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Sclerotinia biggest issue for Manitoba canola growers in 2016

The annual disease survey found this perennial challenge is still the biggest issue for farmers

Sclerotinia was the biggest issue for Manitoba canola growers last year, according to results from the 2016 disease survey.

It showed that over 90 per cent of the 105 Manitoba fields surveyed had some level of sclerotinia infection.

The incidence of the disease – the number of plants infected in the field – was between 15 and 20 per cent across the province. The general rule of thumb is that a 0.5 per cent incidence is the level at which yield loss is predicted.

“Actual yield loss varies depending on the severity but I’d guess that the majority of the canola fields surveyed was sprayed with a fungicide,” said Holly Derksen, plant pathologist with Manitoba Agriculture in her presentation of the disease survey results at the Manitoba Agronomist Conference (MAC) in Winnipeg last December. “Imagine what it would have looked like if there had not been any fungicide applications out there.”

What tells the real story, however, is the severity of sclerotinia infection last year, which was the highest since 2011. Measured on a scale of zero (lowest) to five (highest), the provincial average severity rate in 2016 was 3.2, with the highest in the southwest at 3.9, followed by the central region at 3.6.

“A two rating could affect about a quarter of the seed formation, at three it could affect about half of the seed formation, and at four, about three-quarters of the seed formation,” said Derksen. “Having an average severity of 3.9 in the southwest is quite significant, obviously. The highest provincial average we have seen before this year was 2.6 so we are quite a bit higher than we had seen for severity in sclerotinia previously.”

Managing in 2017

With any disease it’s important to know your risk. There are a number of risk calculators available online (see sidebar) that can help farmers assess the risk of sclerotinia and forecast the likely economic impact it might have for their operation.

Crop density, past rainfall, the forecast for rain, and regional risk – which for Manitoba is basically anywhere – are important to know, and rotation can help to reduce the risk of infection, but knowing previous disease levels – especially for sclerotinia – is vital.

“Rotation plays a role but the sclerotinia survive in the soil for up to seven years or longer, so rotation isn’t going to have a huge impact unless you have a 10-year rotation, but those previous disease levels do have an impact,” said Derksen. “That’s important when you’re calculating your risk. It’s the top 1-1/2 inches of soil that matters most, and at 25 per cent saturation, ascospores will germinate in five hours, so it doesn’t need to be soaking wet.”

The ideal conditions for ascopore germination is around 25 C, and cooler temperatures — around 20 C — are optimum for lesion development.

“Once you have ascospores affecting your crop, it doesn’t need to be quite as warm for that lesion to spread, cooler is even better,” said Derksen. “Relative humidity over 85 per cent is best for ascospore release but once you get past that early point it doesn’t matter as much if the humidity is that high.”

Hitting the recommended stage for a fungicide application – at the 20 to 50 per cent bloom stage – can be tricky.

“In 2016, we had a very stagey crop so it made it tricky for farmers to get the right stage for their entire fields,” said Derksen. “The flowering period went on and on, and the conditions stayed ripe for infection, so there were growers who made a second fungicide application even later than at 50 per cent bloom. For those growers, I imagine a lot of them don’t regret that decision.”

Sclerotinia-tolerant varieties can also be a part of an integrated pest management strategy. Resistant varieties can’t get the job done alone however, and a fungicide application is still very important.

Blackleg too

Blackleg was the second-most-prevalent disease in Manitoba canola fields in 2016, with around 82 per cent of the fields surveyed showing some level of infection.

Blackleg is present every year in Manitoba fields but usually at a low incident rate. In 2016, the incidence was down a little from the previous year to around 12 per cent, but the aim is to get it even lower, said Justine Cornelsen of the Canola Council of Canada in her presentation at MAC.

New data coming out of Alberta shows how much yield loss farmers can expect as they move up the blackleg disease severity scale, which begins at zero where plants are clean and there is no disease, up to five where there is complete blackening of canola plants causing premature death.

“If farmers move from a zero to one rating, they will lose anywhere from 17 to 24 per cent yield per plant,” said Cornelsen. “With a rating of just two they could lose up to 40 per cent yield. The average severity here in Manitoba is around 1.5, so farmers can expect to lose roughly 30 per cent of yield per plant. This year in Manitoba, farmers were losing around 1.5 to two bushels per acre.”

Looking back over the past 20 years, the prevalence and incidence of blackleg has been slowly increasing for several reasons. One is tighter canola rotations, which may be challenging the durability of resistant varieties. There are basically two different types of resistance genes that offer protection for the canola plant in different ways. A major, qualitative gene stops the disease as soon as it attacks but it’s race specific, so has to match the race of the blackleg pathogen in the field to work. A minor, quantitative gene is broader, so it doesn’t matter what race of the pathogen is present, it will slow down the disease as it moves through the plant, reducing yield loss. A majority of our blackleg-resistant canola varieties employs a combination of major and minor genes.

“Research being done at the University of Manitoba is showing a shift in the disease within our fields,” said Cornelsen. “We’re seeing a shift from avirulent to virulent pathogens. The avirulent pathogen is the ‘key’ to the ‘lock’ that opens the defence response within the plant and stops infection at the site. A virulent pathogen is aggressive and doesn’t fit the ‘lock’ quite right, so the defence response never gets started in the plant, allowing for infection. The minor gene starts pulling its own weight when the virulent form starts spreading throughout the plant. It helps to slow the rate and spread of infection and this is why we have been seeing low incidence and severity rates for blackleg.”

Canada rates varieties for disease as resistant, moderately resistant, moderately susceptible, or susceptible. Australia uses a major gene labelling system so growers know what resistance gene they’re using. “They are able to then rotate their genes, so if they’re seeing a shift in the pathogen, they can move to a different resistance group and use another variety to fight and match what’s going on their field. We are in the early stages of developing a similar rating model for Canada.”

First defence

Meanwhile, best management practices (BMPs) remain the first line of defence for farmers struggling with blackleg in their fields. “The BMPs are pretty well the same for a lot of our canola diseases,” said Cornelsen. “Learning to identify the disease and getting out and checking is crucial, and agronomists should take their growers along with them, because we see a lot of misdiagnosis with blackleg attributed to sclerotinia, verticillium and other diseases.”

For resistant varieties to work well, it’s crucial to make sure farmers are fighting the right pathogen. “That’s where resistance stewardship comes in,” she said. “We don’t want to abuse the genetics that we have out there, similar to what we’ve seen in Alberta with clubroot. We need to be monitoring and managing our resistant genes and varieties.”

More diverse crop rotations, as new crops like soybeans become more popular, is good news to help break up disease cycles. “Biodiversity is so crucial, and if we can extend our rotations, it’s going to help a lot of our crops manage their own disease or pest problems,” said Cornelsen.

Early fungicide applications are vital to be effective against blackleg.

“They need to get into the field at a cotyledon to two leaf,” said Cornelsen. “Although the general recommendation is two to six leaf, if they miss the initial blackleg infection, it’s already too late. From cotyledon to six-leaf stage is when they will see yield loss at the end of the season, so that’s why an early application is important. But it’s a tough sell because you’re pretty well spraying the dirt.”

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