Seed & Chemical
Multiple fungicide applications for sclerotinia – a good idea?
By Bruce Barker
High canola prices over the last few years, coupled with very good growing conditions, had some growers and agronomists looking at multiple fungicide applications to control sclerotinia in canola. While canola prices are currently much lower in the spring of 2014 and penciling in a second fungicide application may be difficult, there are some circumstances where a second application of a fungicide makes economic sense.
Typically, about 50 per cent of canola acres are sprayed with one fungicide application for sclerotinia control. In 2013, 227,000 acres of canola received two fungicide applications for sclerotinia control – a fraction of the almost 20 million acres planted to canola.
“There are scenarios where a second application of a fungicide might make sense. The 2012 growing season was a good example, where the weather conditions were very favourable for disease development for an extended period of time,” says Clint Jurke, Canola Council of Canada agronomist in Saskatoon.
That year, the weather leading up to and through the flowering period was quite wet, favouring sclerotinia development. The wet, humid weather also prolonged the flowering period, providing a longer period for the disease to develop. Ideal canola growing weather is also ideal sclerotinia weather.
Apothecia spores grow and feed on dead flower petals that drop on leaves and stems. The petals provide the food source for spores to germinate and grow. Moist conditions that keep leaves and stems wet provide the environmental conditions necessary for infection. Rainfall or heavy dew can provide these conditions.
The Canola Council of Canada has a scouting and risk assessment tool that provides guidance on the need for fungicide application. Generally, the window of application for a fungicide is 20 to 50 per cent bloom, with optimum timing at 30 per cent bloom. The objective of the fungicide application is to cover as many petals as possible to prevent the spores from germinating and growing. However, the fungicide is only active on the petals that are covered, so using one fungicide application is a balance between waiting for enough flowers to emerge, but not waiting too long and allowing some petals to already fall and allow the disease to start developing.
Split or multiple applications may improve control
In most years, a well-timed fungicide application at 20 to 30 per cent blossom stage will provide optimum sclerotinia control. But in a year where extended periods of rain, high humidity or heavy dew are forecast and the risk factors are high, using a split or multiple application of a fungicide may be warranted, especially in years of high canola prices.
Two fungicides are registered for a split application using lower rates than if only one application was made. Fungicides containing iprodine active ingredient, Overall and Rovral, are registered for a split application using a lower rate at 20 per cent and 50 per cent bloom, as opposed to one higher rate at 20 to 50 per cent bloom. Acapela fungicide from DuPont is also registered for a split application at a lower rate. However, Todd Friday, pulse and oilseed market segment manager with DuPont, does not recommend a split application with Acapela.
“With Acapela, our company’s position is that if you are going to make a second application, you should rotate to a different group of fungicides, rather than applying a second application of Acapela,” says Friday. “Sustainability has a very high profile with DuPont, and rotating fungicides will help to maintain the usefulness of these products.”
That recommendation also stands for DuPont’s other sclerotinia fungicide, Vertisan. Its label states that under high disease pressure, a second application can be made, but to switch to a fungicide with a different mode of action.
Lance fungicide from BASF is registered for a sequential application. Jason Leitch, fungicide brand manager with BASF in Mississauga, Ont., says that while Lance is registered for two applications seven to 14 days apart, “it is always a good practice to rotate groups if a second application (back to back) is being made for the same disease in the same crop.
“Always use full label rates in order to control the disease or risk the fungicide becoming ineffective in control and adding selection pressure for the development of disease resistance,” adds Leitch.
Jurke says that most growers are targeting sclerotinia at the 20 to 30 per cent stage with a full rate of fungicide, and then monitoring the crop and weather forecast to see if a subsequent application would be warranted. That is the practice that Friday and Leitch also recommend.
“Target 20 to 30 per cent bloom, which is the most effective window for sclerotinia control. Then look at the yield potential of the crop, the weather forecast, the crop microclimate, consider past crop rotations and disease risk, and make a decision to make a second application based on those risk factors,” says Friday.