The early drought conditions we have been experiencing in Prairie Provinces has resulted in some believing that a fungicide application may not be necessary to control Sclerotinia stem rot in canola. However, moist soil conditions and warm temperatures (15 to 25° C) during flowering can result in substantial spore production, particularly in fields with high inoculum loads available for possible infection. Additionally, fields with high plant density in which spore production and flowering are synchronized are at the highest risk. A risk assessment tool developed in Sweden can be useful in assessing the risk of Sclerotinia infection in canola, click here for the tool. The risk assessment tool should be used when 75% of the plants have at least three open flowers. Fields that rank at 40 or above with this assessment tool are at higher risk of infection and may benefit from an application of fungicide.
If the risk assessment tool indicates a high risk of Sclerotinia stem rot, when is the best time to apply a fungicide?
Depending on the particular fungicide, the application window is generally between 20 and 50% bloom with 30% flowering as the optimal plant stage. To determine the stage of bloom of the field, sample several locations in the field. Count the number of open flowers on the main stem and consult the chart below to estimate the percentage of bloom of the field. Keep in mind that under usual conditions, the field can move from first flower to 10% bloom in about two to four days.
| Flowering Percentage of Field |
Open Flowers on Main Stem |
|
Brassica napus - Canola |
Brassica rapa - Canola |
| 10% |
10 |
6 to 7 |
| 20% |
14 to 16 |
10 to 12 |
| 30% |
20 |
14 to 16 |
The goal of a fungicide application is to cover as many of the petals as possible and that is usually associated with a field at 30% bloom, which is when the most flowers are open at one time. It is critical to cover as many petals as possible with the fungicide to maximize the possibility of success in controlling the disease.
How long is the application window for a fungicide?
Depending on weather, the window from first flower to 20 to 30% bloom is four to eight days, so if the grower does not have the spray equipment, contacting a custom applicator should be done as soon as the decision is made to apply a fungicide.
The field is very uneven in development, how can the grower determine when to apply a fungicide?
In at least three to five locations across the field, look at 100 plants in each area. Use the stage of growth that best represents the majority of the plants across the field. If a single application is to be made, base your timing of application on the flower stage of the majority of plants. A split application of fungicide may be necessary depending on the range of growth stages that are in the field and if future weather conditions dictate the need for a second application for later flowering plants. It is important to realize that definitive recommendations are not possible in this situation.