Timely application of fungicide is the key to controlling sclerotinia stem rot in canola, but knowing when to spray is tricky, says Kelly Turkington, a plant pathologist with Agriculture Canada at the Lacombe research station.
“There has been a lot of work done on crop rotation and there is very little relation between crop rotation and sclerotinia levels in canola,” said Turkington.
That’s partly because the sclerotinia fungus produces airborne spores and can readily move from one field to another.
“What’s happening in your adjacent field or your neighbour’s adjacent field could have an impact on your risk of sclerotinia,” Turkington told a recent meeting here.
The best way to understand sclerotinia is to look at the host, the pathogen, and the environment and how the three of them relate to each other, he said.
Key things to look for in terms of the host are canopy closure and flowering date period, and whether or not they coincide with favourable weather conditions. High lodging potential within a crop will increase the risk of sclerotinia and plant-to-plant spread of the disease. Producers can also make decisions about spraying by finding out the history of the sclerotinia pathogen in their field or area.
Petals are a key part of the disease cycle.
“If you’ve got about 35 to 45 per cent of those petals infested with sclerotinia, that’s going to be an indication of a risk that might warrant fungicide application,” said Turkington. “These levels can change quite dramatically over the flowering period.”
Yield potential is a reflection of canopy density. Lower yields have a less-dense canopy and a lower risk, said Turkington. Fertility regime can also have an impact on disease control.
“If you’re really pushing for maximum yield and over-fertilizing, that will greatly increase your risk,” he said.
Crops with lower yield potential will not respond as well to fungicide applications as those with higher yield potentials.
“If your crop has a yield potential of much less than 30 or 35 bushels per acre, the risk in terms of sclerotinia stem rot is low and spraying is not likely to provide an economic benefit,” Turkington said.
Risk increases when the pathogen is present in a high-yield crop in an area with frequent precipitation.
If the weather conditions are dry and the temperatures are in the 30 to 35 C range over seven to 14 days, the risk decreases. Frequent showers and moderate temperatures in July up the risk, but heavy precipitation will not be favourable towards the development of the disease. Conditions that promote petal sticking will encourage infestation from infected petals. Irrigation may also have an impact.
The decision on when to apply fungicide depends on when the inoculant becomes available.
“If you’ve got average rainfall throughout June, with good moisture conditions, sclerotinia will start that germination process in mid-June as the crop canopy covers the soil surface,” Turkington said. “There will be sclerotinia spores floating around in the air when the crop gets into the early bloom period. Application at the early bloom stage will be important to look at.”
In very dry conditions, there will be a delay in the germination of sclerotinia until moisture comes. If that moisture comes in late June, most of the inoculum from sclerotinia fungus will hit the canola crops when they are in full bloom, Turkington said.
“In this case, application towards full bloom will be better.”
It’s best to apply fungicide before seeing symptoms of the disease. Fungicide will not provide any benefit once the disease has infected the crop. When applying fungicide, producers need to ensure good coverage of the canola canopies.
“Some of the chemical companies talk about good petal coverage, but you need to actually get that chemical into the crop canopy so you’re getting product down into the leaf axils and leaf bases, the areas where the sclerotinia fungus is going to infect,” Turkington said.