If you had a tough time with sclerotinia in your canola crop this past year, you’re not alone.
“It is still probably one of the most frustrating diseases that producers deal with,” says Barb Ziesman, provincial specialist in plant disease with Saskatchewan Agriculture.
The environment was perfect for disease development across Saskatchewan, says Ziesman. Saskatchewan Agriculture’s annual canola disease survey found sclerotinia stem rot in 92 per cent of the canola crops surveyed, she adds.
Province-wide, disease incidence for sclerotinia averaged 24 per cent in all fields surveyed. Farmers in west-central Saskatchewan suffered the highest disease incidence, averaging 43 per cent. East-central farmers saw the lowest, averaging 18 per cent. Disease incidence measures the percentage of plants observed with disease symptoms within a field.
Ziesman says they chose fields at random for the disease survey, so there’s a chance that farmers had applied fungicide.
Asked why a canola grower might have a wreck with sclerotinia even after applying fungicide, Ziesman says there are two issues around fungicide application.
First a grower must decide whether or not to apply fungicide. Ziesman says that’s often the easier decision. The soil needs to be wet at least in the top couple of centimetres for about 10 days for the sclerotia to germinate, grow into apothecia, and produce spores, says Ziesman. Canola growers can find a sclerotinia checklist to help them make that decision at www.saskcanola.com/research/riskcalculator.php.
“The second one is when do I apply that fungicide?” It’s a small window, Ziesman says, and control problems often come down to timing.
Flower petals are the first infection point in canola, which is why fungicide is applied at flowering. Sclerotinia needs something weak or dying to use as an energy source before it can infect living leaf and stem tissue, Ziesman says.
“If you apply your fungicide at 10 to 20 per cent bloom, those petals are going to fall,” says Ziesman. But if the ascospores come later, those infected petals won’t have fungicide on them. The trick is to spray when the pathogen is most present and risk is highest, while protecting flowers and lower leaves, she says.
As a rule of thumb, farmers should look at how much moisture they’ve had, when the moisture has come, and the crop canopy. The timing of rain is important, Ziesman says. If farmers don’t get any rain until 10 to 20 per cent bloom, the risk might shift to later in the season, she says.
“Watching the external environment is one thing, but you also need to pay attention to what’s going on in your crop,” says Ziesman. Farmers with dense crop canopies need to be vigilant. If pants are wet after walking through the field in the middle of the day, there’s probably enough moisture for sclerotinia spores to germinate and infect plants.
Prolonged flowering leaves plants susceptible to sclerotinia for a longer period. A farmer might time the application perfectly for the first round of ascospores, Ziesman says. But if more ascospores are released, plants can still be infected.
A longer crop rotation will help by cutting the amount of inoculum in the field, Ziesman says. But it’s not a silver bullet. Unlike most pathogens, sclerotinia infects a broad range of plants. Most broadleaves are susceptible, Ziesman says, and the disease infects over 400 host plants, including some weeds. Spores can also travel from other fields.
Ziesman also suggests picking a variety with a high rating for lodging resistance. A lodged crop holds more moisture, which is conducive to disease growth. “So you’ll often see a higher disease severity when a field is lodged.”
Farmers can also seed sclerotinia-tolerant varieties. “The important thing to remember is that it is tolerant, and not resistant. And what that means is that the plant is still going to get infected,” says Ziesman.
Sclerotinia-tolerant plants will suffer less yield loss and infection if disease pressure is low to moderate, she explains. “When disease pressure is high, it can still suffer yield losses and benefit from a fungicide application and other management practices.”
“There’s a risk of resistance with any fungus,” says Ziesman. Short rotations and constant use of a fungicide increase risk, she adds. Ziesman says farmers can try rotating fungicide groups within the field, even with other crops. For example, if a farmer is always controlling sclerotinia in canola and peas, it’s a good idea to track the modes of action.
“The more you use a single mode of action, the higher the chance that you’re going to put selection pressure on the pathogen,” says Ziesman.
Farmers applying fungicide twice within the same season might want to use a different mode of action that year as well, she adds. “It’s definitely a recommendation in some other crops where fungicide-resistant pathogen populations have been identified.”
Researchers have studied fungicide sensitivity in some pathogens, but there are too many pathogens to create a broad program, such as the herbicide-resistant weed programs, Ziesman says.
Farmers can use test strips to monitor fungicide resistant on their own. Leaving an untreated strip could provide the first clue that a pathogen is no longer sensitive to the fungicide. It will also show whether the fungicide application was effective. Test strips also provide good information to look back at when farmers need to make similar decisions in future years, Ziesman adds.