that way.

Cornelsen says scouting for blackleg can be tricky since the disease can show itself in many different ways.

“Blackleg is really unique in that way because the visual symptoms really change from cotyledon stage to full, mature, podded-out canola, so you’ve got to be looking for different signs through the growing season,” she says.

It’s important to understand other canola diseases can have similar symptoms, so it’s a good idea to try to train your eye to spot the key diagnostic features that distinguish blackleg.

The first signs of blackleg on plants are pycnidia and pseudothecia, the tiny fruiting structures you’ll also find on infected canola residue, and small beige lesions on leaves. As the season progresses, you’ll start to see cankers forming or constriction occurring on the plant stems. Toward the end of the season, you can estimate how much infection has occurred by cutting the base of the stem open and seeing how much blackening there is.

The Canola Council of Canada has a blackleg disease severity rating scale you can use to assess infections and calculate yield loss. You can find it along with many other scouting tips on its website.

Checking blackleg levels in infected fields is something producers should always try to do before heading out to swath or straight cut their canola crop, adds Cornelsen. The information can be used to estimate yield losses as well as help with field and crop planning decisions.

“That is the time to really look for the disease because there’s not much you can do throughout the growing season,” she says. “If you can quantify the damage prior to harvesting it, it’s just going to allow you to better plan for the future and how you’re going to handle that crop or that field going forward.”

Crop rotation

Resistant hybrids aren’t the only way to fight blackleg. Short rotations provide a great breeding ground for the L. maculans pathogen, so it only makes sense to try to extend rotations so there’s more time between canola crops and more opportunity for microbes to break down canola residue.

“Crop rotation is one of the, if not the, most important things when we look to manage blackleg properly,” Cornelsen says. “If you can extend out your crop rotation, so grow canola at least a minimum of one every three years, having that two-year break with canola in your rotation is really going to reduce the inoculum.”

Crop diversity is also part of the answer. Cornelsen says using a longer rotation that includes more crops reduces the chances of resistance developing within the pathogen population, since there’s less selection pressure.

Fungicides

Another tool for managing blackleg are fungicides, which are available as seed treatments or as foliar applications.

Cornelsen maintains fungicides generally don’t provide a lot of economic benefit because disease-resistant hybrids do the heavy lifting when it comes to blackleg control. An exception, she says, are high-risk situations where environmental conditions or a tight canola rotation favour disease development and blackleg resistance has broken down.

“Where you see fungicide pay off is when that resistant gene or source you’ve been using in the field is overcome,” she says. “That is where you’ll see some reduction.”

Cornelsen says the problem with relying too much on foliar fungicides is they’re typically applied too late in a canola plant’s development to do much good.

“Timing is everything because blackleg comes in so early,” she explains. “The critical window for blackleg infection is from cotyledon to the two-leaf stage. That is the time plants need to be coated and protected.

“If you’re applying (fungicide) at the six-leaf stage or later, even into flowering, all you’re doing is minimizing late-season infections. But those late-season infections aren’t the ones that contribute to yield loss.”

Fungicide seed treatments generally do an effective job of preventing blackleg from showing up in seed lots. However, most of them don’t protect the plant during its early-growth stage when they’re most susceptible to L. maculans infection, Cornelsen says, and that’s because they’re not systemic.

A couple of new seed treatments have been released in the last year or two that offer a solution, she notes.

One is Saltro from Syngenta. This canola fungicidal seed treatment contains a new active ingredient called Adepidyn, which Syngenta says provides early-season protection against airborne blackleg infection at the plant’s most vulnerable stage.

The other is Vercoras from BASF. This new canola seed treatment combines an insecticide with four fungicide active ingredients, including one BASF says offers control of airborne blackleg through the critical early-season infection period.

Cornelsen says Saltro and Vercoras are products canola producers might want to look at, especially if they are dealing with ideal conditions for disease development and the resistance genes they are using just aren’t working.

“A product like this would pay off in the end because it’s protecting (canola) at that critical window for infection,” she says.

– This article was originally published in the April 19, 2022 issue of Grainews.